Limit test

Limit test

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  LIMIT TEST By, Revathi Gnanavelou,M.Pharm., Assistant Professor Department of Pharmaceutical Chemistry SVCP
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  LIMIT TEST  Limittests are quantitative tests or semi-quantitative tests which are designed to detect and limit / control small quantities of impurities present in the substance.  All the limit tests that are prescribed in the pharmacopoeias are based on the comparison of standard turbidity or colour with that of the sample under test.  Usually the limits are prescribed in parts per million (PPM).  For the preparation of standard turbidity or colour the pharmacopoeias prescribe the limit of particular impurities for particular substances and it varies for different compounds.  The amount of test samples to be taken is mentioned in the individual monograph of the pharmacopoeias.
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  3 TYPES • Testingas prescribed, there is no color, opalescence or precipitate Negative test indicate the absences of Large impurity Test In Which There Is No Visible Reaction • Compare the amount of impurity in the substance with a standard of known concentration and determine whether impurity is within or the excess of the limit prescribed` Comparison Methods • Amount of impurity present in actually determined and compared with the numerical limit given in Pharmacopoeia Quantitative Determination
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   Quantitative Determination Limitsof soluble matter Limits of Insoluble matter Limits of moisture and volatile matter Limits of residue on ignition Limits of non- volatile matter Loss on ignition & Ash values
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  1. Limit Testfor Chlorides • A solution of the substance is acidified with nitric acid, diluted to definite volume and treated with silver nitrate and the opalescence so produced is compared with that of standard opalescence containing known amount of sodium chloride solution. Cl- + AgNO3 → AgCl + NO3- • Presence of nitric acid prevents the precipitation caused by silver carbonate or silver hydroxide which may result due to alkaline impurities in the solution.
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   PRINCIPLE • Limittest for chloride is based on the well-known reaction between silver nitrate and soluble chlorides  forming precipitate of silver chloride which is insoluble in nitric acid. • The test solution becomes turbid, the extent of turbidity depending upon the amount of silver chloride produced which in turn depends upon the amount of chloride present in the test sample.
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  • The opalescenceproduced depends upon the amount of chloride present in the given sample. • It is compared with the opalescence produced in a standard solution containing the known quantity of chloride similarly treated if the opalescence in the sample is less that in standard. Its passes the test. • If it is more, it fails the limit test which is done in Nessler’s cylinder and viewed transversely against a black background.
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   EXPERIMENT
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  PROCEDURE Standard Solution TestSolution Dissolve chloride/unknown sample 5ml Distilled water 10 ml Dil.Nitric acid Make up-to 50ml distilled water 1 ml of Silver nitrate solution COMPARISON
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  2. Limit Testfor Sulphate PRINCIPLE • This is based on the reaction between barium chloride and soluble sulphates in presence of dilute hydrochloric acid. • An opalescence is produced by the precipitation of barium sulphate and compared with the opalescence produced in a standard containing a known quantity of sulphate and similarly treated. SO4 − + BaCl2 → BaSO4 + 2Cl− • Test substance passes the limit test, if the opalescence in it is less intense than that in the standard. If opalescence is found to be more, then it fails the test.
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  • The bariumsulphate reagent used contains barium chloride, sulphate free- alcohol and traces of potassium sulphate • The potassium sulphate increases the sensitivity of the test by giving ionic concentrations in the reagent which just exceed the solubility product of barium sulphate. • 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 substance under test. • Presence of alcohol helps to prevent super saturation and a more uniform turbidity is formed.
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  COMPARISON
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  EXPERIMENT
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  3) Limit Testfor Iron PRINCIPLE • This depends upon the reaction of iron with thioglycolic acid in the presence of citric acid and ammonia when a pale pink to deep reddish purple colour is produced. • Citric acid forms a complex with iron and prevents its precipitation by ammonia. • The colour produced is due to the formation of a ferrous compound with thioglycolic acid.
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  • This isstable in excess of air and fades when exposed to air due to oxidation. • The original state of oxidation is immaterial as thioglycolic acid is a reducing agent and reduces ferric ion to ferrous. • Only because of this advantage this test has been selected as the limit test, since other tests such as the ammonium thiocyanate test give reaction with only one type of iron like the ferric . • The thioglycolic acid test is also considered to be more sensitive. • Ferrous thioglycollate is colourless in acid or neutral solutions. • The colour develops only in the presence of alkali.
