UGC NET FORENSIC SCIENCE
Unit IV Importance
2019–2024 PYQs: ~2–4 Questions per exam — High Importance
SECTION 1: PYQ ANALYSIS — LAST 5 YEARS (2019–2024)
Based on analysis of UGC NET Forensic Science papers from 2019 to 2024, the topic “Analysis of Methanol and Denaturants” consistently appears in Unit IV (Forensic Toxicology). Questions are typically 2–4 per cycle, focusing on qualitative tests, GC parameters, denaturant types, toxicity mechanisms, and forensic significance.
Frequently Tested Sub-Topics (Year-wise Trend)
| Year / Session | Key Focus Areas Tested |
|---|---|
| 2019 (June & Dec) | Chromotropic acid test for methanol; types of denaturants; methylated spirit definition |
| 2020 (Sep) & 2021 | GC column parameters for methanol; Schiff's reagent test; toxicity signs |
| 2022 (June & Oct) | Metabolism of methanol (ADH pathway); formic acid role; forensic significance in illicit liquor |
| 2023 (Feb & June) | Detection methods comparison; denaturant composition; latent period of poisoning |
| 2024 | Lethal dose; antidote mechanism; differentiation methanol vs ethanol |
PYQ Key Insight
Questions are predominantly based on:
- Colour reaction in Chromotropic Acid Test
- Metabolic pathway of methanol
- Types/composition of denaturants
- GC parameters for quantitative analysis
Mastering these four pillars covers approximately 90% of exam questions from this topic.
Common PYQ Question Types
| Type | Frequency | Examples |
|---|---|---|
| Qualitative Test Identification | Very High | Colour produced in Chromotropic acid test; Schiff's reagent principle |
| Denaturant Composition | High | Components of methylated spirits; BIS standard denaturants |
| Metabolic Pathway MCQs | High | Enzyme involved; toxic metabolite identified |
| GC Operating Parameters | Medium | Column type, temperature, detector used for methanol |
| Toxicity & Clinical Signs | Medium | Latent period, minimum lethal dose, target organ |
| Legal / Forensic Significance | Medium | Role in illicit liquor cases; excise offences |
SECTION 2: METHANOL — CHEMISTRY & PROPERTIES
2.1 Chemical Identity
| Property | Details |
|---|---|
| IUPAC Name | Methanol |
| Common Names | Methyl alcohol, Wood alcohol, Wood naphtha, Wood spirit, Carbinol |
| Chemical Formula | CH₃OH (or CH₄O) |
| Molecular Weight | 32.04 g/mol |
| Physical State | Colourless, volatile liquid |
| Boiling Point | 64.7°C (close to ethanol's 78.37°C — separation difficult) |
| Density | 0.791 g/mL at 20°C |
| Odour | Faint alcoholic odour (similar to ethanol) |
| Miscibility | Completely miscible with water and ethanol |
| Flash Point | 11°C (highly flammable) |
| Refractive Index | 1.3284 |
Exam Tip
Methanol is called “wood alcohol” because it was originally obtained by destructive distillation of wood. Its near-identical odour and appearance to ethanol is the primary reason for accidental and fraudulent poisonings.
2.2 Sources of Methanol
- Industrial production: CO + 2H₂ → CH₃OH (ZnO–Cr₂O₃ catalyst, 250–350°C, 50–100 atm)
- By-product of fermentation: Demethylation of pectins (<0.1% v/v in legal spirits)
- Destructive distillation of wood: Produces crude methanol (pyroligneous acid)
- By-product in manufacture of: Formaldehyde, MTBE, acetic acid, DMT
- Illicit liquor: Added to reduce cost or formed due to poor distillation
2.3 Industrial Uses of Methanol
- Solvent in paints, varnishes, shellac
- Fuel additive and fuel cell component (M85 blend)
- Raw material for formaldehyde, acetic acid, MTBE
- Antifreeze agent (windshield washer fluid)
- Denaturant in methylated spirits
- Pesticide and herbicide production
- Photocopier fluid, paint strippers, carburetor cleaner
SECTION 3: DENATURANTS — TYPES, COMPOSITION & ANALYSIS
3.1 Definition and Purpose of Denaturation
Denatured alcohol is ethanol intentionally mixed with toxic, bitter, or foul-smelling chemicals to make it unfit for drinking and exempt it from beverage excise taxes.
