Tartaric Acid Identification: Key Facts at a Glance
• Physical Properties: Physical Appearance · Solubility · Flame Test · Melting Point (170–172 °C) · Tartramide m.p. (195 °C) · Tartaranilide m.p. (180 °C) · Molecular Formula (C₄H₆O₆) · MW (150.09 g/mol) · pKₐ₁ (2.98) · pKₐ₂ (4.34) · Optical Rotation [α]D²⁰ (+12°)
• Chemical Tests & Coverage: Litmus · NaHCO₃ Test · Ester Test · FeCl₃ (buff-yellow) · Bromine Water · Acidified KMnO₄ · Tollens’ Silver Mirror · Fenton’s Reagent (intense blue) · CaCl₂ Precipitation · Cream of Tartar Test · Lassaigne’s Test · Conc. H₂SO₄ Test · Dry Heating · Control Test · Tartramide · Tartaranilide · Mixed Melting Point · Chemical Structure · Stereochemistry · Optical Isomers · Applications · Safety · FAQs · MCQs · Viva Questions
What is tartaric acid? — Identity, formula & structure
Tartaric acid (IUPAC name: 2,3-dihydroxybutanedioic acid; molecular formula C₄H₆O₆) is a naturally occurring dicarboxylic α-hydroxy acid found abundantly in grapes, tamarinds, and several other fruits. It is the primary acid responsible for the characteristic tartness of unripe grapes and for the white crystalline deposits (argol or wine lees) found on the inside of wine barrels. Its systematic IUPAC name is 2,3-dihydroxybutanedioic acid. Louis Pasteur’s 1848 resolution of its racemate using sodium ammonium tartrate crystals made tartaric acid the founding compound of stereochemistry — a historical distinction that makes it uniquely important in organic chemistry education.
|
Property |
Value |
|
IUPAC name |
2,3-Dihydroxybutanedioic acid |
|
Common names |
Cream of tartar acid; L-(+)-tartaric acid; dihydroxysuccinic acid |
|
Molecular formula |
C₄H₆O₆ |
|
Molecular weight |
150.09 g/mol |
|
Functional groups |
2 × –COOH (carboxyl), 2 × –OH (secondary hydroxyl) |
|
Structural class |
Dicarboxylic alpha-hydroxy acid |
|
Degree of unsaturation |
2 (from two C=O in –COOH groups; no C=C double bonds, no rings) |
|
CAS number |
87-69-4 (L-form); 147-73-9 (meso); 133-37-9 (DL-racemic) |
|
E number (food) |
E334 |
|
Acid character |
Diprotic — two ionisable –COOH groups; pKa₁ = 2.98, pKa₂ = 4.34 |
Physical properties
L-(+)-tartaric acid (CAS 87-69-4) has a melting point of 170–172 °C, water solubility of 133 g/100 mL at 20 °C, specific optical rotation [α]D²⁰ of +12.0°, and a molecular weight of 150.09 g/mol. It is a diprotic acid with pKₐ₁ = 2.98 and pKₐ₂ = 4.34. The meso form (CAS 147-73-9) melts at 165–166 °C and the racemic DL form (CAS 133-37-9) at 206 °C. Complete physical constants for all three stereoisomeric forms are summarised in the table below.
|
Property |
L-(+)-form |
meso-form |
DL-racemic |
|
Melting point |
170–172 °C |
165–166 °C |
204–206 °C |
|
Optical rotation [α]D²⁰ |
+12.0° |
0° (internal compensation) |
0° (external cancellation) |
|
Density |
1.788 g/cm³ |
1.666 g/cm³ |
1.697 g/cm³ |
|
Solubility in water (20 °C) |
133 g/100 mL |
125 g/100 mL |
20.6 g/100 mL |
|
CAS |
87-69-4 |
147-73-9 |
133-37-9 |
|
Property |
Value (L-form unless stated) |
|
Appearance |
White crystalline powder or colourless crystals; odourless |
|
Boiling point |
Decomposes ~400 °C (no clean boiling point) |
|
Solubility in Ethanol |
Sparingly soluble |
|
pKa₁ |
2.98 |
|
pKa₂ |
4.34 |
|
pH (0.1 M solution) |
approximately 2.2 |
|
Taste |
Strongly sour, clean acidic (no bitterness) |
|
Acid type |
Diprotic — two ionisable –COOH groups |
Systematic Identification and Qualitative Analysis of Tartaric Acid — Complete Lab Procedure
The qualitative analysis of tartaric acid follows a systematic procedure covering preliminary physical tests, functional group tests for –COOH and –OH, elemental detection, confirmatory tests specific to tartaric acid (CaCl₂ precipitation and cream of tartar), derivative preparation, and mixed melting point confirmation. Each step is designed to progressively narrow the identity of the unknown compound before arriving at a definitive conclusion.
Note: The table summarises what each test detects, what it rules out, and the expected result for tartaric acid. Full procedural details — quantities, conditions, safety notes, and interpretation of borderline results — are provided on the page dedicated to each test. Readers are strongly encouraged to study the relevant page before attempting any procedure in the laboratory.
