Limit Test for Iron in Pharmaceuticals
A limit test is described as a semi-quantitative or quantitative test for identifying and controlling small amounts of impurity that is likely to be present in a given substance. The reasons why limit tests are conducted include:1. To detect the harmful quantities of impurities2. To detect both unavoidable and avoidable quantities of impuritiesThe limit test for iron is usually performed to show that iron content doesn't exceed the specified limit in the individual monograph - with respect to micrograms of iron per every gramme of the test substance. The iron limit test is typically designed to help in the determination of the acceptable limit of iron compounds contained in a sample as impurities. A standard iron solution comprising 40 μg of iron is the one used as far as the comparison of color is concerned.The test is A572GR50 STEEL normally built upon iron reaction in an ammoniacal solution containing thioglycolic acid. The latter forms a pink or deep reddish purple coloured complex of thioglycollate. Comparison of the colour generated from a predetermined amount of substance from the iron limit test is normally viewed vertically using a standard - ferritic ammonium sulphate. A sample is said to pass the test if the colour obtained from the test solution is lesser dark compared to the standard.
The reddish purple colour produced in the sample solution shouldn't be more than the standard solution. Should the colour generated in a sample solution be less than the standard solution, the sample passes the limit iron test and vice versa. The reasons are pretty simple. Citric acid assists iron precipitation by ammonia through the formation of a complex with it. Ammonia makes solution alkaline.In the absence of air, the Fe (SCH2COOH)2 generated from the ferrous form of iron is relatively stable. However, the colour is normally destroyed by strong alkalies and oxidising agents. The initial state of iron doesn't really matter since thioglycolic acid reduces or rather converts Fe3+ to Fe2+. The limit test for iron should be considered as one of the most sensitive tests. To prevent interference from other metal cations, use 20 percent of citric acid - a complex is formed (when other metal cations are involved).ProcedureTo prepare a solution, dissolve a pre-determined amount of substance in 40 ml of water. Alternatively, you may take 10ml of the solution as instructed in Nessler's cylinder monograph. To this, add 2 ml of 20 percent w/v solution of citric acid (iron free) as well as 0.1 ml thioglycolic acid. Using iron free ammonia solution, mix and make alkaline and then dilute it to 50 ml using water.
The resulting solution should be left to stand for approximately five minutes. For the purpose of the standard, dilute 2 ml of standard iron solution simultaneously, with 40 ml of water and then add a similar amount of reagent like the one in the sample. As already mentioned, a sample has passed the test if the colour produced isn't considered more intense than the standard. Colour comparisons of the two solutions should be done against a white background- for the purpose of generating more accurate results. Thioglycollic acid has proven to be a more sensitive reagent when it comes to iron, relative to ammonium thiocyanate.