- Legal basis (India): Denaturing of Spirit Rules, 1972; BIS IS:4117-1973
- Tax aspect: Undenatured ethanol — ~150% duty; Denatured ethanol — ~10%
- Forensic significance: Removal of denaturants to obtain potable alcohol is an excise offence
3.2 Types of Denatured Alcohol
| Type | Description |
|---|---|
| Completely Denatured Alcohol (CDA) | Fixed formula; cannot be reconverted; used for fuel/cleaning |
| Specially Denatured Alcohol (SDA) | Specific industrial formulation (cosmetics, pharma) |
| Methylated Spirit | Ethanol + 5–10% methanol ± pyridine + dye |
| Denatured Rectified Spirit (DNS) | Indian industrial spirit (BIS IS:4117-1973) |
3.3 Common Denaturants and Their Properties
| Denaturant | Purpose | Key Detection |
|---|---|---|
| Methanol | Primary toxic agent | Chromotropic acid test (violet) |
| Pyridine | Fishy odour; toxic | UV at 362 nm |
| Denatonium benzoate | Extreme bitterness | HPLC |
| Isopropyl alcohol | Toxic; foul taste | GC-FID |
| Acetone | Foul smell | GC; DNP test |
| MEK | Unpleasant odour | GC-FID; DNPH derivative |
| Benzene | Carcinogen | GC-MS |
| Castor oil | Oily/nauseating | Saponification; TLC |
| Methyl violet | Visual marker | UV-Vis |
| Naphtha | Foul odour | GC; distillation |
3.4 Standard Denaturation Formulas
British Formula (CDA)
- 90 parts ethanol
- 9.5 parts methanol
- 0.5 parts crude pyridine
- Mineral naphtha: 3.75 L / 1000 L
- Methyl violet: 1.5 g / 1000 L
European Union Formula (2013)
- 3 L isopropyl alcohol
- 3 L methyl ethyl ketone
- 1 g denatonium benzoate (per 100 L ethanol)
Indian Formula (BIS IS:4117-1973)
- Methanol, acetone, pyridine, aromatic hydrocarbons, colouring matter
- 0.5% v/v crude pyridine in DNS
- State excise may modify formulation
3.5 Forensic Analysis of Denaturants
Detection of Pyridine
- Spectrophotometric method with trichloroacetic acid in NaOH
- Heat at 85°C for 75 minutes
- λmax = 362 ± 1 nm (crimson complex)
- Beer's law range: 275–1472 μg / 10 ml
- Calibration: Y = 0.001x – 0.158 (r = 0.997)
- GC method also applicable
Detection of Methyl Violet
- Immediate purple/violet colour
- UV-Vis at ~590 nm
- TLC confirmation
4.1 Chromotropic Acid Test (Most Important — Frequently Asked)
PYQ Alert
The Chromotropic Acid Test is the most frequently asked qualitative test for methanol. Focus areas: colour produced, reagents, oxidation principle, formaldehyde interference.
Principle
Methanol is oxidised to formaldehyde by KMnO₄ in phosphoric acid. Excess permanganate is removed by NaHSO₃. Formaldehyde reacts with chromotropic acid in conc. H₂SO₄ to produce a violet/purple complex.
Stepwise Procedure
- 4.5 mL sample + 0.5 mL ethanol
- Add 2 mL 3% KMnO₄ + 2 mL H₃PO₄ → keep 10 min
- Decolourise MnO₂ using NaHSO₃
- Add 5 mL 0.05% chromotropic acid
- Slowly add 15 mL conc. H₂SO₄
- Observe colour on cooling
Result Interpretation
| Observation | Interpretation |
|---|---|
| Violet / Purple colour | Methanol PRESENT |
| No colour | Methanol ABSENT |
| Colour intensity | Semi-quantitative indication |
Important Exam Points
- Ethanol → acetaldehyde → no violet colour
- Specific for methanol among common alcohols
- Formaldehyde gives false positive
- Detection limit ≈ 1250 mg/L
- Chromotropic acid = 1,8-dihydroxynaphthalene-3,6-disulphonic acid
4.2 Schiff’s Reagent Test
Formaldehyde formed after oxidation reacts with Schiff’s reagent to give a stable magenta colour. Acetaldehyde gives only a faint unstable pink.