|
Step |
Test Name |
What It Detects / Rules Out |
Expected Result for Tartaric Acid |
|
1 |
Physical Appearance |
Pure compound vs impure; organic vs inorganic; presence of chromophore |
Colourless/white crystalline solid; odourless; no colour (no extended conjugation or transition-metal involvement) |
|
2 |
Solubility in water |
Polarity; –OH group presence; aliphatic vs aromatic profile |
Freely soluble; clear colourless solution (three –COOH + two –OH enable extensive H-bonding) |
|
3 |
Dry heating test |
Hydroxy acid character; distinguishes from simple dicarboxylic acids |
Charring and odour of burnt sugar (α-hydroxy acid undergoes intramolecular dehydration and carbonisation) |
|
4 |
Flame test (ignition) |
Organic vs inorganic; aromatic vs aliphatic |
Clean blue-yellow flame; no smoky/sooty flame (no benzene ring; aliphatic compound confirmed) |
|
5 |
Litmus / pH paper |
Acidic compound |
Blue litmus turns red; pH ≈ 2.2 (consistent with a diprotic acid; pKa₁ 2.98) |
|
6 |
NaHCO₃ test |
–COOH group; number of acid groups |
Brisk effervescence; 2 mol CO₂ per mol acid; gas turns lime water milky |
|
7 |
Ester test (esterification) |
–COOH group confirmed; rules out sulfonic acid |
Fruity odour of diethyl tartrate on heating with EtOH / conc. H₂SO₄ |
|
8 |
Conc. H₂SO₄ test |
α-hydroxy acid character; dehydration behaviour |
Immediate charring and effervescence (dehydration + oxidation of α-hydroxy acid; not seen with simple dicarboxylic acids) |
|
9 |
Elemental detection (Lassaigne’s test) |
Presence of N, S, Cl, Br, & I |
Negative for all: no N, S, or halogens — compound contains C, H, and O only (consistent with C₄H₆O₆) |
|
10 |
Bromine water test |
C=C unsaturation; aromatic ring |
No decolourisation (negative — fully saturated; no C=C present) |
|
11 |
Acidified KMnO₄ (Baeyer’s test) |
C=C double bond vs –OH oxidation |
Slow decolourisation (–OH groups oxidised; no C=C; slow rate distinguishes from alkenes which decolourise instantly) |
|
12 |
FeCl₃ test |
Phenolic –OH vs aliphatic –OH; α-hydroxy acid character |
Buff-yellow colouration; no deep violet (aliphatic –OH only; deep violet would indicate phenol) |
|
13 |
Ammoniacal AgNO₃ test (Tollens’) |
Reducing character; α-hydroxy acid (distinguishes from non-reducing dicarboxylic acids) |
Silver mirror formed on test tube walls (Ag⁺ reduced to Ag⁰ by the α-hydroxy acid) |
|
14 |
Fenton’s reagent test |
Tartaric acid fingerprint (H₂O₂ + excess NaOH) |
Intense blue colour; highly specific for tartaric acid — citric acid and malic acid do not give this response |
|
15 |
Melting point determination |
Purity; compound identity (first estimate); stereoisomeric form |
M.p. 170–172 °C (L-form); dl-form 206 °C; meso-form 165–166 °C |
|
16 |
CaCl₂ precipitation |
Tartrate ion (primary confirmatory) |
White crystalline Ca-tartrate precipitate in cold solution — key contrast: calcium citrate only precipitates on heating |
|
17 |
KCl / K⁺ test (cream of tartar) |
Tartrate ion (secondary confirmatory) |
White KHC₄H₄O₆ (cream of tartar) precipitate — uniquely specific for tartaric acid among common organic acids |
|
18 |
Literature survey |
Compound identity cross-check against physical and chemical constants |
Literature match: aliphatic, saturated, diprotic α-hydroxy acid, m.p. 170–172 °C → L-(+)-tartaric acid (Sources: Merck Index, CRC Handbook, PubChem) |
|
19 |
Control test |
Validates CaCl₂ / Fenton’s / KCl results against authentic standard |
Authentic L-tartaric acid gives identical results to unknown in CaCl₂, Fenton’s, and KCl tests |
|
20 |
Derivative I — amide (tartramide) |
Literature comparison; compound identity (independent of acid m.p.) |
White crystalline solid; amide derivative m.p. 195 °C (verify vs Vogel’s / lit.) |
|
21 |
Derivative II — anilide (tartaranilide) |
Literature comparison; second independent physical checkpoint |
White crystalline solid; anilide derivative m.p. 180 °C (verify vs Vogel’s / lit.) |
|
22 |
Mixed melting point |
Positive identity confirmation (gold standard) |
No depression when mixed with authentic L-tartaric acid standard — conclusively confirms identity |
This is the real skill of qualitative organic analysis: it is not about running every test in the textbook, but about reading the molecular structure and selecting tests that carry real diagnostic weight. Once that habit is established, identifying any unknown organic compound becomes a logical exercise rather than a guessing game.
Materials and Reagents for Qualitative Analysis of Tartaric Acid
⦁ Tartaric acid sample (unknown)
⦁ Distilled water
⦁ NaHCO₃ solution (sodium bicarbonate, ~5% aqueous)
⦁ Freshly prepared lime water (Ca(OH)₂ solution) — for CO₂ confirmation
⦁ Blue and red litmus paper
⦁ Absolute (anhydrous) ethanol — for ester test
⦁ Concentrated H₂SO₄ — for ester test and dry heating test
⦁ Bromine water (dilute aqueous solution of Br₂)
⦁ Acidified KMnO₄ solution (dilute) — Baeyer’s reagent
⦁ Neutral FeCl₃ solution (~1% aqueous, freshly prepared) — for FeCl₃ test
⦁ Calcium chloride solution (CaCl₂, ~5% aqueous) — for calcium tartrate precipitate test
⦁ Ammonium hydroxide solution (NH₄OH) — for neutralisation in CaCl₂ test
⦁ Potassium chloride solution (KCl, saturated) — for cream of tartar (potassium hydrogen tartrate) test
⦁ Freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent) — for silver mirror test
⦁ Hydrogen peroxide solution (H₂O₂, ~3%) — for Fenton’s reagent test
⦁ Sodium hydroxide solution (NaOH, ~2M) — for Fenton’s reagent test (added after H₂O₂)
⦁ Concentrated aqueous ammonia — for tartramide (amide derivative) preparation
⦁ Aniline + pyridine or dilute NaOH — for tartaranilide (anilide derivative) preparation
⦁ Melting point apparatus with sealed capillary tubes
⦁ Delivery tube assembly — for passing CO₂ into lime water
⦁ Authentic (known) L-(+)-tartaric acid sample — for control test and mixed melting point
Qualitative Analysis of Tartaric Acid — Procedure & Observations
This section details the step-by-step procedure, observations, and inferences for the qualitative identification of tartaric acid in the laboratory. Tests covered include the NaHCO₃ test, FeCl₃ test, bromine water test, acidified KMnO₄ test, Tollens’ silver mirror test, Fenton’s reagent test, CaCl₂ precipitation, and cream of tartar test — each recorded with its expected observation and chemical inference in the format required in undergraduate practical examinations.
Note: The table summarises what each test detects, what it rules out, and the expected result for tartaric acid. Full procedural details — quantities, conditions, safety notes, and interpretation of borderline results — are provided on the page dedicated to each test. Readers are strongly encouraged to study the relevant page before attempting any procedure in the laboratory.
