INTRODUCTION:

Limit refers to quantity that may be present in the substance while test refers to analyse or estimate. Limit test is either a quantitative or semi quantitative tests to analyse the presence of inorganic impurities that are present in the compound which can be done by comparison of both test and standard either by turbidity or opalescence.^1–5

Types of Limit tests:

It is of three types,

a. Limit test involving no visible reaction

In this type, reaction takes place in which visible changes such as colour, precipitate and opalescence are not seen. These negative results only denote the absence of undesirably large amount of the impurity.

b. Comparison method

In this method, the standard containing known amount of impurities is compared with test where it produces visible changes like colour, precipitate and opalescence.

This comparison is set up at the same time and under the same conditions of the test experiment. Here, the amount of the impurity present in the sample is compared with the standard and it is possible to find out whether the impurity is within the prescribed limit or not. Examples, Limit test for Chloride, Sulphate and Iron etc.

c. Quantitative method

This method is usually applied in special cases where limit test cannot be applied directly. Here, the amount of the impurity present is actually determined and compared with the numerical limit given in the pharmacopoeia. Examples, Ash value, Limits of Soluble matter, Insoluble matter, Non-volatile matter, Moisture and Volatile matter and Residue on ignition.

Impurity:

It means the presence of foreign matter that can detoriate the original quantity of the compound. The impurities can be organic, inorganic, microbial, dust etc. ^1–3

Characteristics of Impurities:

· Some of the impurities which may be toxic or harmless at low concentration.

· Some impurities may have the physiochemical properties like colour, odour, etc. of the compound. ^2,5,6

Sources of Impurities:

· Raw materials used in manufacturing of the compounds.

· The chemicals used in the processing of the compounds.

· May be produced during various stages of manufacturing.

· May be due to atmospheric attack.

· Denaturation of the compounds during storage.

· Due to adulteration.

· Impurities produced due to errors in package.

The limit tests can be concluded based on the comparison of the turbidity or opalescence of the test and the standard. If the test is less turbid than the standard, it passes the test. ^1,2 The apparatus required to perform the limit test includes

· Nessler’s cylinder - The pair of glass tubes with uniform diameter of 24mm and capacity of 50ml. ⁶

Test tube holder.

Stirrer

1. Limit test for Chloride

(a) Aim - The aim is to carry out the Limit test for Chloride.

(b) Principle - The principle behind the limit test for chloride is based on chemical reaction between soluble chloride ions and silver nitrate in presence of dilute nitric acid. The insoluble silver chloride precipitate is formed and the test solution becomes turbid. The extent of turbidity or opalescence depends upon the amount of chloride present in the test sample. The turbidity of the test is compared with the standard.^7,10–13

(c) Procedure:

Take a pair of Nessler's cylinders and label as test and standard.

Test: Take specific amount of sample in Nessler’s cylinder and dissolve with small amount of water. 10ml of dilute nitric acid is added into that. Makeup to 50ml with distilled water and add 1ml of 0.1M silver nitrate. Stir immediately with glass rod and do not disturb for 5 minutess and protect it from light.

Standard: Take 10ml of standard chloride solution (25 ppm Cl) in Nessler’s cylinder and dilute with 5ml of distilled water. 10ml of dilute nitric acid is added into that. Makeup to 50ml with distilled water and add 1ml of 0.1M silver nitrate. Stir immediately with glass rod and do not disturb for 5 minutes and protect it from light.^{7, 9–11}

The opalescence so obtained from the test is compared with the opalescence obtained from the standard solution containing a known quantity of chloride against a dark background. The standard opalescence is produced from the amount of chloride which is prescribed as the limit of chloride impurity present in the test sample. If the opalescence obtained from the sample is less than the standard opalescence, the sample passes the limit test for chloride. If the opalescence obtained from the sample is more than the standard opalescence, the sample fails the limit test for chloride.

(d) Reason:

Silver nitrate is used to produce the precipitate of silver chloride (the precipitate is produced when it reacts with the chloride ion).

2. Limit test for Sulphate:

(a) Aim - To perform the Limit test of Sulphate.