- Positive → stable magenta colour
- Negative → no stable colour
- Acetaldehyde pink disappears after H₂SO₄
4.3 Comparison of Qualitative Tests
| Parameter | Chromotropic Acid | Schiff’s Test |
|---|---|---|
| Colour | Violet/Purple | Magenta |
| With Ethanol | No colour | Faint pink (fades) |
| Sensitivity | Higher | Lower |
| Specificity | High | Moderate |
| Time | ~20 min | ~40 min |
| Exam Importance | PRIMARY | Secondary |
4.4 Other Qualitative Tests
- Iodoform test: Positive for ethanol, negative for methanol
- Dichromate test: Not specific
- Permanganate test: Not specific
Key Differentiation Point
Ethanol → positive iodoform test (yellow CHI₃)
Methanol → negative iodoform test
SECTION 5: QUANTITATIVE ANALYSIS OF METHANOL BY GAS CHROMATOGRAPHY
5.1 Why GC is the Gold Standard
GC-FID enables simultaneous separation and quantification of methanol, ethanol and other volatiles in a single run.
5.2 GC Operating Parameters
Method 1 (Standard)
| Parameter | Specification |
|---|---|
| Column | Porapak (80–100 mesh) |
| Temperature | 160°C |
| Carrier Gas | N₂ (50 mL/min) |
| Detector | FID |
| Internal Standard | n-Propanol |
Method 2 (Temperature Programmed)
| Parameter | Specification |
|---|---|
| Column | 0.3% Carbowax 20M on Carbopack C |
| Program | 35°C → 175°C at 5°C/min |
| Carrier Gas | N₂ (30 mL/min) |
| Detector | FID |
5.3 Internal Standard Method
- Internal standard: n-Propanol
- Corrects injection and drift errors
- Concentration = Peak area ratio × IS concentration × response factor
5.4 GC-MS for Confirmation
- m/z 32 (M⁺), 31 (base peak), 29
- Used in medico-legal cases
- Definitive identification
5.5 Other Analytical Methods
| Method | Notes |
|---|---|
| Density | Not specific alone |
| Refractive Index | Supplementary |
| UV Spectrophotometry | Chromotropic acid based |
| HPLC | Less common |
| Enzymatic kits | Rapid screening |
| Near-IR | Non-destructive QC |
5.6 Permissible Limits of Methanol
| Matrix / Regulation | Limit |
|---|---|
| IS:3752 (India) | ≤50 mg/100 mL absolute alcohol |
| EU spirits | ≤10 g/hL |
| US TTB | ≤1500 mg/L |
| WHO | <100 mg/L |
| OIV wine | ≤200 mg/L |
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SECTION 6: TOXICOLOGY OF METHANOL — FORENSIC SIGNIFICANCE
6.1 Route of Exposure
- Ingestion: Most common route — oral consumption of illicit/adulterated liquor, windshield washer fluid, industrial solvents
- Inhalation: Occupational exposure in factories; high vapour concentration can cause toxic blood levels
- Dermal absorption: Possible with prolonged skin contact with concentrated methanol
- Methanol is rapidly absorbed from the GI tract; maximum blood levels within 30–60 minutes of ingestion
6.2 Metabolism of Methanol — THE CRITICAL PATHWAY
EXAM ALERT
The metabolic pathway of methanol is one of the MOST FREQUENTLY ASKED topics. Know every step, enzyme, metabolite, and organ involved. Questions often ask: 'Which enzyme?' or 'Which metabolite causes blindness?' or 'Which metabolite causes metabolic acidosis?'
