The Study of Non-biological (not from living things) Evidence
Shabina Fatma, Kiran Kumari
Department of Forensic Science, Jharkhand Raksha Shakti University, Ranchi, Jharkhand, India – 834008.
*Corresponding Author E-mail: shabinafatma2022@gmail.com
Abstract:
Forensic chemistry is the application of chemistry and its subfield, forensic toxicology, in a legal setting. A forensic chemist can assist in the identification of unknown materials found at a crime scene. Specialists in this field have a wide array of methods and instruments to help identify unknown substances.
KEYWORDS: Forensic chemistry, Forensic toxicology.
INTRODUCTION:
Forensic chemistry is the study of non-biological (not from living things) evidence found at crime scenes. Forensic chemists use several types of technology to do their jobs1. Forensic chemists use laser ablation inductively coupled plasma mass spectrometry to study glass shards found at a crime scene.2
Forensic chemistry involves using scientific methods to investigate physical evidence.
Tests for acid attacks
Cation and anion tests
Trap cases
Alcohol Detectionin Drink Driven Cases
Salt analysis (also known as systematic qualitative analysis or qualitative inorganic analysis) involves the identification of the cation and anion of an inorganic salt. This is done by conducting a series of tests in a systematic manner and using the observations to confirm the absence or presence of specific cations and anions. 3
Experiment: Add a few drops of dilute H2SO4 (sulfuric acid) to a small quantity of the salt in a test tube (If nothing happens, move on to preliminary test for group 2 anions).4
|
Anion |
Positive Result |
|
Carbonate (CO32-) |
Colourless and odourless gas that turns limewater milky |
|
Sulphite (SO32-) |
Colourless, pungent-smelling gas |
|
Sulphide (S2-) |
Colourless gas which smells like rotten eggs |
|
Nitrite (NO2–) |
Light brown gas having a pungent smell |
Experiment: Take a small chunk of the salt in a test tube and add a few drops of concentrated H2SO4 (sulfuric acid) to it (If nothing happens, move on to preliminary test for group 2 anions).
|
Anion |
Positive Result |
|
Chloride (Cl–) |
White, pungent-smelling fumes that intensify when a glass rod dipped in ammonium hydroxide is brought to the mouth of the test tube |
|
Bromide (Br–) |
Reddish-brown fumes |
|
Iodide (I–) |
Violet fumes |
|
Acetate (CH3COO–) |
Pungent fumes that smell like vinegar |
|
Nitrate (NO3–) |
Brown fumes with a pungent smell |
|
Oxalate (C2O4–) |
Bubbly effervescence of a combination of carbon monoxide and carbon dioxide gas |
The sulphate and phosphate ions (group 3 anions) do not have any preliminary tests. If no positive preliminary tests are obtained for group 1 and group 2 anions, you can directly jump to confirmatory tests for the sulphate and phosphate anions.
*Note: Water extract of the salt can be prepared by dissolving a pinch of the inorganic salt in a few drops of water.
|
Anion |
Confirmatory Test |
Positive Observation |
|
Carbonate (CO32-) |
Add magnesium sulphate (MgSO4) to the water extract of the salt |
Formation of a white precipitate. |
|
Sulphite (SO32-) |
Add aqueous barium chloride (BaCl2) to the water extract |
Formation of a white precipitate which disappears when diluting hydrochloric acid (HCl) is added. |
|
Sulphide (S2-) |
(i) Add sodium nitroprusside to the water extract (ii) Add aqueous lead acetate to the water extract |
(i) The solution turns purple or violet (ii) Formation of a black precipitate |
|
Nitrite (NO2–) |
Boil a mixture of the water extract and dilute H2SO4. Now add solid potassium iodide and starch solution to it |
The solution develops a deep blue colour |
|
Chloride (Cl–) |
Add silver nitrate (AgNO3) to the water extract |
Formation of a white precipitate which is soluble in ammonium hydroxide (NH4OH) |
|
Bromide (Br–) |
Add silver nitrate to the water extract |
Formation of a yellow precipitate which is partially soluble in ammonium hydroxide |
|
Iodide (I–) |
Add silver nitrate to the water extract |
Formation of a yellow precipitate which is insoluble in NH4OH |
|
Nitrate (NO3–) |
Mix the water extract with iron (II) sulphate solution (FeSO4) and add one drop of concentrated nitric acid (HNO3) along the side of the test tube. |
Formation of a brown ring at the junction of the acid and the solution. |
|
Acetate (CH3COO–) |
Add concentrated H2SO4 and some ethanol to the salt. |
Development of a fruity smell (due to the formation of an ester) |
|
Oxalate (C2O4–) |
Add acetic acid and calcium chloride to the water extract and boil the solution. |
Formation of a white precipitate that dissolves upon the addition of dilute HNO3 |
|
Sulphate (SO42-) |
Add aqueous barium chloride (BaCl2) to the water extract |
Formation of a white precipitate which is insoluble in concentrated hydrochloric acid (HCl) |
|
Phosphate (PO43-) |
Add dilute nitric acid (HNO3) and ammonium molybdate ((NH4)2MoO4) to the water extract and boil the resulting solution. |
Formation of a yellow, crystalline precipitate |
The original solution (OS) of the salt can be prepared by mixing a small amount of the inorganic salt with water and acid.
