Analysis of Ethyl Alcohol in Forensic Samples
Built using PYQ trends + forensic toxicology + analytical chemistry for last-day revision.
PYQ Trend
- Pyknometer – alcohol in beverages
- Widmark factor
- Blood : breath ratio
- Preservative in blood
- Gold standard method
- ADH principle
- Stages of intoxication
Forensic Significance of Ethyl Alcohol
Ethyl alcohol is one of the most frequently encountered toxicological substances in forensic casework. Its analysis is essential not only for determining intoxication but also for interpreting behaviour, cause of death, criminal responsibility, and medico-legal reconstruction of events.
1. Drunken Driving (Road Traffic Offences)
- Alcohol causes CNS depression leading to impaired judgment, delayed reaction time, reduced coordination, and visual disturbance.
- Forensic laboratory determines Blood Alcohol Concentration (BAC), which is the primary legal evidence.
- Legal limit in India: 30 mg / 100 mL blood.
- Breath alcohol analysis is used for screening in the field, while blood analysis is used for confirmatory evidence in court.
- Back calculation using Widmark equation helps estimate BAC at the time of the incident.
- Important issues in interpretation:
- Time gap between incident and sampling
- Absorption vs elimination phase
- Recent drinking (post-incident consumption defence)
Forensic Opinion: Whether the person was under the influence at the time of driving.
2. Homicide and Suicide Investigations
- Alcohol is frequently detected in victims as well as accused persons.
- It helps in interpreting:
- State of mind
- Level of consciousness
- Possibility of resistance or defence
- Risk-taking behaviour
- In violent deaths, alcohol may:
- Facilitate aggression (offender intoxication)
- Reduce defensive injuries (victim intoxication)
- Cause accidental falls misinterpreted as homicide
- In suicide:
- Alcohol lowers inhibition and increases impulsivity
- Common in poisoning, hanging, firearm deaths
- Vitreous humour alcohol is important in decomposed bodies.
Forensic Importance: Helps reconstruct events and assess behavioural capability.
---3. Drug-Facilitated Crimes (DFC)
- Alcohol is the most common substance used in drug-facilitated sexual assault.
- Acts as:
- Sedative
- Amnesic agent (anterograde amnesia)
- Often combined with:
- Benzodiazepines
- GHB
- Antihistamines
- Forensic challenges:
- Rapid metabolism
- Delay in reporting
- Volatile nature
- Specimens of choice:
- Blood (early cases)
- Urine (longer detection window)
Forensic Role: Correlating intoxication with the victim’s inability to consent.
---4. Industrial and Occupational Deaths
- Determination of alcohol is required in:
- Workplace accidents
- Machinery injuries
- Falls from height
- Factory explosions
- Helps in deciding:
- Worker negligence
- Employer liability
- Insurance compensation
- Even moderate BAC can significantly impair:
- Motor coordination
- Judgment
- Risk perception
Medico-legal Importance: Financial liability and legal responsibility.
5. Illicit Liquor Poisoning
- Occurs due to consumption of spurious alcohol contaminated with:
- Methanol
- Higher alcohols (fusel oil)
- Industrial solvents
- Forensic examination involves:
- Identification of ethanol and methanol by GC
- Quantification of congeners
- Comparison with seized samples
- Common in:
- Mass poisoning cases
- Rural and economically weaker populations
- Characteristic findings in methanol poisoning:
- Metabolic acidosis
- Optic nerve damage → blindness
- High fatality
Forensic Role:
- Establish cause of death
- Link victims with source of liquor
- Provide evidence for criminal prosecution
6. Interpretation in Post-mortem Toxicology
- Need to differentiate between:
- Ante-mortem consumption
- Post-mortem formation
- Important factors:
- Putrefaction
- Microbial activity
- Specimen type
- Reliable specimens in decomposed bodies:
- Vitreous humour
- Urine
- Muscle
7. Behavioural and Criminal Responsibility
- Alcohol intoxication is evaluated in relation to:
- Competency to give consent
- Criminal intent
- Fitness to drive or operate machinery
- Helps court in deciding:
- Degree of impairment
- Voluntary vs involuntary intoxication
UGC NET FORENSIC – EXAM FOCUS
- BAC and legal interpretation
- Role in DFC (most common substance)
- Vitreous humour in decomposed bodies
- Industrial medico-legal importance
- Illicit liquor → methanol differentiation
One-line Revision:
Alcohol analysis in forensics is used to assess intoxication, reconstruct events, determine liability,
evaluate consent and behaviour, establish cause of death, and link victims to crime or source.