METABOLIC PATHWAY OF METHANOL
Methanol (CH₃OH)
↓ Alcohol Dehydrogenase (ADH) — Liver [rate-limiting step]
Formaldehyde (HCHO) ← HIGHLY TOXIC
↓ Aldehyde Dehydrogenase (ALDH) — Liver & Erythrocytes [very fast, formaldehyde barely detectable]
Formic Acid / Formate ← PRIMARY TOXIC METABOLITE (Causes Blindness & Metabolic Acidosis)
↓ Tetrahydrofolate (THF) / Folate-dependent pathway [slow in humans]
CO₂ + H₂O (excreted — harmless)
6.3 Key Enzymes and Their Significance
| Enzyme | Role in Methanol Metabolism |
|---|---|
| Alcohol Dehydrogenase (ADH) | Oxidises methanol → formaldehyde (rate-limiting step). Also oxidises ethanol → acetaldehyde (10x faster than methanol). Ethanol COMPETITIVELY INHIBITS ADH for methanol — basis of antidote treatment. |
| Aldehyde Dehydrogenase (ALDH) | Oxidises formaldehyde → formic acid. Very rapid — formaldehyde does not accumulate in serum. |
| Tetrahydrofolate (THF/Folate pathway) | Converts formate → CO₂ + H₂O. Very slow in humans (unlike rodents) — explains accumulation of formate and species-specific toxicity. |
| Catalase (CYP2E1, minor pathway) | Minor oxidative pathway in peroxisomes; limited contribution |
6.4 Mechanism of Toxicity
(A) Formic Acid / Formate Toxicity:
- Formate inhibits Cytochrome c Oxidase (Complex IV) of the mitochondrial electron transport chain
- This causes inhibition of oxidative phosphorylation → ATP deficiency → cellular hypoxia
- Results in metabolic acidosis with elevated anion gap (formate accumulation)
- Optic nerve (retrolaminar portion) and retinal cells are particularly sensitive to formate-induced ATP deficiency
- Formic acid causes demyelination of optic nerve → impaired visual signal transmission → blindness
- Bilateral putaminal necrosis (basal ganglia damage) is characteristic radiological finding on MRI
(B) CNS Depression:
- Methanol itself acts as a CNS depressant (similar to ethanol) but is less potent per gram
- Initial symptoms mimic mild ethanol intoxication
6.5 Clinical Features of Methanol Poisoning
Latent Period — CRITICAL EXAM POINT:
KEY FACT: There is a latent period of 12–24 hours (sometimes up to 30 hours) between ingestion and onset of severe symptoms. This is because methanol itself is not highly toxic — its toxic metabolites (formaldehyde → formic acid) take time to accumulate. If ethanol is co-ingested, the latent period is even longer (up to 36 hours) because ethanol competitively inhibits ADH.
| Phase | Timing | Symptoms |
|---|---|---|
| Early / CNS Phase | 0–6 hours post-ingestion | Mild inebriation, headache, nausea, vomiting, dizziness (similar to mild ethanol intoxication) |
| Latent Phase | 6–24 hours | Apparent recovery; symptom-free interval; metabolites accumulating silently |
| Toxic / Ocular Phase | 12–36 hours | Visual disturbances — blurring, photophobia, 'snow-field vision', loss of visual acuity; metabolic acidosis; Kussmaul breathing |
| Severe Phase | 24–72 hours | Blindness, coma, convulsions, cardiovascular collapse, death |
6.6 Dose-Response Data (Exam Numbers to Remember)
| Dose / Level | Effect |
|---|---|
| 3.16 g (approximately 4 mL) | Minimum dose causing irreversible optic nerve damage |
| 10 mL of pure methanol | Can cause permanent blindness |
| 30 mL of pure methanol | May be fatal |
| Oral LD₅₀ (humans) | Estimated at 56.2 g (≈ 1 g/kg body weight) |
| Minimum lethal dose (adults) | 0.3–1 g/kg body weight |
| Toxic blood level | >20 mg/dL (>6.25 mmol/L) |
| Serious blood level | >40 mg/dL (>12.5 mmol/L) |
| Potentially lethal blood level | >150 mg/dL |
| Formate level causing visual toxicity | ≥3.7 mmol/L serum formate |
| Formate level — lethal outcome likely | ≥17.5 mmol/L serum formate |
6.7 Post-Mortem Findings
- Cerebral oedema and congestion
- Bilateral hemorrhagic necrosis of putamen — pathognomonic finding
- Pulmonary oedema
- Hyperaemia of optic disc; retinal oedema
- Gastrointestinal hyperaemia and haemorrhage
- Odour of formic acid on post-mortem dissection (characteristic)
- Detection of methanol in vitreous humour, blood, urine, brain — vitreous is best post-mortem sample (protected from putrefaction)