(b) Principle - This is based on the reaction between the soluble sulphate and barium chloride in the presence of dilute hydrochloric acid. Opalescence is produced because of the formation of the product barium sulphate. Hence the formed opalescence of test is compared with the standard containing a known quantity of sulphate similarly treated against a dark background. The standard opalescence is produced from the amount of sulphate which is prescribed as the limit of sulphate impurity present in the test sample. If the opalescence obtained from the sample is less than the standard opalescence, the sample passes the limit test for sulphate. If the opalescence obtained from the sample is more than the standard opalescence, the sample fails the limit test for sulphate.

(c) Procedure - Take a pair of Nessler’s cylinders and name it as Test and Standard.

Test: Transfer 5ml of 25%W/V of Barium chloride solution in Nessler’s cylinder. Take 1.5 ml of Ethanolic standard solution into it, stir well and allow it to rest for 1 minute. Add 15 ml of sample solution and add 0.15 ml of 5M Acetic acid into Nessler’s cylinder. Makeup to 50 ml with purified water, stir and then set aside for 5 minutes.

Standard: Transfer 5ml of 25%W/V of Barium chloride solution in Nessler’s cylinder. Take 1.5 ml of Ethanolic standard solution into it, stir well and allow it to rest for 1 minute. Add 15 ml of standard sulphate solution and add 0.15 ml of 5M Acetic acid into Nessler’s cylinder. Makeup to 50 ml with purified water, stir and then set aside for 5 minutes. ^{7, 9–11}

(d) Reasons:

· The barium sulphate reagent used contains barium chloride, sulphate free alcohol and potassium sulphate and is made in situ.

· Sulphate free alcohol prevents super saturation and more uniform turbidity is formed.

· Potassium sulphate increases the sensitivity of the test.

· The ionic concentration in the reagent is so adjusted that the solubility product of barium sulphate is exceeded.

· Barium sulphate present in the reagent in a small quantity acts as a seeding agent for precipitation of barium sulphate if sulphate is present in the substances under test.

3. Limit test for Iron:

(a) Aim -To perform the Limit test for Iron.

(b) Principle - The principle is based on the reaction of iron with thioglycolic acid in the presence of citric acid in ammoniacal solution to produce pale pink to deep reddish purple. The colour produced is due to the formation of ferrous compound with thioglycolic acid (a coordination compound). This is stable in the absence of air and fades when exposed to air due to oxidation. The thioglycolic acid is a reducing agent and reduces ferric to ferrous iron. Ferrous thioglycolate is colourless in neutral or acid solution. The colour development takes place in the presence of alkali. The reactions are

(c) Procedure - Take a pair of Nessler’s cylinders and name it as test and standard.

Test: Dilute specific quantity of the sample with 20ml of water in Nessler’s cylinder. Add 2ml of 20% W/V of Iron free citric acid and 0.1ml Thioglycolic acid and stir. Add 2ml of 20% Iron free Ammonia to make the solution alkaline. Dilute to 50ml with water and allow to stand for 5minutes.

Standard: Dilute 2ml of standard iron solution with water in Nessler’s cylinder. Add 2ml of 20% W/V of Iron free citric acid and 0.1ml Thioglycolic acid and stir. Add 2 ml of 20% Iron free Ammonia to make the solution alkaline. Dilute to 50 ml with water and allow to stand for 5 minutes.^{7, 9–11}

The intensity of the colour in the test solution should be less than that in the standard so that it may be declared to be passed the test. If the intensity of the colour is more than the standard, it fails the test. (Note: Intensity of the colour in the test and the standard should be immediately compared after allowing to stand for 5minutes. If there is any delay, the colour will be faded due to oxidation and the test will not be reliable).

(d) Reason:

· Citric acid forms iron complex and prevents precipitation by ammonia.

· Thioglycolic acid is used for oxidation of ferrous ion to ferric ion.

4. Limit test for Heavy metals:

(a) Aim - To perform the Limit test for Heavy metal.

(b) Principle - The principle behind limit test for the heavy metals is based on the interaction between heavy metal ions and hydrogen sulphide solution. Thus, metal sulphide is produced, which have colours from dark brown to black. The heavy metals used are lead, mercury, bismuth, arsenic, antimony, tin, cadmium, silver, copper and molybdenum. Many heavy metals are tested and fixing a standard for comparison is difficult. For this, standard solution containing a definite quantity of lead nitrate is chosen. This quantity is called as the heavy metals limit and is expressed as parts of lead (by weight) per million parts of the substance. The usual limit for heavy metals according to Indian Pharmacopoeia is 20ppm. The sulphides formed and distributed as colloidal form which shows brown colour. The test is compared with the colour produced by the standard using lead solution as a heavy metal.