|
Step |
Test |
Procedure |
Observation |
Inference |
|
Steps 1–4: Preliminary Tests (Physical Observations) |
|
1 |
Physical appearance |
Examine a small sample visually; waft cautiously for odour. |
White crystalline solid; odourless; no colour. |
No chromophore; no aromatic conjugation; consistent with a pure aliphatic organic acid. |
|
2 |
Solubility in water |
Add 0.5 g to 5 mL distilled water; shake at room temp; warm gently and re-check. |
Freely soluble; clear colourless solution. |
Polar compound; two –COOH groups and two –OH groups assist hydration via H-bonding. Aliphatic profile. |
|
3 |
Dry heating test |
Place a small quantity in a dry test tube. Heat gently over a Bunsen flame; note all changes. |
Charring; odour of burnt sugar produced |
α-Hydroxy acid undergoes intramolecular dehydration and carbonisation on dry heating. Distinguishes tartaric acid from simple dicarboxylic acids (e.g. oxalic sublimates cleanly). |
|
4 |
Flame test (ignition) |
Heat a spatula-tip on a nichrome wire in a non-luminous Bunsen flame. |
Burns with a clean blue-yellow flame; no sooty/smoky flame. |
Organic compound confirmed; absence of sooty flame confirms no benzene ring (aliphatic compound). |
|
Steps 5–8: Functional Group Tests — Carboxylic Acid (–COOH) |
|
5 |
Litmus / pH paper |
Dip pH paper into the aqueous solution. |
Blue litmus turns red; pH paper shows ≈ 2.2. |
Acidic compound — consistent with a dicarboxylic acid (pKa₁ 2.98). Not specific alone; further tests required. |
|
6 |
NaHCO₃ test |
Add solid NaHCO₃ to aqueous tartaric acid solution. Pass evolved gas through lime water via delivery tube. |
Brisk effervescence; CO₂ produced; lime water turns milky. |
–COOH confirmed; 2 mol CO₂ per mol acid (dicarboxylic acid). CO₂ evolves because tartaric acid (pKa₁ 2.98) is stronger than carbonic acid. |
|
7 |
Ester test (esterification) |
Heat aqueous solution with ethanol and 2 drops conc. H₂SO₄; cautiously note odour on pouring into cold water. |
Fruity odour of diethyl tartrate detected. |
Two esterifiable –COOH groups present; confirms dicarboxylic acid. Rules out sulfonic acid. |
|
8 |
Conc. H₂SO₄ test |
Add a few drops of conc. H₂SO₄ to a small quantity of tartaric acid in a test tube. Warm gently (fume cupboard). |
Immediate charring and effervescence. |
α-Hydroxy acid dehydrated by conc. H₂SO₄ — carbonisation + CO/CO₂ evolution. Not seen with simple dicarboxylic acids (e.g. succinic, malonic). |
|
Steps 9–11: Elemental Detection & Unsaturation / Hydroxyl Group Tests |
|
9 |
Elemental detection (Lassaigne’s test) |
Fuse with sodium metal; plunge into water; filter. Test extract for N (FeSO₄/FeCl₃/HCl), S (nitroprusside; Pb-acetate), halogens (AgNO₃/dil. HNO₃). |
N: no Prussian blue. S: no violet/black ppt. Halogens: no ppt with AgNO₃. |
N, S, and halogens absent. Compound contains C, H, O only — consistent with C₄H₆O₆. |
|
10 |
Bromine water test |
Add 2 drops Br₂/water to the aqueous solution; shake. |
NO decolourisation — bromine water retains orange-red colour. |
Negative result. Tartaric acid is fully saturated; no C=C present. This confirms KMnO₄ decolourisation (Step 11) is from –OH oxidation, not C=C. |
|
11 |
Acidified KMnO₄ (Baeyer’s test) |
Add 2 drops acidified KMnO₄ (cold) to the aqueous solution; shake. |
Slow decolourisation of purple KMnO₄. |
Secondary aliphatic –OH groups are oxidised slowly. Slow rate confirms no C=C — alkenes decolourise instantly. |
|
Steps 12–15: Hydroxyl Group & α-Hydroxy Acid Tests |
|
12 |
FeCl₃ test |
Add 2–3 drops neutral 5% FeCl₃ solution to aqueous tartaric acid. |
Buff-yellow colouration; no deep violet; no precipitate. |
Aliphatic –OH groups present. Absence of deep violet confirms NO phenolic –OH (phenols give intense violet with FeCl₃). |
Laboratory Note: Use freshly prepared, neutral FeCl₃ solution. Acidic reagent suppresses complex formation; alkaline solutions may produce Fe(OH)₃ precipitates. The buff-yellow is less intense than phenol’s violet — compare with a control
|
13 |
Ammoniacal AgNO₃ test (Tollens’) |
Add freshly prepared ammoniacal AgNO₃ solution to tartaric acid solution; warm gently in water bath. |
Silver mirror formed on walls of test tube. |
α-Hydroxy acid reduces Ag⁺ to Ag⁰. Distinguishes tartaric acid from non-reducing dicarboxylic acids (e.g. succinic, malonic). Note: reduction is slower than with aldehydes — warm gently and allow 5–10 minutes; a grey precipitate rather than a clean mirror may form. |
Laboratory Note: Use freshly prepared Tollens’ reagent only. Never store prepared reagent as explosive silver nitride may form. Clean the test tube with dilute HNO₃ after use.
|
14 |
Fenton’s reagent test |
Add freshly prepared ammoniacal AgNO₃ solution to tartaric acid solution; warm gently in water bath. |
Intense blue colour produced. |
Highly specific for tartaric acid. Citric acid and malic acid do not give this response. Acts as a fingerprint test. Note: In qualitative analysis, ‘Fenton’s reagent’ refers to H₂O₂ + excess NaOH in the presence of trace iron — distinct from the industrial Fenton process (Fe²⁺/H₂O₂). |
|
15 |
Melting point determination |
Pack finely powdered dry compound into a capillary tube; heat in melting point apparatus at 1–2 °C/min. |
M.p. 170–172 °C (L-form). |
Purity confirmed (sharp range). L-form 170–172 °C; dl-form 206 °C; meso-form 165–166 °C. Narrows identity before literature search. |
|
Steps 16–17: Confirmatory Tests for Tartaric Acid ★ |
|
16 |
CaCl₂ precipitation ★ Primary confirmatory ★ |
Neutralise solution with NH₃ (2–3 drops); add CaCl₂ solution (0.5 mL); mix and allow to stand 5 min in cold. |
White crystalline precipitate forms in cold solution; insoluble in water; dissolves in dilute HCl. |
Calcium tartrate (CaC₄H₄O₆) — positive confirmation of tartrate ion. Key contrast: calcium citrate only precipitates on heating, not in cold. |
|
17 |
KCl / K⁺ test (cream of tartar) ★ Secondary confirmatory |
Concentrate the aqueous solution by gentle heating; add saturated KCl; cool in ice bath; scratch inside of flask. |
White crystalline precipitate of cream of tartar. |
Potassium hydrogen tartrate (KHC₄H₄O₆) — uniquely specific to tartaric acid among common organic acids. No other common aliphatic acid gives this precipitate. |
|
Steps 18–22: Literature Survey, Control Test, Derivatives & Mixed Melting Point |
|
18 |
Literature survey |
Compile all observations: aliphatic, saturated, diprotic, α-hydroxy acid; m.p. 170–172 °C. Search Merck Index, CRC Handbook, PubChem, SDBS. |
Literature match found. |
Aliphatic, saturated, diprotic α-hydroxy acid with m.p. 170–172 °C → L-(+)-tartaric acid (C₄H₆O₆, MW 150.09 g/mol) confirmed from literature. |
|
19 |
Control test |
Repeat CaCl₂ , FeCl₃ test, Fenton’s, and KCl tests using authentic L-tartaric acid under identical conditions. Compare side-by-side. |
Authentic sample gives identical results to unknown in all three tests. |
Confirms results are genuine, reproducible characteristics of tartaric acid — not artefacts of reagent impurity or procedure variation. |
|
20 |
Derivative I — amide (tartramide) |
React tartaric acid with excess NH₃; evaporate gently; recrystallise from water. |
White crystalline solid obtained; m.p. 195 °C. |
Diamide confirms dicarboxylic acid reactivity. m.p. 195 °C matches literature for tartramide — independent physical checkpoint. |
|
21 |
Derivative II — anilide (tartaranilide) |
React tartaric acid with 2 equiv. aniline; heat at 120 °C; cool; filter; recrystallise from EtOH. |
White crystalline solid obtained; m.p. 180 °C. |
Dianilide confirms dicarboxylic acid rreactivity. m.p. 180 °C matches literature. Two independent derivative m.p. values eliminate coincidence. |
|
22 |
Mixed melting point (gold standard) |
Mix equal quantities of unknown and authentic L-tartaric acid; pack into capillary tube; determine melting point. |
Mixture melts at 170–172 °C — no depression, no elevation. |
No depression confirms both samples are the same compound. Gold-standard identity confirmation. (Depression would indicate different compounds via eutectic formation.) |
Chemical Reactions of Tartaric Acid
In the previous section, you recorded what you observed in the laboratory and what conclusion each observation leads to. This section goes one step deeper — explaining the chemistry behind each obsr3ervation through balanced chemical equations, so you understand not just what happens, but why it happens at the molecular level.