Physicochemical Properties of Ethyl Alcohol
Understanding the physicochemical properties of ethyl alcohol is essential in forensic toxicology because these properties form the basis of its analysis, detection, preservation, distribution in the body, and interpretation of results. Many analytical methods and specimen handling protocols are directly derived from these properties.
| Property | Value | Forensic Importance |
|---|---|---|
| Boiling point | 78.37 °C | Low boiling point makes ethanol highly volatile → basis of distillation, headspace gas chromatography (HS-GC), and breath analysis. |
| Specific gravity | 0.789 | Lower than water → principle of pyknometer method, hydrometer, and alcoholometer for estimation in beverages. |
| Volatility | High | Responsible for:
|
| Water solubility | Completely miscible | Causes rapid absorption from:
|
| Oxidation | Ethanol → Acetaldehyde → Acetic acid | Biological oxidation by Alcohol dehydrogenase (ADH) → principle of enzymatic method and dichromate chemical method. |
1. Volatility
- Ethanol readily vaporizes at room temperature.
- This property is the basis of:
- Breath alcohol testing
- Headspace gas chromatography
- Microdiffusion techniques
- Forensic precautions:
- Samples must be stored in airtight, fully filled vials
- Avoid repeated opening
- Use of preservative (sodium fluoride)
- Loss of alcohol due to evaporation may lead to false low BAC.
2. Specific Gravity (Density)
- Density of ethanol is lower than water.
- As alcohol concentration increases → density of solution decreases.
- Analytical applications:
- Pyknometer method
- Hydrometer / alcoholometer
- Distillation followed by SG measurement
- Limitation:
- Not reliable for complex mixtures containing sugars, salts, or higher alcohols.
3. Complete Miscibility with Water
- Ethanol mixes with water in all proportions.
- Forensic significance:
- Rapid absorption into bloodstream
- Distribution proportional to tissue water content
- High concentration in:
- Blood
- Brain
- Vitreous humour
- This property explains:
- Blood as the best specimen for legal purposes
- Use of Widmark equation
- Lower concentration in adipose tissue
4. Oxidation Characteristics
- In the body:
- Ethanol → Acetaldehyde (by ADH)
- Acetaldehyde → Acetic acid (by ALDH)
- In chemical analysis:
- Potassium dichromate oxidizes ethanol → colour change orange to green.
- This forms the basis of:
- Dichromate method
- ADH enzymatic method
5. Low Molecular Weight & Rapid Diffusion
- Allows ethanol to:
- Cross biological membranes easily
- Reach equilibrium between blood and breath
- This is the basis of the blood : breath ratio (2100 : 1).
6. Hygroscopic Nature
- Ethanol absorbs moisture from air.
- Affects accurate weighing and density measurement if not properly handled.
FORENSIC APPLICATION SUMMARY
| Property | Application |
|---|---|
| Volatility | Breath analyser, HS-GC |
| Low boiling point | Distillation methods |
| Specific gravity | Pyknometer, hydrometer |
| Water miscibility | Widmark distribution |
| Oxidation | Dichromate & ADH methods |
UGC NET – MOST EXPECTED MCQ AREAS
- Property responsible for breath alcohol analysis → Volatility
- Principle of pyknometer → Specific gravity
- Distribution in body → Water miscibility
- ADH method principle → Oxidation of ethanol
- Need for airtight vial → Volatile nature
One-line Revision:
Volatility → breath test & HS-GC
Low density → pyknometer
Water miscibility → Widmark distribution
Oxidation → ADH & dichromate methods
Analysis of Ethyl Alcohol in Beverages – Screening Tests
Screening tests are preliminary tests used for the rapid detection of the presence of ethyl alcohol in beverages and suspected liquid samples. These tests are qualitative in nature and are followed by confirmatory quantitative methods such as distillation–specific gravity or gas chromatography.
In forensic laboratories, screening is important for:
- Sorting large numbers of seized samples
- Detecting illicit liquor
- Deciding whether further quantitative analysis is required
1. Odour Test
Principle: Ethanol is a volatile liquid having a characteristic spirituous odour.
Procedure
- The sample is gently warmed or shaken.
- Vapours are cautiously wafted towards the nose.
Observation
- Characteristic alcoholic smell indicates possible presence of ethanol.
Forensic Significance
- Very rapid and non-destructive.
- Used for preliminary examination of seized beverages.
- Helps in detecting fermentation in suspected samples.
Limitations
- Subjective and not specific.
- Cannot differentiate between:
- Ethanol
- Methanol
- Higher alcohols
- Flavoured beverages may mask the odour.
Exam Focus: Odour test is only a presumptive test, not confirmatory.