(c) Procedure – The Indian Pharmacopoeia has been prescribed four methods depending upon the type of metals involved.

Method A: This method is used for substances which are giving clear, colourless solution, while preparing a test.

Take a pair of Nessler’s cylinders and name it as test and standard.

Test: Take 25ml of sample solution which is prepared as prescribed in the monograph in Nessler’s cylinder. To maintain the pH between 3 to 4 add dilute Acetic acid or Aqueous Ammonia and dilute to about 35 ml water and stir well. Add 10ml of newly prepared Hydrogen sulphide solution, stir, makeup to 50ml with water and allow to stand for 5 minutes.

Standard: Take 1ml of Lead Standard solution (20ppm Pb) in Nessler’s cylinder, which will be diluted to 25ml with water. To maintain the pH between 3 to 4 add dilute Acetic acid or Aqueous Ammonia and dilute to about 35 ml water and stir well. Add 10 ml of newly prepared Hydrogen sulphide solution, stir, makeup to 50 ml with water and allow to stand for stir well.^10,13,14

Both the solutions will be viewed downwards against a white background. The intensity of the colour produced in the test should not be more than the intensity of the colour produced by the standard. If it is, it passes the test. If it is not, it fails the test.

Method B: This method is used for substances which do not give clear, colourless solutions and also used for substances that will interfere with precipitation of the contaminating heavy metals as their sulphides.

Take a pair of Nessler’s cylinders and name it as test and standard.

Test: The sample is taken in a suitable crucible. Add sufficient sulphuric acid to wet the sample, ignite carefully at a low temperature to get thoroughly charred. 2ml of nitric acid and 5 drops of sulphuric acid to the charred mass. Heats cautiously until white fumes are no longer evolved. That’s how the organic matter present in the sample is destroyed by the addition of nitric acid and sulphuric acids and ignition at 500-600ºC in a muscle furnace until the carbon is completely burnt off. Cool, add 4ml of hydrochloric acid, cover, digest on a water bath for 15 minutes, uncover and slowly evaporate to dryness on a water bath. Moisten the residue with 1 drop of hydrochloric acid, add 10ml of hot water and digest for 2 minutes. Add ammonia solution dropwise until the solution is just alkaline to litmus paper, dilute to 25ml with water and adjust with dilute acetic acid to a pH between 3 and 4. Filter, if necessary, rinse the crucible and the filter with 10ml of water, combine the filtrate and washings in a 50ml Nessler’s cylinder, dilute with water to about 35ml and mix. Add 10ml of newly prepared Hydrogen sulphide solution, stir, makeup to 50ml with water and allow to stand for 5minutes.

Standard: It is prepared in the same way as

Method A. ^{10, 13, 14}

Method C: This method is used for substances which are giving clear, colourless solution when they are dissolved in sodium hydroxide solution. In this method, alkaline condition is used for the precipitation of metallic sulphide.

Take a pair of Nessler’s cylinders and name it as test and standard.

Test: Test solution is prepared as given in the individual monograph in Nessler’s cylinder. To this, 20 ml of water and 5 ml of sodium hydroxide solution are added. Then make up to 50 ml and mixed. 5 drops of sodium sulphide solution is added to this and mixed. Then it is allowed to stand for 5 minutes.

Standard: Take 1ml of Lead Standard solution (20 ppm Pb) in Nessler’s cylinder and 5 ml of dilute sodium hydroxide solution are taken in a 50 ml Nessler’s cylinder. Then make up to 50 ml and mixed. 5 drops of sodium sulphide solution is added to this and mixed. Then it is allowed to stand for 5 minutes.

Both the solutions will be viewed downwards against a white surface. The intensity of the colour produced in the test should not be more than the intensity of the colour produced by the standard. If it is, it passes the test. If it is not, it fails the test.

Method D: This method is used for certain substances in which other impurities may interfere with the usual sulphide test. Here, the heavy metals are allowed to react with thioacetamide at pH 3.5 producing a brown colour.

Take a pair of Nessler’s cylinders and name it as test and standard.

Test: Test solution is prepared as given in the individual monograph and from that 12ml is pipetted into a Nessler’s cylinder. 2ml of acetate buffer of pH 3.5 and 1.2ml of thioacetamide reagent are added. Mixed and allowed to stand for 2 minutes.