Reactions covered include acid–base (NaHCO₃ test), esterification, oxidation (KMnO₄ and Tollens’), precipitation (CaCl₂ and cream of tartar), and the Fenton’s reagent fingerprint test — each linked directly to a functional group or structural feature of tartaric acid.
Colour Legend
|
✅ Positive result |
⚠️ Negative result |
🧪 Derivative |
🔍 Specific fingerprint |
|
R1 |
NaHCO₃ Test |
✅ Positive result |
|
Eq. |
C₄H₆O₆ + 2 NaHCO₃ → Na₂C₄H₄O₆ + 2 H₂O + 2 CO₂↑ |
|
Note |
Acid–base reaction. pKa₁ 2.98 — tartaric acid is strong enough to protonate HCO₃⁻ and release CO₂ gas. Two moles of CO₂ evolved per mole of acid (diprotic). CO₂ confirmed by lime water: CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O. |
|
Phenols (pKa ≈ 10) cannot protonate bicarbonate — this test specifically confirms a carboxylic acid, not a phenol. |
|
R2 |
Ester Test (Fischer Esterification) |
✅ Positive result |
|
Eq. |
C₄H₆O₆ + 2 C₂H₅OH ⇌ diethyl tartrate + 2 H₂O [conc. H₂SO₄, Δ] |
|
Note |
Fischer esterification. Both –COOH groups react with ethanol under acid catalysis. Equilibrium driven by excess ethanol. Product (diethyl tartrate) has a sweet, fruity odour. Sulfonic acids do not esterify under these mild conditions, ruling them out. |
|
R3 |
Calcium Chloride Test — forms in cold (Primary Confirmatory) |
✅ Positive result |
|
Eq. |
C₄H₆O₆ + CaCl₂ + 2 NH₃ → CaC₄H₄O₆↓ + 2 NH₄Cl |
|
Note |
White precipitate of calcium tartrate (CaC₄H₄O₆) forms in cold solution after neutralisation with NH₃. The precipitate is insoluble in water but dissolves in dilute HCl. Confirms tartrate ion. |
|
Key contrast: calcium citrate (from citric acid) only precipitates on heating, NOT in cold. This thermal distinction is the single most diagnostic test for distinguishing tartaric acid from citric acid. |
|
R4 |
KCl Test — Cream of Tartar (Secondary Confirmatory) |
🔍 Specific fingerprint |
|
Eq. |
C₄H₆O₆ + KOH → KHC₄H₄O₆↓ + H₂O |
|
Note |
Potassium hydrogen tartrate (KHC₄H₄O₆) — cream of tartar — precipitates as white crystals on concentrating the solution and adding saturated KCl. This is uniquely specific to tartaric acid among common organic acids; no other common aliphatic acid gives an insoluble potassium hydrogen salt under these conditions. |
|
R5 |
Bromine Water Test |
⚠️ Negative result |
|
Eq. |
No reaction — Br₂ (orange-brown) remains unchanged. |
|
Note |
Tartaric acid is fully saturated; no C=C double bond is present. Bromine water is NOT decolourised. Degree of unsaturation = 2 (from two C=O of –COOH groups; no C=C). |
|
Key contrast: calcium citrate (from citric acid) only precipitates on heating, NOT in cold. This thermal distinction is the single most diagnostic test for distinguishing tartaric acid from citric acid. |
|
R6 |
Acidified KMnO₄ (Baeyer’s Test) — slow –OH oxidation |
✅ Positive result |
|
Eq. |
C₄H₆O₆ + [O] → diketotartaric acid + CO₂ (slow) |
|
Note |
The secondary aliphatic –OH groups on C-2 and C-3 are slowly oxidised by KMnO₄. Purple colour decolourises slowly. The slow rate is diagnostic: C=C double bonds decolourise KMnO₄ instantly via syn dihydroxylation. Must be read alongside R5 (bromine water negative) to confirm saturated –OH oxidation. Note: diketotartaric acid is the initial oxidation product; under prolonged or vigorous conditions, further oxidation to oxalic acid and CO₂ may occur. |
|
R7 |
Amide Derivative — Tartramide (Derivative I) |
🧪 Derivative |
|
Eq. |
C₄H₆O₆ + 2 NH₃ → C₄H₈N₂O₄ + 2 H₂O |
|
Note |
Direct aminolysis of both –COOH groups with excess NH₃. Product: diamide of tartaric acid (tartramide). Recrystallise from water. m.p. 195 °C (literature). Independent physical checkpoint — does not rely on the acid’s own melting point. |
|
Two derivatives are always prepared: any two different compounds can coincidentally share one derivative melting point, but sharing two independent melting points is extremely unlikely. |
|
R8 |
Anilide Derivative — Tartaranilide (Derivative II) |
🧪 Derivative |
|
Eq. |
C₄H₆O₆ + 2 C₆H₅NH₂ → C₄H₄O₂(CONHC₆H₅)₂ + 2 H₂O |
|
Note |
Direct reaction of tartaric acid with 2 equivalents of aniline at 120 °C. Dianilide (tartaranilide) formed. Recrystallise from ethanol. m.p. 180 °C (literature). Second independent physical checkpoint confirming dicarboxylic acid reactivity. |
Quick Reference Summary — Chemical Tests for Tartaric Acid Identification
|
# |
Reaction |
Key reagent(s) |
Result |
What it confirms |
|
R1 |
NaHCO₃ test |
NaHCO₃ / lime water |
Brisk CO₂ effervescence; lime water milky |
–COOH confirmed; 2 mol CO₂ per mol acid |
|
R2 |
Ester test |
Ethanol + conc. H₂SO₄, heat |
Sweet fruity odour (diethyl tartrate) |
Dicarboxylic acid; rules out sulfonic acid |
|
R3 |
CaCl₂ test |