SECTION 7: TREATMENT OF METHANOL POISONING
7.1 Antidotes — Mechanism-Based
| Antidote / Treatment | Mechanism of Action |
|---|---|
| Ethanol (Ethyl alcohol) | COMPETITIVE INHIBITOR of ADH → preferentially binds ADH (10x higher affinity for ethanol) → blocks metabolism of methanol to toxic products → methanol excreted unchanged by kidneys. Loading dose: 0.8 g/kg IV, maintain blood ethanol at 100–200 mg/dL. |
| Fomepizole (4-Methylpyrazole) | SELECTIVE, REVERSIBLE ADH INHIBITOR → same mechanism as ethanol but without CNS effects (no inebriation, no hypoglycaemia). Preferred over ethanol; given IV. Loading dose: 15 mg/kg. |
| Folinic Acid (Leucovorin) | ENHANCES FOLATE PATHWAY → accelerates conversion of formate to CO₂ + H₂O → reduces toxic formate accumulation. Given IV every 4–6 hours for 24–48 hours. |
| Sodium Bicarbonate (IV) | CORRECTS METABOLIC ACIDOSIS → alkalinises blood → reduces ionisation of formate → promotes renal excretion → reduces retinal penetration of formate |
| Haemodialysis | Removes methanol and formate directly from blood → most effective elimination; indicated if serum methanol >50 mg/dL or severe metabolic acidosis or visual symptoms |
EXAM RECALL
The antidotes for methanol poisoning in order of importance: (1) Fomepizole [preferred] or Ethanol [alternative] → ADH inhibition, (2) Folinic acid → enhance formate clearance, (3) Sodium bicarbonate → correct acidosis, (4) Haemodialysis → direct elimination.
7.2 Why Ethanol Protects Against Methanol
Alcohol dehydrogenase (ADH) has approximately 10–20 times greater affinity (lower Km) for ethanol than for methanol. Therefore, when both are present, ethanol saturates the enzyme and blocks methanol from being metabolised to toxic products. Unmetabolised methanol is excreted slowly by kidneys and lungs. This is the basis for co-ingestion of ethanol providing a degree of natural protection, and explains why individuals who drank ethanol along with methanol-contaminated liquor were less affected than those who drank methanol alone.
SECTION 8: FORENSIC SIGNIFICANCE IN ILLICIT LIQUOR CASES
8.1 Methanol in Illicit Liquor — The Indian Context
- Methanol poisoning from illicit (hooch) liquor is a major forensic public health issue in India
- Major poisoning outbreaks have occurred repeatedly in states like Uttar Pradesh, Bihar, West Bengal, Gujarat, Tamil Nadu, and others
- In illicit distillation, methanol is produced from pectin-rich raw materials (mahua, sugarcane, jaggary); poor temperature control in distillation does not effectively separate methanol from ethanol
- Methanol is sometimes deliberately added to illicit liquor to increase volume/potency (unscrupulous adulteration)
- Forensic scientists are called upon to: (1) establish cause of death, (2) identify source batch, (3) quantify methanol in samples, (4) testify in court
8.2 Sample Collection for Forensic Analysis
| Sample | Notes |
|---|---|
| Suspected liquor sample | Original sealed bottle preferred; take multiple aliquots |
| Blood (ante-mortem) | Fluoride-oxalate tube to prevent fermentation; refrigerate |
| Vitreous Humour (post-mortem) | Best post-mortem sample — resists putrefaction, protected from GI bacteria; reflects true blood level |
| Urine | Useful; methanol excreted slowly by kidneys |
| Gastric lavage/contents | Useful if collected early |
| Liver tissue (post-mortem) | For methanol and metabolite detection |
| Brain tissue (post-mortem) | For putaminal lesion correlation |
| Exhaled breath (ante-mortem) | Headspace GC analysis possible |
8.3 Medico-legal Importance
- Methanol poisoning is treated as a homicide if liquor was deliberately adulterated
- Manufacturer/seller of illicit liquor causing death can be charged under IPC Section 304 (Culpable Homicide) or Section 304A (Death by negligence)
- The Excise Acts of respective States govern penalties for producing/selling illicit liquor
- Mass casualty methanol poisoning events require systematic forensic investigation linking victims to common source
- Chain of Custody of liquor samples must be maintained for evidence admissibility in court