Experiment: Add a few drops of dilute hydrochloric acid (HCl) to the original solution.
|
Cation |
Positive Result |
|
Lead (Pb2+) |
Formation of a white precipitate |
Experiment: Add a few drops of dilute HCl and hydrogen sulphide (H2S) to the original solution.
|
Cation |
Positive Result |
|
Copper (Cu2+) |
A black precipitate is formed |
· Ferrous salts (featuring Fe2+ ions) are green in colour and ferric salts (featuring Fe3+ ions) are brown in colour.
· For aluminium (Al3+ ion), a gelatinous white precipitate is obtained when the solid ammonium chloride (NH4Cl) and excess ammonium hydroxide are added to the original solution.
Experiment: Add solid NH4Cl and excess NH4OH to the original solution. Now pass H2S gas through it.
|
Cation |
Positive Result |
|
Cobalt (Co2+) and Nickel (Ni2+), conduct both confirmatory tests |
Formation of a black precipitate |
|
Manganese (Mn2+) |
A skin-coloured precipitate is formed |
|
Zinc (Zn2+) |
A greyish-white precipitate is formed |
Experiment: Add ammonium carbonate ((NH4)2CO3), ammonium chloride (NH4Cl), and ammonium hydroxide to the original solution. If you obtain a white precipitate, the cation might be a group 5 cation. Add dilute acetic acid (CH3COOH) to dissolve the white precipitate. Now do the following tests in the same order followed in the tabular column provided below.
|
Cation |
Positive Result |
|
Barium (Ba2+) |
A yellow precipitate is formed when K2CrO4 is added to the solution. |
|
Strontium (Sr2+) |
A white precipitate is formed when aqueous (NH4)2SO4 is added to the solution. |
|
Calcium (Ca2+) |
A white precipitate is formed when aqueous (NH4)2C2O4 (ammonium oxalate) and NH4OH are added to the solution. |
|
Cation |
Confirmatory Test |
Positive Observation |
|
Ammonium (NH4+) |
Add sodium hydroxide to the original solution and treat it with Nessler’s reagent (K2HgI4) |
Formation of a yellow or brown precipitate |
|
Lead (Pb2+) |
(i) Add potassium iodide (KI) to the original solution (ii) Add potassium chromate (K2CrO4) to the original solution |
Formation of a yellow precipitate (for both the tests) |
|
Copper (Cu2+) |
In the preliminary test, a black precipitate is obtained from the addition of dilute HCl and H2S to the original solution. Isolate the black precipitate and add excess ammonium hydroxide to it |
A blue coloured solution is formed |
|
Iron (Fe3+) |
Add concentrated nitric acid to the original solution and heat it. A brown precipitate will form. Add HCl and potassium ferrocyanide (K4[Fe(CN)6]) to it |
A blue precipitate is formed |
|
Aluminium (Al3+) |
A gelatinous white precipitate is obtained when NH4Cl and excess NH4OH are added to the original solution. Isolate the precipitate and dissolve it in HCl. Now add blue litmus and NH4OH to it drop-by-drop |
A floating, blue layer is formed on the surface of the clear solution |
|
Cobalt (Co2+) |
Add solid NH4Cl and excess NH4OH to the original solution and pass H2S gas through it. Dissolve the resulting blue residue in water and add dilute CH3COOH and KNO2 to it. Now warm the mixture. |
A yellow precipitate is obtained. |
|
Nickel (Ni2+) |
Add solid NH4Cl and excess NH4OH to the original solution and pass H2S gas through it. Dissolve the resulting yellow residue in water (you will now obtain a green-coloured solution). Add NaOH and bromine water to this mixture and boil it. |
A black precipitate is formed |
|
Manganese (Mn2+) |
To the skin-coloured precipitate obtained in the preliminary test, add dilute HCl and boil off the H2S. Now add NaOH. |
Formation of a white precipitate that turns brown or black when bromine water is added |
|
Zinc (Zn2+) |
Add NaOH (in excess) to the greyish-white precipitate obtained in the preliminary test. |
The white precipitate is dissolved |
|
Barium (Ba2+), Strontium (Sr2+), and Calcium (Ca2+) |