NH₃ then CaCl₂ in cold |
White crystalline precipitate in cold |
Tartrate ion (primary confirmatory); contrast with citric acid (heating needed) |
|
R4 |
KCl / cream of tartar |
Sat. KCl; cool; scratch |
White KHC₄H₄O₆ crystals |
Uniquely specific for tartaric acid |
|
R5 |
Bromine water |
Br₂ / water |
No decolourisation |
Fully saturated; no C=C present |
|
R6 |
Acidified KMnO₄ |
KMnO₄ / dil. H₂SO₄ |
Slow decolourisation |
Aliphatic –OH oxidised; no C=C (slow rate is diagnostic) |
|
R7 |
Amide derivative |
Excess NH₃; evaporate; recrystallise |
White solid; m.p. 195 °C |
Tartramide — independent physical checkpoint |
|
R8 |
Anilide derivative |
Aniline at 120 °C; recrystallise EtOH |
White solid; m.p. 180 °C |
Tartaranilide — second independent checkpoint |
Result Summary — Tartaric Acid Identification
All physical and chemical observations are consistent with the identity of the compound as L-(+)-tartaric acid (C₄H₆O₆).
The sample was a colourless crystalline solid, freely soluble in cold water, with a characteristic charring and burnt sugar odour on dry heating. The flame test confirmed aliphatic character (clean blue-yellow non-sooty flame). Litmus, NaHCO₃, and ester tests confirmed dicarboxylic acid (–COOH) functionality. The bromine water test was negative (no C=C unsaturation), while KMnO₄ showed slow decolourisation via –OH oxidation — together confirming a saturated aliphatic structure. The FeCl₃ test gave a buff-yellow colour (aliphatic α-hydroxy acid, not phenolic). The ammoniacal AgNO₃ test gave a silver mirror, confirming reducing character. Fenton’s reagent produced an intense blue colour, highly specific for tartaric acid. The CaCl₂ test produced a white calcium tartrate precipitate in cold solution, and the KCl test produced cream of tartar — both uniquely characteristic of tartaric acid. The concentrated H₂SO₄ test showed immediate charring and effervescence. The melting point of 170–172 °C (L-form), together with amide derivative (m.p. 195 °C) and anilide derivative (m.p. 180 °C), provided independent physical confirmation. The mixed melting point showed no depression, conclusively confirming identity.
Conclusion — Tartaric Acid Identified
Based on the combined physical and chemical evidence — aliphatic character (clean non-sooty flame), saturation (bromine water negative; KMnO₄ slow via –OH oxidation), dicarboxylic acid functionality (NaHCO₃ vigorous CO₂, litmus, ester tests), positive FeCl₃ (buff-yellow; aliphatic α-hydroxy acid), silver mirror (Tollens’ reagent; reducing α-hydroxy acid), intense Fenton’s blue (highly specific fingerprint), whole-compound fingerprints (CaCl₂ white ppt in cold; cream of tartar with KCl), charring on dry heating and with conc. H₂SO₄, matching literature melting point (170–172 °C; L-form), confirmed derivative melting points (tartramide 195 °C; tartaranilide 180 °C), and absence of mixed melting point depression — the given unknown compound is conclusively identified as:
L-(+)-Tartaric Acid (C₄H₆O₆)
IUPAC name: (2R,3R)-2,3-dihydroxybutanedioic acid | MW: 150.09 g/mol | m.p. 170–172 °C (L-form) | dl-form m.p. 206 °C
Important note: Mixed melting point test — gold standard identity confirmation
The mixed melting point (or mixture melting point) test is the classical gold standard for confirming that an unknown compound is identical to a known authentic sample. It requires a separately sourced authentic sample of L-tartaric acid for comparison.
|
What is mixed |
Expected result if SAME compound |
Expected result if DIFFERENT compound |
|
Unknown alone |
MP = 170–172 °C |
MP = 170–172 °C |
|
Authentic L-tartaric acid alone |
MP = 170–172 °C |
MP = 170–172 °C |
|
1:1 mixture of unknown + authentic |
MP = 170–172 °C |
MP depressed 10-30 °C below both pure samples |
Theory: why depression occurs
When two different compounds are mixed, each acts as an impurity in the other, lowering both melting points (eutectic depression). The degree of depression is related to the mole fraction of impurity by the van’t Hoff equation. If no depression is observed, the two samples are the same compound — because neither acts as an impurity in the other’s lattice.
For tartaric acid specifically: mixing L-tartaric acid with meso-tartaric acid would show significant depression, confirming they are different compounds despite both giving zero optical rotation in the meso case. This is one practical example where the mixed MP test distinguishes isomers that a single-compound melting point cannot resolve.
Laboratory Worksheet — Record Your Observations for Tartaric Acid Identification
Complete this table during your practical session. ★ = confirmatory tests specific to tartaric acid.