- Forensic expert's testimony on method of analysis, quantitative results, and comparison with permissible limits is critical
SECTION 9: COMPLETE ANALYSIS PROTOCOL FOR LIQUOR SAMPLES
9.1 Standard Operating Procedure (SOP)
A. Qualitative Analysis:
- 1. Test for Ethanol: Iodoform Test — yellow precipitate (CHI₃) confirms ethanol presence
- 2. Test for Methanol: Chromotropic Acid Test — violet colour confirms methanol
- 3. Test for Methanol: Schiff's Reagent Test — magenta colour confirms methanol
- 4. Test for Copper & Iron: Flame test or colorimetric methods (relevant in some hooch poisoning cases)
- 5. Test for Furfural: Aniline acetate test — red colour; furfural from grain husks indicates poor distillation
B. Quantitative Analysis:
- 6. Determination of Ethanol by Gas Chromatography (FID, internal standard n-propanol)
- 7. Determination of Methanol by Gas Chromatography (same or separate run)
- 8. Additional determinations (as per IS:3752-1988): Ash, Total acidity, Volatile acidity, Fixed acidity, Esters, Furfural, Copper, Iron, Aldehydes
C. Sample Preparation:
- Distillation: If sample is turbid or coloured, distillation is done first to obtain clear distillate for testing
- Dilution: High-strength samples diluted with distilled water before testing
- Internal standard addition: Known concentration of n-propanol added before GC injection
9.2 Other Determinations in Liquor Analysis
| Parameter | Significance / Method |
|---|---|
| Total acidity | Measures all acid equivalents; titrimetry with NaOH |
| Volatile acidity (as acetic acid) | Indicator of poor fermentation/vinegar contamination; steam distillation + titration |
| Fixed acidity | Non-volatile acids; difference between total and volatile acidity |
| Esters (as ethyl acetate) | Flavour compounds; GC analysis |
| Furfural | From pentose sugars/grain husks; indicates country spirit; aniline acetate test (red colour) |
| Copper | From copper stills; atomic absorption spectrophotometry (AAS) |
| Iron | From iron vessels; AAS or colorimetry |
| Higher alcohols (fusel oils) | n-propanol, isobutanol, amyl alcohol; GC analysis |
SECTION 10: METHANOL vs ETHANOL — KEY DIFFERENCES
| Property | METHANOL (CH₃OH) | ETHANOL (C₂H₅OH) |
|---|---|---|
| Molecular Formula | CH₃OH (MW = 32) | C₂H₅OH (MW = 46) |
| Boiling Point | 64.7°C | 78.37°C |
| Common Name | Wood alcohol | Grain alcohol |
| Iodoform Test | NEGATIVE | POSITIVE (yellow CHI₃) |
| Chromotropic Acid Test | POSITIVE (violet) | NEGATIVE (no violet) |
| Main toxic metabolite | Formic acid (via formaldehyde) | Acetaldehyde (less toxic) |
| Primary enzyme | ADH (10x slower than ethanol) | ADH (faster metabolism) |
| Key toxicity feature | Metabolic acidosis + blindness | CNS depression; hangover |
| Lethal dose | ~30 mL (pure) | ~3–4 L (pure) |
| Antidote | Ethanol, Fomepizole | No specific antidote |
| GC RT on Porapak column | Elutes before ethanol | Elutes after methanol |
| Permissible in spirits | Max 50 mg/100 mL pure alcohol | Primary constituent |
| Origin in illicit liquor | Pectin fermentation; added adulterant | Fermentation of sugars |
Section 11: Practice MCQs — PYQ Pattern
Q1. Which reagent gives a VIOLET colour when reacted with methanol in the presence of concentrated sulphuric acid?
(A) Schiff's reagent
(B) Chromotropic acid
(C) Ninhydrin
(D) Nessler's reagent
Answer: (B) | Chromotropic acid (1,8-dihydroxynaphthalene-3,6-disulphonic acid) gives a characteristic violet/purple colour with formaldehyde (from oxidised methanol) in concentrated H₂SO₄. This is the basis of the Chromotropic Acid Test for methanol.
Q2. The primary toxic metabolite responsible for blindness in methanol poisoning is:
(A) Formaldehyde
(B) Formic acid / Formate
(C) Acetaldehyde
(D) Formalin
Answer: (B) | Formic acid (formate ion) is the PRIMARY toxic metabolite. It inhibits cytochrome c oxidase in the mitochondria of optic nerve cells, causing ATP deficiency, optic nerve oedema and ultimately irreversible blindness. Formaldehyde is an intermediate that is rapidly converted to formic acid.