---2. Iodoform Test
Principle: Ethanol on oxidation forms acetaldehyde, which gives a positive iodoform reaction with iodine in alkaline medium.
Reaction:
Ethanol → Acetaldehyde → Iodoform (CHI3)
Reagents Required
- Iodine solution
- Sodium hydroxide (NaOH)
Procedure
- Add iodine solution to the sample.
- Add sodium hydroxide dropwise.
- Warm gently.
Positive Result
- Formation of yellow crystalline precipitate of iodoform.
- Characteristic antiseptic smell.
Chemical Basis
- Compounds containing the CH3–CHOH– group give this test.
Forensic Significance
- Simple and rapid chemical test for ethanol in beverages.
- Used in preliminary identification of alcoholic content.
Limitations
- Not specific for ethanol.
- Also given by:
- Acetaldehyde
- Isopropyl alcohol
- Methyl ketones
- Hence it is a presumptive test only.
Forensic Laboratory Approach
- Screening test → indicates presence of alcohol.
- Positive samples → subjected to quantitative analysis.
- Final report is based on confirmatory method (GC).
Comparison of Screening Tests
| Test | Nature | Result | Specificity |
|---|---|---|---|
| Odour test | Physical | Characteristic smell | Low |
| Iodoform test | Chemical | Yellow precipitate | Moderate (not specific) |
UGC NET – MCQ FOCUS
- Iodoform test → yellow crystals of CHI3
- Iodoform test detects → CH3–CHOH group
- Both odour and iodoform tests → screening / presumptive tests
- Not confirmatory for ethanol
One-line Revision:
Odour test → rapid but subjective
Iodoform test → yellow CHI₃ precipitate → presumptive for ethanol → needs GC confirmation
Odour test → rapid but subjective
Iodoform test → yellow CHI₃ precipitate → presumptive for ethanol → needs GC confirmation
Quantitative Methods for Estimation of Ethyl Alcohol
Quantitative estimation determines the percentage strength of alcohol in beverages and seized samples. In forensic science, accurate measurement is essential for:
- Detecting illicit liquor
- Quality control of alcoholic beverages
- Determination of tax liability
- Confirmatory toxicological reporting
These methods are based on specific gravity, oxidation reactions, or chromatographic separation.
---1. Pyknometer Method (Specific Gravity Method) ⭐ PYQ Favourite
Principle: Determination of alcohol content from the specific gravity of ethanol–water mixture.
Procedure (Concept)
- Weigh empty pyknometer.
- Fill with distilled water → weigh.
- Fill with sample → weigh.
- Calculate specific gravity.
- Alcohol percentage obtained from standard tables.
Forensic Significance
- Simple and accurate for binary mixtures (ethanol + water).
- Commonly asked in UGC NET examinations.
Limitation
- Not suitable for beverages containing:
- Sugars
- Salts
- Higher alcohols
- Requires prior distillation for complex samples.
Exam Focus: Pyknometer → based on specific gravity.
---2. Hydrometer / Alcoholometer
Principle: Alcohol concentration is inversely proportional to density of the solution.
Working
- Instrument floats at different levels depending on density.
- Direct reading gives alcohol percentage.
Applications
- Field and excise laboratories.
- Quick estimation of spirit strength.
Limitation
- Not accurate for coloured or viscous liquids.
- Affected by dissolved solids.
3. Distillation followed by Specific Gravity
Principle: Ethanol is separated by distillation due to its low boiling point (78.37 °C), and the distillate is analysed for specific gravity.
Why Distillation is Required?
- Removes:
- Sugars
- Colouring agents
- Non-volatile impurities
- Provides ethanol–water mixture for accurate SG measurement.
Forensic Importance
- Standard method in many forensic and excise laboratories.
- Improves accuracy of pyknometer/hydrometer methods.
4. Dichromate Oxidation Method (Spectrophotometric Method)
Principle: Ethanol is oxidized by acidified potassium dichromate to acetic acid.
Reaction:
Orange dichromate → Green chromium (Cr³⁺)
Measurement
- Decrease in dichromate concentration is measured spectrophotometrically.
- Intensity of colour change ∝ alcohol concentration.
Advantages
- Relatively sensitive.
- Can be used for biological samples also.
Limitations
- Other oxidizable substances may interfere.
- Less specific than GC.
Exam Focus: Orange → Green colour change.
---5. Headspace Gas Chromatography (HS-GC) – GOLD STANDARD ⭐
Principle: Separation and quantification of ethanol based on its partition into vapour phase (headspace) followed by gas chromatographic analysis.
Key Features
- Highly specific and accurate.