|
Step |
Test Name |
Key Reagent(s) |
Your Observation |
Result (+ / −) |
What It Confirms |
|
1 |
Physical appearance |
Visual |
|||
|
2 |
Solubility in water |
Distilled water |
|||
|
3 |
Dry heating test |
Bunsen flame |
|||
|
4 |
Flame test (ignition) |
Nichrome wire |
|||
|
5 |
Litmus / pH paper |
Litmus paper |
|||
|
6 |
NaHCO₃ test |
NaHCO₃ / lime water |
|||
|
7 |
Ester test |
EtOH / conc. H₂SO₄ |
|||
|
8 |
Conc. H₂SO₄ test |
Conc. H₂SO₄ |
|||
|
9 |
Lassaigne’s test |
Na fusion |
|||
|
10 |
Bromine water test |
Br₂ / water |
|||
|
11 |
Acidified KMnO₄ test |
KMnO₄ / dil. H₂SO₄ |
|||
|
12 |
FeCl₃ test |
FeCl₃ solution |
|||
|
13 |
Tollens’ test |
Ammoniacal AgNO₃ |
|||
|
14 |
Fenton’s reagent test |
H₂O₂ + NaOH |
|||
|
15 |
Melting point determination |
MP apparatus |
|||
|
16 |
CaCl₂ test ★ |
NH₃ + CaCl₂ |
|||
|
17 |
Cream of tartar test ★ |
Sat. KCl |
|||
|
18 |
Literature survey |
Merck / PubChem |
|||
|
19 |
Control test |
Authentic sample |
|||
|
20 |
Tartramide derivative |
Excess NH₃ |
|||
|
21 |
Tartaranilide derivative |
Aniline / 120 °C |
|||
|
22 |
Mixed melting point |
Authentic + unknown |
Uses and Applications of Tartaric Acid
|
Industry / Field |
Application |
Specific Role |
|
Food & Beverage Industry |
Acidulant and antioxidant |
Primary acidulant in wine production; naturally present in grapes. Added to baking powder (with NaHCO₃ to produce CO₂). Approved food additive (E334). |
|
Baking |
Cream of tartar (KHC₄H₄O₆) |
Potassium bitartrate used as a raising agent, stabiliser for egg whites, and to prevent sugar crystallisation in candies. |
|
Pharmaceuticals |
Excipient and active ingredient |
Component of effervescent drug formulations. Tartrate salts used as counterions for drugs (e.g. metoprolol tartrate, antimony potassium tartrate). |
|
Photography (historical) |
Fixer and reducing agent |
Potassium sodium tartrate (Rochelle salt) used in early photographic development. Now largely replaced by modern fixers. |
|
Analytical Chemistry |
Standard acid and complexing agent |
Used as a primary standard in acid–base titrations. Forms complexes with metals; used in Fehling’s solution (sodium potassium tartrate) to prevent Cu(OH)₂ precipitation. |
|
Cosmetics |
pH adjuster and skin conditioner |
Used in skin creams, toners, and anti-ageing formulations as an α-hydroxy acid (AHA) for mild exfoliation and skin renewal. |
|
Organic Synthesis |
Chiral resolving agent |
L-(+)-tartaric acid and its salts are classical reagents for resolving racemic mixtures into enantiomers (chiral resolution). Key in pharmaceutical synthesis of chiral drugs. |
|
Electroplating |
Complexing agent |
Rochelle salt (sodium potassium tartrate) used in electroplating baths to keep metal ions in solution and ensure uniform deposition. |
Safety Precautions
General Laboratory Safety
- Wear a laboratory coat, safety goggles, and nitrile gloves throughout the entire experiment.
- Work in a well-ventilated laboratory. Use a fume cupboard for all steps involving concentrated H₂SO₄, SOCl₂, aniline, and Tollens’ reagent preparation.
- Never pipette by mouth. Use a pipette filler or dropper for all liquid reagents.
- Dispose of all chemical waste in designated containers. Do not pour FeCl₃, KMnO₄, or heavy-metal solutions down the sink.
- Never store prepared Tollens’ reagent (ammoniacal AgNO₃) — explosive silver nitride may form on standing. Prepare fresh immediately before use and destroy any excess with dilute HNO₃.
- Know the location of the eyewash station, emergency shower, fire extinguisher, and first-aid kit before starting.
Per-Reagent Hazard and Precaution Table
|
Reagent |
Hazard Classification |
Specific Risk |
Precaution |
|
Tartaric acid |
Low hazard (irritant) |
May cause mild skin, eye, and respiratory irritation. Not acutely toxic. |
Avoid inhaling dust. Wear gloves. Handle normally on open bench. |
|
Concentrated H₂SO₄ |
CORROSIVE • OXIDISING |
Severe, potentially permanent burns to skin, eyes, and mucous membranes. Reacts violently with water. |
Always add acid slowly and carefully. Use fume cupboard. Wear acid-resistant gloves and goggles. |
|
H₂O₂ (30%) |
OXIDISING • CORROSIVE |
Strong oxidiser; can cause burns and support combustion. Harmful to skin and eyes. |
Wear gloves and goggles. Keep away from organic matter and metals. Use in fume cupboard. |
|
NaOH solution |
CORROSIVE |
Causes burns to skin and eyes. Corrosive at >2% concentration. |
Wear gloves and goggles. Avoid contact with skin. Flush immediately with water if contact occurs. |
|
Acidified KMnO₄ |
OXIDISING • IRRITANT |
Strong oxidising agent. Manganese compounds are toxic by ingestion. |
Keep away from flammable solvents. Wear gloves. Dispose in heavy-metal waste. |
|
Ammoniacal AgNO₃ |
TOXIC (vapour) • CORROSIVE |
AgNO₃ is corrosive and stains skin/clothing. NH₃ vapour is toxic. Explosive if stored. |
Prepare fresh only. Use in fume cupboard. Wear gloves and goggles. Destroy immediately after use with dil. HNO₃. |
|
FeCl₃ solution |
IRRITANT |
Irritating to skin and eyes. Mildly corrosive. |
Wear gloves and goggles. Dispose in heavy-metal waste container. |
|
Aniline |
TOXIC • HARMFUL |
Readily absorbed through skin. Causes methaemoglobinaemia. Suspected carcinogen. |
Use in fume cupboard at all times. Wear double gloves. Seek medical advice immediately if skin contact occurs. |
|
SOCl₂ (if used for derivatives) |
HIGHLY CORROSIVE • TOXIC |
Reacts violently with water; releases HCl and SO₂. Severe burns. |
Use ONLY in a dry fume cupboard. Ensure all glassware is completely dry. Wear acid-resistant gloves. |
First Aid Summary
|
Exposure Route |
Immediate First Aid Action |
|
Eye contact with any chemical |
Immediately irrigate with large volumes of flowing water for at least 15 minutes, holding eyelids open. Seek medical attention. |
|
Skin contact — H₂SO₄ or NaOH |
Remove contaminated clothing. Wash with copious water for at least 15 minutes. Seek medical attention. |
|
Skin contact — aniline |
Remove contaminated clothing immediately. Wash with soap and water for at least 15 minutes. Seek medical attention — systemic toxicity risk. |
|
Skin contact — AgNO₃ |
Wash immediately with water. AgNO₃ stains skin brown/black — apply sodium thiosulfate solution to remove stain. |
|
Inhalation — NH₃ or SOCl₂ |
Move to fresh air immediately. If breathing is difficult, administer oxygen. Seek medical attention urgently. |
|
Ingestion |
Do NOT induce vomiting. Rinse mouth with water. Seek medical attention immediately and bring the chemical’s safety data sheet (SDS). |
Where is tartaric acid found? — natural sources & biosynthesis
Tartaric acid is biosynthesised in plants from ascorbic acid (Vitamin C) via oxidative cleavage of the C4–C5 bond. Grapes are the dominant commercial source; wine lees (argol — white crystalline tartrate deposits on wine barrel walls) are the industrial extraction feedstock. Tartaric acid accumulates as the free acid in grape juice and as potassium hydrogen tartrate (cream of tartar) in aged wine.