Flame test: Make a paste of the salt by mixing it with a few drops of concentrated hydrochloric acid. Now skim off some of the paste with a glass rod and expose it to a Bunsen Burner’s flame. |
Ba2+: Green-coloured flame Sr2+: Crimson red flame Ca2+: Brick red flame |
|
Magnesium (Mg2+) |
Add NH4Cl, (NH4)3PO4 (ammonium phosphate) and excess NH4OH to the original solution. |
A white precipitate is obtained. |
TRAP CASES
Phenolphthalein is used, during trap cases as proof of acceptance of bribe. Phenolphthalein powder is spread on the currency notes and other objects which are likely to come in contact with hands of the perpetrator resulting in its transfer (based on Locard‘s principle of exchange) [It states that whenever two objects come in contact, there is always transfer or exchange of any material.]
Analysis Of Phenolphthalein:
· pH Test: the pH of the solution is examined with the pH paper. More than pH 9 (range 8.3-10) with pink /red color shows the positive test for the presence of phenolphthalein.
· Acid –Alkali Test: On addition of few drops dilute hydrochloric acid (HCl), pink colour disappears. Now add few drops of dilute solution of sodium hydroxide (NaOH) in water, the pink colour reappears. Appearing and disappearing of pink colour indicates the positive test for the presence of phenolphthalein.
Preparing an Extract for Further Confirmation
Appropriate amount of hand wash in a 100 ml beaker is taken. This solution is acidified with dilute hydrochloric acid. The solution is extracted with 3×30 ml of diethyl ether. The extracted residue was crystallized with hot water as white crystalline mass. The ether extract is concentrated for further analysis
Mobile Phase:
· Benzene: Dioxane: Acetic acid (75: 15: 10)
· Chloroform: Acetone (4:1)
· Ethyl acetate: Methanol: Ammonia (80:10:5)
· Stationary phase: Silica Gel G
Visualizing reagent:
· UV light
· Exposed to ammonia vapor
· Spray with neutral ferric chloride solution.
· Iodine fumes
· With 1 % solution of potassium permanganate in 0.25 M sulfuric acid.
The ether extract is loaded on a silica gel plate. Standard phenolphthalein was used for comparing. The plate was developed in a suitable solvent system and then sprayed with the suitable spraying reagent given below. TLC plate was exposed to ammonia vapor for visual development of undecomposed phenolphthalein as pink spot. Alternatively all the spots should be visualized by iodine fumes treatment of the developed plate and compared with the standards.
RESULTS AND DISCUSSION:
The different tests were performed for the presence of cations like ammonium, copper, iron, etc, anions like carbonate, chloride, nitrite, nitrate, etc.
Formation of white precipitate on addition of magnesium sulfate to the water extract of the salt showed the presence of carbonate. Similarly, formation of brown ring at the junction of the acid and the solution, upon mixing the water extract with iron sulfate solution and nitric acid showed the presence of nitrate. Obtaining different colour precipitates for the presence of cations and anions on performing different chemical tests have been mentioned in the above table.
REFERENCES:
1. I.C.F. Poole, S.K. Poole; Analytical Chemistry. Acta, 216: 1989;109-145.
2. M.L. Puttermans et al: 'Analytical Chemistry'; 67; 1984.
3. Saferstein R; 'Criminalistics-An Introduction to forensic Science', 6th Edn. Prentice Hall, New Jersey; 1998.
4. Lorna AN, Gabrielle ED, Karen SS (2023). Drug stability in forensic toxicology. Wiley interdisciplinary reviews. E1481.
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Received on 27.07.2023 Modified on 02.09.2023 Accepted on 23.09.2023 ©A&V Publications All right reserved Research J. Science and Tech. 2023; 15(4):211-214. DOI: 10.52711/2349-2988.2023.00035 |
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