- Separates:
- Ethanol
- Methanol
- Isopropanol
- Acetone
- Uses internal standard (e.g., n-propanol or t-butanol).
Forensic Importance
- Confirmatory method for:
- Beverages
- Blood alcohol
- Illicit liquor analysis
- Provides:
- High precision
- High sensitivity
- Legal defensibility in court
Exam Focus: Gold standard → Headspace GC.
---COMPARISON OF METHODS
| Method | Principle | Accuracy | Forensic Use |
|---|---|---|---|
| Pyknometer | Specific gravity | Good | Simple beverages |
| Hydrometer | Density | Moderate | Field estimation |
| Distillation + SG | Boiling point + SG | High | Routine lab method |
| Dichromate method | Oxidation reaction | Moderate | Chemical estimation |
| HS-GC | Chromatographic separation | Very high | Confirmatory (Gold standard) |
UGC NET – MOST EXPECTED MCQs
- Pyknometer → estimation based on specific gravity
- Hydrometer works on → density principle
- Distillation used for → removal of non-volatile पदार्थ
- Dichromate method → orange to green colour change
- Gold standard → Headspace Gas Chromatography
- Internal standard in GC → improves accuracy and precision
One-line Revision:
SG → Pyknometer
Density → Hydrometer
Boiling point → Distillation
Oxidation → Dichromate
Confirmatory → HS-GC
SG → Pyknometer
Density → Hydrometer
Boiling point → Distillation
Oxidation → Dichromate
Confirmatory → HS-GC
Biological Specimens for Alcohol Analysis
Selection of the appropriate biological specimen is crucial for the accurate estimation and interpretation of alcohol concentration. Different specimens provide different forensic information depending on:
- Time since consumption
- Ante-mortem or post-mortem condition
- Degree of decomposition
- Purpose of analysis (legal / clinical / screening)
1. Blood
Forensic Value: Best and legally accepted specimen.
Why Blood is Preferred?
- Direct correlation with degree of intoxication.
- Used for legal interpretation in drunken driving.
- Basis of Widmark calculations.
Collection
- Preferred site: Femoral vein (post-mortem).
- Avoid heart blood → risk of post-mortem diffusion.
Preservative & Anticoagulant
- Sodium fluoride → prevents microbial production of alcohol.
- Potassium oxalate / EDTA → anticoagulant.
Limitations
- Post-mortem alcohol formation possible.
- Contamination during collection.
Exam Focus: Blood → best legal evidence.
---2. Breath
Forensic Value: Field estimation of alcohol.
Principle
- Based on blood : breath ratio = 2100 : 1.
- Measures alcohol in alveolar air.
Advantages
- Non-invasive.
- Immediate result.
- Used in breath analysers.
Limitations
- Indirect measurement of BAC.
- Affected by:
- Mouth alcohol
- Temperature
- Breathing pattern
Forensic Use: Screening test in drunken driving.
---3. Urine
Forensic Value: Screening and supportive evidence.
Characteristics
- Alcohol concentration usually higher than blood.
- Represents average BAC over time, not current intoxication.
Interpretation
- Cannot be directly correlated with behavioural impairment.
- Useful to detect prior consumption.
Limitations
- Time lag between blood and urine alcohol levels.
- Requires knowledge of voiding time.
Exam Focus: Urine → not reliable for degree of intoxication.
---4. Vitreous Humour
Forensic Value: Most reliable specimen in decomposed bodies.
Why Important?
- Resistant to:
- Putrefaction
- Microbial contamination
- Protected anatomical location.
- Less chance of post-mortem alcohol formation.
Interpretation
- Useful when blood is:
- Putrefied
- Unavailable
- Helps confirm ante-mortem consumption.
Exam Focus: Decomposed body → vitreous humour.
---5. Saliva
Forensic Value: Rapid and non-invasive estimation.
Characteristics
- Alcohol concentration is proportional to free alcohol in blood.
- Reflects recent intake.
Advantages
- Easy collection.
- Useful in roadside testing.
Limitations
- Affected by contamination from recent drinking.
- Less commonly used in forensic laboratories.
SPECIMEN SELECTION IN FORENSIC CASES
| Situation | Best Specimen |
|---|---|
| Drunken driving | Blood |
| Roadside screening | Breath |
| Past consumption | Urine |
| Decomposed body | Vitreous humour |
| Rapid non-invasive test | Saliva |
POST-MORTEM INTERPRETATION ISSUES
- Post-mortem alcohol formation due to microbial activity.
- Diffusion from stomach to heart blood.
- Evaporation loss if sample not preserved.
To avoid misinterpretation:
- Collect peripheral blood.