|
Source |
Tartaric Acid Content |
Form present |
Notes |
|
Tamarinds |
8–10 g / 100 g |
Free acid |
Highest natural concentration; key acidulant in South Asian cuisine |
|
Grapes (unripe) |
0.5–1.0 g / 100 g |
Free acid |
Primary commercial source; content falls as grape ripens |
|
Grapes (ripe) / wine lees |
0.2–0.5 g / 100 g (juice) |
Free acid + K-bitartrate |
Argol deposits on barrel walls are industrial source of tartaric acid |
|
Bananas |
~0.5 g / 100 g |
Free acid |
Present alongside malic acid |
|
Avocados |
~0.08 g / 100 g |
Free acid |
Minor component |
|
Tomatoes |
~0.04 g / 100 g |
Free acid |
Minor; predominant acid is citric |
|
Unripe mangoes |
Trace amounts |
Mixed with citric and malic acids |
Decreases on ripening |
Tartaric acid vs citric acid — how to distinguish them in the lab
Citric acid and tartaric acid are the two most commonly confused organic acids in undergraduate practical examinations. Both are white crystalline solids, both are freely soluble, both give brisk NaHCO₃ effervescence, and both fail the FeCl₃ deep-violet test. Two tests resolve the confusion unambiguously:
Two-test quick distinction (exam answer)
⦁ Test 1 — NaHCO3 stoichiometry: tartaric acid produces 2 mol CO2 per mol acid (dicarboxylic); citric acid produces 3 mol CO2 per mol acid (tricarboxylic). Citric acid effervescence is noticeably MORE vigorous.
⦁ Test 2 — K+ precipitation: add KCl to a concentrated solution. Tartaric acid forms a white crystalline cream of tartar precipitate (KHC4H4O6). Citric acid does NOT form any precipitate with K+ ions.
⦁ These two tests together are sufficient for definitive differentiation in a practical examination.
|
Property |
Tartaric Acid |
Citric Acid |
Malic Acid |
Oxalic Acid |
|
Formula |
C₄H₆O₆ |
C₆H₈O₇ |
C₄H₆O₅ |
C₂H₂O₄ |
|
MW (g/mol) |
150.09 |
192.12 |
134.09 |
90.03 |
|
–COOH groups |
2 |
3 |
2 |
2 |
|
–OH groups |
2 (secondary) |
1 (tertiary) |
1 (secondary) |
0 |
|
Chiral centres |
2 |
1 |
1 |
0 |
|
pKa₁ |
2.98 |
3.13 |
3.40 |
1.25 |
|
pKa₂ |
4.34 |
4.76 |
5.11 |
4.27 |
|
Melting point |
170–172 °C |
153 °C |
130 °C |
189 °C |
|
NaHCO₃ test |
Brisk |
Very vigorous |
Brisk |
Brisk |
|
FeCl₃ colour |
Buff-yellow |
Pale yellow |
No significant colour |
No colour |
|
Ca²⁺ (CaCl₂) ppt |
White ppt |
No ppt |
No ppt |
White ppt |
|
K⁺ (KCl) ppt |
White ppt |
No ppt |
No ppt |
No ppt |
|
Bromine water |
No decolorisation |
No decolorisation |
No decolorisation |
No decolorisation |
|
KMnO₄ (cold) |
Slow decolorisation |
Slow decolorisation |
Slow decolorisation |
Rapid decolorisation |
|
Natural source |
Grapes, tamarinds |
Citrus fruits, lemons |
Apples, stone fruits |
Spinach, rhubarb |
|
E number |
E334 |
E330 |
E296 |
— |
FAQ’s
Viva questions & model answers
|
# |
Question |
Model answer (key points for full marks) |
|
1 |
How would you distinguish tartaric acid from citric acid using exactly two tests? |
Test 1 — NaHCO₃: tartaric gives 2 mol CO₂ per mol (dicarboxylic); citric gives 3 mol CO₂ (tricarboxylic) — citric effervescence is more vigorous. |
|
2 |
Why does meso-tartaric acid show zero optical rotation despite having two chiral carbons? |
The (2R,3S) meso form has an internal plane of symmetry. The rotation produced by C2 (clockwise) is exactly cancelled by C3 (anticlockwise) within the same molecule — internal compensation. This differs from a racemate: the racemate has external cancellation between separate molecules and can in principle be resolved; the meso form cannot |
|
3 |
Write the balanced equation for the reaction of tartaric acid with excess NaOH. |
C₄H₆O₆ + 2 NaOH → Na₂C₄H₄O₆ + 2 H₂O |
|
4 |
What was the significance of Pasteur’s resolution of tartaric acid in 1848? |
Pasteur hand-separated sodium ammonium tartrate crystals into two mirror-image hemihedral forms and showed each solution rotated polarised light in opposite directions. It was: (1) the first resolution of a racemate, (2) proof that optical activity arises from molecular asymmetry (chirality), (3) the founding experiment of stereochemistry, and (4) evidence for the tetrahedral carbon atom (later formalised by van’t Hoff and Le Bel in 1874). |
|
5 |
Why does tartaric acid give buff-yellow with FeCl₃ instead of deep violet? |
Deep violet with FeCl₃ is characteristic of phenolic –OH (e.g. salicylic acid, catechol), which form strongly coloured iron-phenolate complexes. Tartaric acid has aliphatic secondary –OH groups on sp³ carbons; these form only weak iron complexes giving a buff-yellow colour. The absence of deep violet is diagnostically significant — it rules out any phenolic compound |
|
6 |
Why is bromine water a NEGATIVE test for tartaric acid, and why is this result important? |
Tartaric acid is fully saturated — it has no C=C double bonds (degree of unsaturation = 2, from the two C=O groups in the two –COOH moieties). Bromine water decolorises only with: alkenes (addition), alkynes, or phenols (substitution). Tartaric acid reacts with none of these mechanisms, so bromine water stays orange-red. This negative result is important because it distinguishes tartaric acid from maleic or fumaric acid (isomers with C=C) and confirms that KMnO₄ decolorisation in Step 8 is due to –OH oxidation, not C=C oxidation. |
|
7 |
What is the purpose of preparing two derivatives (amide and anilide) rather than one? |
Any two structurally different unknown compounds can coincidentally share a single derivative melting point — this is a known limitation of the method. If two derivatives are prepared and both melting points match the literature values for tartaric acid, the probability of coincidence is statistically negligible. Two independent derivative melting points provide a much more robust confirmation than one. |