Q3. The latent period in methanol poisoning (between ingestion and onset of severe symptoms) is typically:
(A) 1–2 hours
(B) 12–24 hours
(C) 48–72 hours
(D) 5–7 days
Answer: (B) | The latent period is 12–24 hours (sometimes 6–30 hours). This silent interval occurs because methanol itself causes mild CNS depression, but the toxic metabolites take time to accumulate. Co-ingestion of ethanol extends this latent period further.
Q4. In GC analysis of methanol using the standard (IS method), the detector used is:
(A) TCD (Thermal Conductivity Detector)
(B) ECD (Electron Capture Detector)
(C) FID (Flame Ionization Detector)
(D) NPD (Nitrogen-Phosphorus Detector)
Answer: (C) | FID (Flame Ionization Detector) is used for quantification of methanol and ethanol by GC. FID is sensitive to carbon-containing compounds and is the detector of choice for volatile alcohols. Column: Porapak 80–100 mesh; Column temp: 160°C; Carrier gas: N₂ at 50 mL/min.
Q5. Ethanol acts as an antidote in methanol poisoning by which mechanism?
(A) Chelating methanol and preventing absorption
(B) Competitive inhibition of Alcohol Dehydrogenase (ADH)
(C) Enhancing renal excretion of methanol
(D) Alkalinising the blood to neutralise formic acid
Answer: (B) | Ethanol is a COMPETITIVE INHIBITOR of ADH. ADH has approximately 10–20x higher affinity for ethanol than methanol. By preferentially binding ADH, ethanol blocks the conversion of methanol to toxic formaldehyde and formic acid.
Q6. The enzyme responsible for converting formaldehyde to formic acid in methanol metabolism is:
(A) Alcohol dehydrogenase (ADH)
(B) Aldehyde dehydrogenase (ALDH)
(C) Cytochrome c oxidase
(D) Monoamine oxidase (MAO)
Answer: (B) | Aldehyde Dehydrogenase (ALDH) converts formaldehyde → formic acid. This step is very rapid, so formaldehyde does not accumulate in detectable amounts in serum. ADH converts methanol → formaldehyde (the rate-limiting first step).
Q7. Methylated spirit primarily consists of ethanol denatured with:
(A) 5% Acetone and 5% Benzene
(B) 5–10% Methanol (and optionally pyridine)
(C) Denatonium benzoate only
(D) 10% Isopropyl alcohol
Answer: (B) | Methylated spirit consists of ethanol + 5–10% methanol as the primary denaturant. Pyridine (0.5%) and methyl violet dye may also be added. The presence of methanol is what makes it toxic and unsuitable for drinking.
Q8. Which of the following alcohols gives a positive Iodoform test?
(A) Methanol
(B) Isopropanol
(C) Ethanol
(D) Both Ethanol and Isopropanol
Answer: (D) | Ethanol and Isopropanol both give positive Iodoform test (yellow CHI₃ precipitate). Methanol does NOT give Iodoform test — this is a key difference used to differentiate methanol from ethanol.
Q9. The internal standard most commonly used in GC quantification of methanol in liquor samples is:
(A) n-Butanol
(B) n-Propanol (1-Propanol)
(C) tert-Butanol
(D) Benzyl alcohol
Answer: (B) | n-Propanol (1-propanol) is the most commonly used internal standard. It is chosen because it elutes close to but distinct from methanol and ethanol and behaves similarly during sample preparation.
Q10. Characteristic post-mortem finding in fatal methanol poisoning on brain imaging/autopsy is:
(A) Cerebral cortex atrophy
(B) Bilateral haemorrhagic necrosis of the putamen (basal ganglia)
(C) Hippocampal sclerosis
(D) Cerebellar vermis degeneration
Answer: (B) | Bilateral haemorrhagic necrosis of the putamen is the pathognomonic finding. It results from formate-induced mitochondrial dysfunction in the metabolically active basal ganglia cells.