- Preserve with sodium fluoride.
- Analyse vitreous humour.
- Compare multiple specimens.
UGC NET – MOST EXPECTED MCQs
- Best legal specimen → Blood
- Specimen for decomposed body → Vitreous humour
- Field test → Breath
- Urine represents → Past consumption
- Preservative → Sodium fluoride
- Site for post-mortem blood → Femoral vein
One-line Revision:
Blood → legal evidence
Breath → screening
Urine → past intake
Vitreous → decomposed body
Saliva → rapid test
Blood → legal evidence
Breath → screening
Urine → past intake
Vitreous → decomposed body
Saliva → rapid test
Stages of Intoxication
| BAC (mg%) | Stage |
|---|---|
| 0–50 | Subclinical |
| 50–150 | Euphoria |
| 150–250 | Excitement |
| 250–350 | Confusion |
| 350–450 | Stupor |
| >450 | Coma |
Legal Limit of Alcohol in India
As per the Motor Vehicles Act, 1988, a person is considered legally intoxicated if the Blood Alcohol Concentration (BAC) exceeds 30 mg of alcohol per 100 mL of blood (30 mg%).
Equivalent Breath Alcohol Concentration
Based on the blood : breath ratio of 2100 : 1,
30 mg% BAC ≈ 0.03% BAC ≈ 0.014 mg alcohol / 210 L breath
---Medico-Legal Importance
- Primary evidence in drunken driving cases.
- Used to determine fitness to drive.
- Helps the court assess degree of impairment.
- Essential for insurance and compensation claims.
Legal Interpretation of BAC
| BAC (mg%) | Interpretation |
|---|---|
| 0 – 30 | Within permissible limit |
| > 30 | Legally intoxicated in India |
| 50 – 100 | Impaired judgment and reaction time |
| > 100 | Significant motor incoordination |
Even below the legal limit, behavioural impairment may occur, but legal offence is defined only above 30 mg%.
---Specimen Required for Legal Purpose
- Blood → confirmatory and court accepted.
- Breath → screening; must be confirmed by blood analysis.
Role of Forensic Expert in Court
- Report the measured BAC.
- Comment on whether it exceeds legal limit.
- Avoid giving opinion on “guilt”.
Quick Revision – High Yield Forensic Constants
These values are frequently asked in UGC NET, forensic viva, and MCQs. They should be memorised for rapid recall.
| Parameter | Value | Forensic Significance |
|---|---|---|
| Widmark factor (male) | 0.68 | Represents fraction of body weight as total body water → used for BAC calculation. |
| Widmark factor (female) | 0.55 | Lower due to higher body fat percentage. |
| Elimination rate | 15–20 mg% / hour | Used for back calculation of BAC at the time of incident. |
| Preservative | Sodium fluoride | Prevents microbial production of alcohol after collection. |
| Anticoagulant | Potassium oxalate / EDTA | Prevents clotting of blood sample. |
| Gold standard method | Headspace Gas Chromatography | Confirmatory, highly specific and legally defensible. |
| Blood : breath ratio | 2100 : 1 | Basis of breath alcohol analysis. |
| Legal BAC limit (India) | 30 mg% | Threshold for drunken driving offence. |
Widmark Equation (Conceptual Importance)
BAC = A / (r × W)
- A = amount of alcohol consumed (g)
- r = Widmark factor
- W = body weight (kg)
Used for:
- Back calculation in drunken driving cases
- Estimating amount of alcohol consumed
UGC NET – MOST EXPECTED MCQs
- Legal BAC in India → 30 mg%
- Widmark factor (male) → 0.68
- Elimination rate → 15–20 mg% per hour
- Preservative for blood alcohol → Sodium fluoride
- Gold standard → HS-GC
- Blood : breath ratio → 2100 : 1
30-Second Revision:
Legal limit India → 30 mg%
Widmark → 0.68 (male), 0.55 (female)
Elimination → 15–20 mg% / hr
Preservative → NaF
Gold standard → HS-GC
Breath ratio → 2100 : 1
Legal limit India → 30 mg%
Widmark → 0.68 (male), 0.55 (female)
Elimination → 15–20 mg% / hr
Preservative → NaF
Gold standard → HS-GC
Breath ratio → 2100 : 1
PYQ Practice
Pyknometer is used for: Estimation of ethyl alcohol in beverages
Preservative in blood: Sodium fluoride
Widmark factor (male): 0.68
Gold standard for BAC: Headspace GC
Blood : breath ratio: 2100 : 1
1-Day Revision Strategy
- Read theory once
- Memorize tables
- Solve PYQs
- Revise quick values