|
8 |
What does no melting point depression in the mixed melting point test confirm? |
No depression means the two samples (unknown and authentic standard) are the same compound. When two different compounds are mixed, each acts as an impurity in the other, forming a eutectic mixture with a lower melting point (van’t Hoff depression). Absence of depression proves identity: neither sample acts as an impurity in the other’s crystal lattice, because they are identical. |
|
9 |
What is the equivalent weight of tartaric acid and how is it calculated? |
Equivalent weight = Molar mass / Basicity = 150.09 / 2 = 75.05 g/equivalent. |
|
10 |
How would you confirm that a sample is the L-form of tartaric acid rather than the meso form, given both can appear as white crystals? |
Three approaches: (1) Melting point — L-form melts at 170–172 °C, meso at 165–166 °C. A significant difference. (2) Polarimetry — L-form gives [α]D = +12°; meso form gives 0° (due to internal symmetry). (3) Mixed melting point — mix unknown with authentic L-tartaric acid; no depression confirms L-form; depression indicates meso or D-form. |
Multiple Choice Questions
MCQ 1
1. The correct molecular formula of tartaric acid is:
A. C₄H₆O₄
B. C₄H₆O₆
C. C₆H₈O₇
D. C₄H₄O₅
MCQ 2
2. The melting point of L-(+)-tartaric acid (naturally occurring form) is:
A. 140 °C
B. 153 °C
C. 170–172 °C
D. 206 °C
MCQ 3
3. The IUPAC name of L-(+)-tartaric acid is:
A. (2R,3S)-2,3-dihydroxybutanedioic acid
B. 2-hydroxypropane-1,2,3-tricarboxylic acid
C. (2R,3R)-2,3-dihydroxybutanedioic acid
D. 2,3-dihydroxybutanoic acid
MCQ 4
4. Which pair of tests BOTH confirm that tartaric acid is saturated?
A. Litmus test and FeCl₃ test
B. No decolourisation of bromine water and slow decolourisation of KMnO₄ via –OH oxidation
C. Flame test and ester test
D. Fenton’s test and CaCl₂ test
MCQ 5
5. Tartaric acid reacts with NaHCO₃ to give CO₂. How many moles of CO₂ are produced per mole of tartaric acid?
A. 1 mol
B. 2 mol
C. 3 mol
D. 4 mol
MCQ 6
6. When bromine water is added to tartaric acid, the result is:
A. Immediate decolourisation (C=C present)
B. White precipitate forms
C. Orange-brown colour retained — no decolourisation
D. Purple colour develops
MCQ 7
7. Tartaric acid decolourises acidified KMnO₄ slowly because:
A. It has a C=C double bond that is oxidised instantly
B. The secondary –OH groups are slowly oxidised; no C=C is present
C. The carboxyl groups reduce KMnO₄
D. KMnO₄ reacts with the tartrate ion to form a precipitate
MCQ 8
8. The cream of tartar test for tartaric acid involves:
A. Adding NaOH to form a white sodium tartrate precipitate
B. Adding CaCl₂ in cold; white precipitate forms
C. Concentrating the solution then adding saturated KCl; white KHC₄H₄O₆ precipitate forms
D. Adding H₂O₂ and excess NaOH to produce a blue colour
MCQ 9
9. The number of distinct functional group types in tartaric acid is:
A. One (–COOH only)
B. Two (–COOH and –OH)
C. Three (–COOH, –OH, and C=C)
D. Four (–COOH, –OH, C=C, and benzene ring)
MCQ 10
10. Tartaric acid burns with a clean non-sooty flame because:
A. It contains nitrogen
B. It has a high C:H ratio due to the benzene ring
C. It is a saturated aliphatic compound with no benzene ring; combustion is relatively complete
D. It contains sulfur
MCQ 11
11. The FeCl₃ test on tartaric acid gives:
A. Deep violet/purple coloration (phenol response)
B. Light yellow precipitate
C. Buff-yellow colouration; no deep violet
D. No visible change
MCQ 12
12. The degree of unsaturation (DoU) of tartaric acid (C₄H₆O₆) is:
A. 0
B. 2 (from two C=O groups in the two –COOH moieties)
C. 3
D. 6
MCQ 13
13. Which statement about stereoisomerism in tartaric acid is CORRECT?
A. Tartaric acid has no stereocentres
B. There are two stereoisomers: L and D forms only
C. There are three stereoisomers: L-(+), D-(–), and meso forms; the meso form is optically inactive
D. Tartaric acid shows geometrical isomerism due to C=C
MCQ 14
14. The key difference between the CaCl₂ test for tartaric acid and citric acid is:
A. Tartaric acid gives a blue precipitate; citric acid gives a white precipitate
B. Calcium tartrate precipitates in cold solution; calcium citrate only precipitates on heating
C. Both acids require heating to produce a precipitate
D. Only citric acid reacts with CaCl₂
MCQ 15
15. The melting point of tartramide (amide derivative of tartaric acid) is:
A. 170 °C
B. 180 °C
C. 195 °C
D. 210 °C
MCQ 16
16. The unknown sample is mixed with authentic L-tartaric acid. The mixture melts at 170–172 °C with no depression. This means:
A. The compound is meso-tartaric acid
B. The compound is dl-tartaric acid
C. Both samples are identical — the compound is conclusively identified as L-tartaric acid
D. The test has failed
MCQ 17
17. Fenton’s reagent test for tartaric acid involves:
A. Adding FeSO₄ + H₂O₂ and observing a red colour
B. Adding one drop of H₂O₂ then excess NaOH → intense blue colour
C. Adding KMnO₄ and H₂SO₄ and observing slow decolourisation
D. Adding AgNO₃ in ammonia and observing a silver mirror
MCQ 18
18. The cream of tartar used in baking is:
A. Calcium tartrate (CaC₄H₄O₆)
B. Potassium hydrogen tartrate (KHC₄H₄O₆)
C. Sodium potassium tartrate (Rochelle salt)
D. dl-Tartaric acid
MCQ 19
19. On heating in a dry test tube, tartaric acid gives:
A. Clean sublimation with no residue
B. Sweet fruity ester odour
C. Charring and odour of burnt sugar
D. White fumes of SO₂
MCQ 20
20. The molecular weight of tartaric acid (C₄H₆O₆) is:
A. 132.07 g/mol
B. 150.09 g/mol
C. 192.12 g/mol
D. 168.06 g/mol