Standard: Take 10ml of Lead Standard solution (1ppm Pb or 2ppm Pb) as prescribed in the monograph in Nessler’s cylinder. 2ml of acetate buffer of pH 3.5 and 1.2ml of thioacetamide reagent are added. Mixed and allowed to stand for 2 minutes.

(d) Reason: Hydrogen sulphide is used to produce coloured solution.

5. Limit test for Arsenic:

(a) Aim -To perform the limit test for Arsenic.

(b) Principle – The arsenic impurity in the sample is expressed as ppm (parts per million). The principle of limit test for arsenic is based on the conversion of arsenic into arsenic acid by reduction with zinc and dilute hydrochloric acid, which then reduced to arsenous acid. The nascent hydrogen produced by the reaction of zinc and hydrochloric acid reduces arsenous acid to arsine (arsenious hydride) gas. The test stain (yellow colour) obtained by the reaction of arsenic impurities in the form of arsine gas with mercuric chloride (paper) is compared with the standard stain (yellow colour) obtained by the reaction of known quantity of arsenic with mercuric chloride. ^{1, 3, 6, 15}

The intensity of the stain is compared with a standard stain in daylight. For preparation of standard solution, a specified quantity of standard dilute arsenic solution is used. If the intensity of the sample is less than the standard, the sample passes the test. If the intensity of the sample is more than the standard, the sample fails the test.

(c) Apparatus (Gutzeit):

The apparatus consists of a 100 ml bottle or conical flask closed by means of a rubber or ground glass stopper. A glass tube (about 20cm X 5mm) is passed through this. The lower part of the tube is having an internal diameter of 1.00mm, and 15mm from its tip is a lateral orifice 2 to 3mm in diameter. When the tube is in position in the stopper, the lateral orifice should be atleast 3mm below the lower surface of the stopper. The upper end of the tube has a perfectly flat surface at right angles to the axis of the tube. A second glass tube of the same internal diameter and 30mm long, with a similar flat surface, is placed in contact with the first and is held in position by two spiral springs or clips. Into the lower tube, insert 50-60 mg of lead acetate cotton, loosely packed or a small plug of cotton and a roll piece of lead acetate paper weighing 50 to 60 mg. This is used to trap any hydrogen sulphide that is formed. Between the flat surfaces of the tubes, place a disc or a small square of mercuric chloride paper to cover the orifice of the tube (15mm X 15mm).

(d) Reagents:

All the reagents used in this limit test for arsenic are marked and distinguished by letter “As T”. This indicates that all should be arsenic free and should they conform for the limit test for arsenic. Exceptions are dilute and strong arsenic solutions which are used for standard stains.

(e) Procedure:

Take two 120ml wide mouthed bottles or conical flasks with a label attachment as test and standard.

Test: Weigh accurately 10g of the sample and dissolve in 50 ml of water and transfer to the flask. Add 10ml of stannate hydrochloric acid As T and add 5ml of 1M Potassium iodide in addition to 10g of Zinc As T. Place the cork immediately over the bottle with attachments and immerse the bottle in water bath at suitable temperature for 40 minutes. Allow the reaction to be carried out for 40 min after that check the stain in the Mercury chloride paper.

Standard: Transfer 1ml of arsenic standard solution (10ppm As) in a flask and dilute to 50ml with water. Add 10ml of stannate hydrochloric acid and add 5ml of 1M Potassium iodide in addition to 10g of Zinc As T. Place the cork immediately over the bottle with attachments and immerse the bottle in water bath at suitable temperature for 40 minutes. Allow the reaction to be carried out for 40 min after that check the stain in the Mercury chloride paper.

[Note: For preparation of Arsenic Standard Solution (10ppm As), dissolve 0.330g of arsenic trioxide in 5ml of 2M sodium hydroxide and dilute to 250ml with water. Dilute 1volume of this solution to 100 volumes with water.]

The depth of the colour in the test stain is compared with the standard stain in daylight. If the test stain is more intense than the standard stain, the sample does not pass the test and is considered as substandard. If the test stain is not more intense than the standard stain, the sample passes the test.

6. Modified Limit test for Chloride and Sulphate by using potassium permanganate.

(a) Aim - To perform the modified limit test for chloride and sulphate by potassium permanganate.

