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Limit test for arsenic and lead

The document describes procedures for limit tests to detect arsenic and lead impurities. For arsenic, the test uses Gutzeit apparatus to convert any arsenic present into arsine gas, which is detected by the formation of a yellow stain on mercuric chloride paper. For lead, the test uses dithizone to form a violet-colored lead dithizonate complex if lead is present, and the color is compared to a standard. Both tests involve extraction and color comparison to determine if the level of impurity exceeds the specified limit.

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Limit test for arsenic and Lead MrAshishR. Chaudhari ASST. PROFESSOR, M.PHARM (QA) P.R.PATIL, INSTITUTE OF PHARMACY, TALEGAON (SP), DIST- WARDHA Presented by :
• The original Gutzeit test was quantitative one and the liberated arsine gas is detected by covering the mouth of the flask with filter paper, on which small amount of nitrate crystals were placed. silver The liberated arsine turns silver nitrate complex black. Limit test for arsenic
• In the modified test, filter paper dipped in mercuric chloride solution is used, which yields a yellow colored complex. It produces a stain in the filter paper.
Principle • The arsenic impurity present in the sample is first converted into arsenious acid by the action of reducing agents like potassium iodide, Zinc, HCl and stannous chloride.
• The arsenious acid is then reduced to arsine by the action of nascent hydrogen which is produced by the reaction between Zinc and HCl. • The liberated arsine gas reacts with mercuric chloride paper and forms yellow stain
Gutzeit apparatus
Procedure • The required amount of sample solution is placed in the wide mouthed bottle. • To this add 1 g of potassium iodide and 10 g of granulated zinc AST. • Immediately close the bottle with stopper assembly. • The acid present in the test solution and zinc reacts, produces nascent hydrogen which reacts with arsenious acid present in the sample and liberates arsine gas which passes through the tube and reacts with mercuric chloride paper and produces yellow stain.
• The reaction is maintained for 40 minutes to ensure complete conversion of arsenious acid to arsine gas. • Some heat pressure are produced during the reaction due to the increased amount of gas in the inside of the apparatus. • The rate of reaction is increased by placing the total assembly on a warm surface.
Standard of standard stain • Standard stain is prepared by using dilute arsenic solution [ 0.132 g of arsenic in 100 ml] • I ml is diluted to 100 ml . • The comparison of the stain is done after the completion of the test immediately, so the test and standard should be carried out simultaneously.
Precaution • If the stain present in the filter paper becomes dark, the test should be repeated by using pure reagents. • The most suitable temperature for carry out this test is 40C. • Cotton wool dipped in lead acetate solution is used to trap any hydrogen sulphide gas liberated with arsine gas. • Care must be taken that the filter paper remains quite dry during the reaction.
• During the succeeding tests the tube must be washed with HCl AsT rinsed with water and dried. • All the reagents used for this test should be free from arsenic and mentioned as AsT.
LIMIT TEST FOR LEAD The Lead Limit Test is designed to determine the allowable limit of heavy metal i.e. Lead contained in a sample. • Lead is a most undesirable impurity in medical compounds and comes through use of sulphuric acid, lead lined apparatus and glass bottles used for storage of chemicals. • Common sources of lead impurities are: Equipment used for manufacturing. Storage container. From packaging material
• Principle • Limit test of lead is based on the reaction of lead and diphenyl thiocarbazone (dithizone) in alkaline solution to form lead dithizonate complex which is violet in color. • Lead present as an impurity in the sample reacts with dithizone to form lead thizonate which is responsible for the appearance of violet color in the solution. This reaction will only occur if the given sample contains lead as an impurity even in trace amount.
• In this test, Nessler cylinders are not used, instead it is performed by extraction with the help of separating funnel. • The original color of dithizone in chloroform is green while the lead dithizonate complex is violet in color. The intensity of the violet color of the complex depending upon the quantity of lead present in the solution is compared with that of the standard color produced by treating standard solution containing definite amount of lead in the similar manner. Intensity of this violet color is compared in both the tests and standard solution in a chloroform solvent medium.
PROCEDURE TEST SAMPLE STANDARD SAMPLE REASON Take a sample and transferred in separating funnel Take standard lead solution and transferred in separating funnel. To check impurity of lead present in sample or not Add 6ml Ammonium citrate Add 6ml Ammonium citrate To maintain optimum pH Add 2ml potassium cyanide and 2ml hydroxylamine hydrochloride Add 2ml potassium cyanide and 2ml hydroxylamine hydrochloride Cyanide for complex with all interfering metal in solution Add 2 drop phenol red Add 2 drop phenol drop Act as indicator to develop color Make solution alkaline by adding ammonia Make solution alkaline by adding ammonia It provide basic condition to develop red color Extract with 5ml dithizone until it become green Extract with 5ml dithizone until it become green At least 2-3 extraction done and mix in the end in separating funnel Combined dithizone extracts are shaken for 30 mins with 30 ml of nitric acid and the chloroform layer is discarded Combined dithizone extracts are shaken for 30 mins with 30 ml of nitric acid and the chloroform layer is discarded Acidic layer is separated from chloroform . Nitric acid make lead free from chloroform layer
To the acid solution add 5ml standard dithizone To the acid solution add 5ml standard dithizone Violet color produced due to lead- dithizonate Add 4ml ammonium cyanide Add 4ml ammonium cyanide Maintain optimum pH Shake for 30 minutes and observe the color Shake for 30 minutes and observe the color Compare colour
REAGENT PREPARATIONS: 1. Preparation of standard lead solution (1 ppm Pb): Dissolve 0.4 g of lead nitrate in water containing 2 ml of dilute nitric acid and add sufficient water to produce 250.0 ml. This gives standard lead solution (1% Pb). 2. Standard lead solution (1 ppm Pb) is prepared by diluting 1 volume of standard lead solution (1% Pb) to 1000 volumes with water. 3. Preparation of dithizone extraction solution: Dissolve 30 mg of dithizone in 1000 ml of chloroform and add 5 ml of ethanol (95%). The solution is stored in refrigerator. Before use, the solution is shaken with about half of its volume of 1% v/v nitric acid solution and the acid is discarded. 4. Preparation of dithizone standard solution: Dissolve 10 mg of dithizone in 1000 ml of chloroform.
CONCLUSION • If color produced in the sample solution is less than the standard solution, the sample will pass the limit test of lead and vice versa..