Q11. The preferred drug of choice (first-line antidote) for methanol poisoning is:
(A) Ethanol (IV)
(B) Fomepizole (4-methylpyrazole)
(C) N-acetylcysteine
(D) Atropine
Answer: (B) | Fomepizole (4-methylpyrazole) is the PREFERRED first-line antidote. It is a selective, reversible ADH inhibitor that blocks methanol metabolism without causing inebriation or hypoglycaemia.
Q12. The Indian BIS Standard IS:4117-1973 governs:
(A) Permissible methanol levels in potable spirits
(B) Denaturing specifications for Denatured Rectified Spirit (DNS)
(C) Maximum permitted pyridine in industrial alcohol
(D) Forensic analysis protocol for liquor adulteration
Answer: (B) | BIS Standard IS:4117-1973 specifies the approved denaturants and their concentrations for Denatured Rectified Spirit (DNS) / Denatured Alcohol in India.
SECTION 12: RAPID REVISION — HIGH-YIELD FACTS
TOP 20 HIGH-YIELD FACTS FOR UGC NET
- 1. Methanol = Wood alcohol; Ethanol = Grain alcohol
- 2. Chromotropic acid test: Methanol → VIOLET/PURPLE colour (gold standard qualitative test)
- 3. Schiff's reagent test: Methanol → Magenta/reddish-purple; Ethanol → faint pink that fades
- 4. Iodoform test: POSITIVE for Ethanol & Isopropanol; NEGATIVE for Methanol
- 5. Metabolic pathway: CH₃OH →(ADH)→ HCHO →(ALDH)→ HCOOH →(Folate)→ CO₂ + H₂O
- 6. Rate-limiting enzyme: Alcohol Dehydrogenase (ADH) for methanol oxidation
- 7. PRIMARY TOXIC METABOLITE: Formic acid (Formate) — causes blindness & metabolic acidosis
- 8. Formate inhibits: Cytochrome c Oxidase (Complex IV) in mitochondria
- 9. Latent period: 12–24 hours between ingestion and severe symptoms
- 10. Minimum dose for blindness: 10 mL pure methanol; for death: ~30 mL pure methanol
- 11. LD50 (humans): ~56.2 g (≈ 1 g/kg body weight)
- 12. GC parameters: Column = Porapak 80-100 mesh; Temp = 160°C; Carrier gas = N₂ (50 mL/min); Detector = FID
- 13. Internal standard for GC: n-Propanol (1-propanol)
- 14. Antidote (preferred): Fomepizole (4-methylpyrazole) — ADH inhibitor
- 15. Antidote (alternative): Ethanol — competitive ADH inhibitor; blood level maintained 100–200 mg/dL
- 16. Folinic acid: Enhances formate clearance via folate pathway
- 17. Methylated spirit = Ethanol + 5–10% methanol ± 0.5% pyridine + Methyl violet dye
- 18. Best post-mortem sample for methanol: Vitreous humour
- 19. Pathognomonic brain lesion: Bilateral haemorrhagic necrosis of putamen
- 20. Permissible methanol in Indian potable spirits: ≤50 mg/100 mL absolute alcohol (IS:3752)
SECTION 13: MNEMONICS & MEMORY AIDS
For Methanol Metabolism
MNEMONIC — 'MAF-CO':
M = Methanol → A = ADH (Alcohol Dehydrogenase) → F = Formaldehyde → ALDH → Formic acid → C = Cytochrome oxidase (inhibited) → O = Optic nerve damage
For Denaturants
MNEMONIC — 'MIPBA-D':
M = Methanol, I = Isopropanol, P = Pyridine, B = Benzene (phased out), A = Acetone, D = Denatonium benzoate — these are the common denaturants used in denatured alcohol formulations.
For Antidotes of Methanol
MNEMONIC — 'FLESH':
F = Fomepizole (ADH blocker — first choice), L = Leucovorin/Folinic acid (Folate pathway enhancer), E = Ethanol (ADH blocker — alternative), S = Sodium bicarbonate (acidosis correction), H = Haemodialysis (elimination)
Study Guidance
Priority Order for Exam Preparation:
- (1) Chromotropic Acid Test details
- (2) Metabolic Pathway + Enzymes
- (3) Denaturant composition
- (4) GC parameters
- (5) Toxicity numbers & antidotes
Reference Standards: IS:3752 (liquor analysis), IS:4117-1973 (denatured spirit), BIS, Denaturing of Spirit Rules 1972