(b) Principle – The principle for modified limit test is based on that the Potassium Permanganate is usually deeply coloured substance, and it can be decolourized on adding ethanol. It is reduced to brown precipitate of Magnesium di oxide. The above procedure is called as Pre-treatment. The colourless filtrate is collected (Sample) and limit test is performed.

Procedure for the preparation of Sample solution:

1g of potassium permanganate is dissolved in 50ml of water in hot condition. Add alcohol dropwise until potassium permanganate is decolourised. Filter the precipitate of magnesium dioxide and then collect the filtrate and the limit test is performed.

Procedure:

The above solution is used as sample for the modified limit test for chloride and sulphate using potassium permanganate.

Test for Chloride:

Test: Take specific amount of sample in Nessler’s cylinder and dissolve with small amount of water. Add 10ml of dilute nitric acid into it. Makeup to 50ml with distilled water and add 1ml of 0.1M silver nitrate. Stir immediately with glass rod and do not disturb for 5 mins and protect it from light.

Standard: Take 10ml of standard chloride solution in Nessler’s cylinder and dilute with 5ml of distilled water. Add 10 ml of dilute nitric acid into it. Makeup to 50ml with distilled water and add 1ml of 0.1M silver nitrate. Stir immediately with glass rod and do not disturb for 5 mins and protect it from light

Test for Sulphate:

Test: Transfer 1ml of 25%W/V of Barium chloride solution in Nessler’s cylinder. Take 1.5ml of Ethanoic standard solution into it, stir well and allow it to rest for 1 minute. Add 15ml of sample solution and add 0.15ml of 5M Acetic acid into Nessler’s cylinder. Makeup to 50 ml with purified water. Stir and allow to stand for 5 minutes.

Standard: Transfer 1ml of 25%W/V of Barium chloride solution in Nessler’s cylinder. Take 1.5ml of Ethanoic standard solution into it, stir well and allow it to rest for 1 minute. Add 15ml of standard sulphate solution and add 0.15 ml of 5M Acetic acid into Nessler’s cylinder. Makeup to 50 ml with purified water. Stir and allow to stand for 5 minutes.^4,5

7. Modified Limit test for Chloride and Sulphate by using Sodium bicarbonate:

(a) Aim - To perform the modified limit test for chloride and sulphate by using sodium bicarbonate.

(b) Principle – The principle behind the modified limit test for chloride and sulphate using sodium bicarbonate where the pre-treatment is about neutralising sodium bicarbonate with an appropriate mineral acid. Thus, the resulting neutralized solution is used for limit test (sample).

For limit test for chloride, the sample is dissolved in distilled water and neutralized with nitric acid while for limit test of sulphate the sample is dissolved in distilled water and neutralized with hydrochloric acid. In both the cases, the solution is stirred well, and the effervescence should be allowed to subside.

Procedure for Pre-treatment of Sodium bicarbonate

Steps	Chloride	Sulphate
1	Weigh accurately 1.25 g of sample and dissolve it with 15ml of water.	Weigh accurately 1g of sample and dissolve it with 10ml of water.
2	Add 2ml of con. Nitric acid.	Until the effervescence ceases neutralize with Con. HCl and dilute it with 15ml of water.

Test for Chloride:

The above solution obtained from pre-treatment (in chloride) used is used as sample for the modified limit test using sodium bicarbonate.

Test: Take specific amount of sample in Nessler’s cylinder and dissolve with small amount of water. Add 10 ml of dilute nitric acid into it. Makeup to 50ml with distilled water and add 1ml of 0.1M silver nitrate. Stir immediately with glass rod and do not disturb for 5 mins and protect it from light.

Standard: Take 10ml of standard chloride solution in Nessler’s cylinder and dilute with 5ml of distilled water. Add 10 ml of dilute nitric acid into it. Makeup to 50ml with distilled water and add 1ml of 0.1M silver nitrate. Stir immediately with glass rod and do not disturb for 5 mins and protect it from light

Test for Sulphate:

The above solution obtained from pre-treatment in Sulphate used is used as sample for the modified limit test using sodium bicarbonate.

Test: Transfer 1ml of 25%W/V of Barium chloride solution in Nessler’s cylinder. Take 1.5ml of Ethanoic standard solution into it, stir well and allow it to rest for 1 minute. Add 15ml of sample solution and add 0.15ml of 5M Acetic acid into Nessler’s cylinder. Makeup to 50 ml with purified water. Stir and allow to stand for 5 minutes.