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  The reactions are;
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  PROCEDURE
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  EXPERIMENT
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  4. Limit Testfor Heavy Metals PRINCIPLE • All Metals like Copper, Bismuth, Lead, Mercury, Arsenic, Antimony, Silver etc (except alkali metals and alkaline earth metals) are coloured by sulphide ions (H2S or Na2S) under specified conditions. • The limit test for detecting and limiting the impurity of Heavy metals. • The HM are precipitated as Metal sulphides by the addition of hydrogen sulphide or sodium sulphide solution. • Depends upon the quantity of the metal the colour varies from brown to black.
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  • There arethree methods are prescribed in I.P to determine the presence of heavy metals. • The sample is dissolved in acid or alkali for making a solution. • Method A and B are carried out in acid conditions with hydrogen sulphide reagent and method C involves alkaline medium with the use of sodium sulphide reagent. • Method A  A solution of substance is adjusted to a pH 3 to 4 (by adding ammonia (or) acetic acid) and hydrogen sulphide reagent is mixed with this  And comparison of black colour produced with a standard colour containing a known amount of lead.
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  • Method B(For Organic Compounds)  The substance is ignited well in presence of conc. sulphuric acid and treated with mixture of nitric and sulphuric acids. The resulting solution is digested with dilute hydrochloric acid.  Then extracted with hot water and proceeded as in method A. • Method C  The solution of the substance is treated with sodium hydroxide solution and sodium sulphide reagent.  Then it is compared with that a standard colour. Pb++ + Na2S → PbS + 2Na+ Pb++ + H2S → PbS + 2H+
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  • Method D Is based upon the precipitation of relatively insoluble and characterisitically coloured sulphides of heavy metals when aqueous solutions are treated with alkali metal sulphides (NaSH).  NaSH is generated immediately before use by heating thioacetamide with sodium hydroxide solution.  In this test, there is a formation of brown colour because of the precipitation of metal sulphides at about pH 3.5 in colloidal form which is stabilised by glycerine.
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   The testis done in two 50 ml Nessler’s cylinder, test is prepared in one and standard solution is prepared in other cylinder.  Taking the prescribed volume of std lead solution and reagent solution is added in both , they are diluted to the Mark and mixed after standing for 5 minutes and compared with colour intensity.
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  5. Limit Testfor Arsenic PRINCIPLE • The limit test for arsenic is based on the reduction of the arsenic in the arsenious state to the arsine gas (AsH3) with zinc and hydrochloric acid. • In this test arsenic impurities if at all present is converted in to arsine gas (ASH3) which when contact with a mercuric chloride paper produces yellow stain. Zn + 2HCl → ZnCl2 + 2 [H]+ As 3+  H3AsO4 (Impurity) (Arsenic acid) • The intensity of the stain is proportional to the amount of arsenic present. • Apparatus used for arsenic limit test is called Gutzeit apparatus
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  • A drugsolution is prepared and placed in wide mouthed bottle, potassium iodide, zinc dust, hydrochloric acid, stannous chloride are added into it. • When the sample is dissolved in hydrochloric acid whereby arsenic present as impurity in the sample gets converted to either arsenic or arsenious acid depending upon on its valency state. H3ASO4 + 2 [H] → H3ASO3 + H2O • Then it is further treated with reducing agents like stannous chloride, potassium iodide etc., arsenic acid is reduced to form arsenious acid (pentavalent arsenic is reduced to the trivalent state) • Nascent hydrogen gas is generated by the presence of stannous chloride, hydrochloric acid (stannated hydrochloride) and potassium iodide on arsenic free granulated zinc.
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  • The arsinegas produced in the bottle escapes through the tube and the lead acetate impregnated cotton wool kept in the centre of the tube entraps the hydrogen sulphide if any from the arsine gas. H3ASO3 +6 [H] → ASH3 + 3H2O • When arsine gas escapes through glass tube and reacts with mercuric chloride paper kept in the clips and produces yellow stain. The reaction is allowed to proceed for forty minutes maintained at 400C. 2ASH3 + HgCl2 → Hg (ASH3)2 + 2HCl
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  • The presenceof stannous chloride and hydrochloric acid ensures rapid reaction between acid and potassium iodide and produces nascent hydrogen gas. • Depth of the yellow stain depends upon the amount of arsenic present in the sample, which is less intense in colour when compared with that of standard stain produced. • It passes the limit test • Permitted limit for arsenic is 1 PPM
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