Standard: Transfer 1ml of 25%W/V of Barium chloride solution in Nessler’s cylinder. Take 1.5ml of Ethanoic standard solution into it, stir well and allow it to rest for 1minute. Add 15ml of standard sulphate solution and add 0.15ml of 5M Acetic acid into Nessler’s cylinder. Makeup to 50ml with purified water. Stir and allow to stand for 5 minutes.^4,5

8. Modified Limit test for Chloride and Sulphate by using Sodium benzoate:

(a) Aim - To perform the modified limit test for chloride and sulphate by using sodium benzoate.

(b) Principle – The principle is based on the reaction of sodium benzoate with hydrochloric acid which is further converted to benzoic acid. It is insoluble in water and solution is filtered. This filtrate is used to carry out the modified limit test.

Procedure of Preparation of Sample:

10g of sodium benzoate is taken and dissolved in sufficient quantity of CO₂ free water and makeup to the volume of 100ml with distilled water. From the above solution 25ml of solution is taken and to this 15ml of distilled water is added along with 10ml of dilute nitric acid. Filter the solution and perform the limit test for chloride and sulphate using sodium benzoate.

Test for Chloride:

The above solution obtained from pre-treatment in chloride used is used as sample for the modified limit test using Sodium benzoate.

Test for Sulphate:

The above solution obtained from pre-treatment in Sulphate used is used as sample for the modified limit test using sodium benzoate.

Test: Transfer 1ml of 25% W/V of Barium chloride solution in Nessler’s cylinder. Take 1.5ml of Ethanoic standard solution into it, stir well and allow it to rest for 1 minute. Add 15ml of sample solution and add 0.15ml of 5M Acetic acid into Nessler’s cylinder. Makeup to 50 ml with purified water. Stir and allow to stand for 5 minutes.

Standard: Transfer 1ml of 25% W/V of Barium chloride solution in Nessler’s cylinder. Take 1.5ml of Ethanoic standard solution into it, stir well and allow it to rest for 1minute. Add 15ml of standard sulphate solution and add 0.15 ml of 5M Acetic acid into Nessler’s cylinder. Makeup to 50ml with purified water. Stir and allow to stand for 5 minutes.^4,5

Sodium benzoate Benzoic acid

Inference:

The turbidity / opalescence is viewed against the dark background. If the turbidity produced in the test is less than the standard, the sample passes the limit test.

The turbidity/opalescence is viewed against the dark background. If the turbidity produced in the standard is less than the test, the sample fails the limit test.

REFERENCES:

1. J.S. Qadry, S.Z. Qadry. Textbook of Inorganic Pharmaceutical and Medicinal Chemistry. 3-10

2. P. Gundu Rao. Pharmaceutical and Medicinal Inorganic Chemistry. 417- 423

3. L.M. Atherden. Bentley and Driver’s Textbook of Pharmaceutical Chemistry. 8th ed.;110-118

4. bspublications.net. [cited 2022 Nov 16]; Available from: https://www.bspublications.net/downloads/059cc8f84560f2_Ch-1_Subba%20Rao_Practical%20Pharmaceutical%20In-organic%20Chemistry.pdf

5. Ankur Choudhary. Limit tests in Pharmaceuticals.

6. Prof. V.N. Rajasekaran. Text Book of Pharmaceutical Inorganic Chemistry. 101-114, 416-418

7. Anees Ahmad Siddiqui and Seemi Siddiqui. Experimental Pharmaceutical Chemistry. Pg. No; 82-93

8. Dr. Anees Ahmed Siddiqui, Dr. Mohammed Ali. Practical Pharmaceutical Chemistry. Pg. No; 64-71

9. Indian Pharmacopoeia. 2018;1. Pg. No; 138-140

10. Habeeb Ahmed, Mohammed Ehtesham NR and ASM. Pharmaceutical Importance and Significance of Limit Test

11. Dr.A.V. Kasture, S.B. Gokhale. Practical pharmaceutical chemistry. 12. G.R. Chatwal. Pharmaceutical Chemistry Inorganic. 2018th ed. Vol. 1.Pg. No; 31-41, 47-61

Received on 27.05.2023 Modified on 13.10.2023

Asian J. Pharm. Tech. 2024; 14(1):23-30.

DOI: 10.52711/2231-5713.2024.00006