Understanding Primary and Secondary Standards in Chemical Analysis: Properties and Uses, Exams of Molecular Structure

Download Understanding Primary and Secondary Standards in Chemical Analysis: Properties and Uses and more Exams Molecular Structure in PDF only on Docsity! Standards can be divided into two types: ➢ Primary standard ➢ Secondary standard Primary standard A primary standard is a chemical or reagent which has certain properties such as- (a) It is extremely pure – A primary standard material should be extremely pure which means that it should be a chemical of high grade of purity, preferably 99.98%. In a pahramceutical analysis laboratory we come across chemicals of different grade of purity. If we check the label we will notice a number with percentage termed as purity. So when a chemical has purity of 99.98% or more it is a suitable material to be used as primary standard. (b) It is highly stable – It should be highly stable which means it usually does not react easily when kept in its pure form or it should have very low reactivity. This is important because if a reagent reacts easily with atmospheric oxygen or water or changes its property over time then it is unreliable and such a unstable and unreliable chemicals can never be used as standard. (c) It is anhydrous- It should be anhydrous which means that it does not contain any water molecule in its molecular structure. For example, in a pharmaceutical analysis laboratory we come across same chemical with different number of water molecules attached with it e.g. magnesium sulphate (MgSO4), which is also called Epsom salt. The Epsom salt which is found in drug store is a chemical with formula MgSO4.7H2O. Therefore if we want to prepare a primary standard of magnesium sulphate we should purchase an anhydrous MgSO4 preferably an analytical reagent grade chemical and with purity greater than 99.98%. (d) It is less hygroscopic in nature- The chemical preferably should be less hygroscopic i.e. on opening the container it should not absorb water molecules from atmosphere. (e) It has very high molecular weight- It has very high molecular weight compared to its other similar forms. For example Epsom salt. Take 1 gram of MgSO4 for making a primary standard and name it as salt A. Now take 1 gram of MgSO4.7H2O for common uses and name it as salt B. Now if we compare the actual weight (Molecular weight) of magnesium sulphate to make a standard solution for both chemicals then it is found that- CHEMICALS MOLECULAR WEIGHT MgSO4 108 MgSO4.7H2O 234 In first case, molecule of salt A the weight of actual MgSO4 will be 108 atomic mass unit. But in second case, molecule of salt B the weight of actual MgSO4 will be 108 out of its total weight of 234 atomic mass unit. 108 gram salt A (MgSO4) will give 108 gram of MgSO4 So, 1 gram MgSO4 salt will give = 108/108 = 1 gram of MgSO4 But 234 gram salt B (MgSO4.7H2O) will give 108 gram of MgSO4 So, 1 gram MgSO4.7H2O salt will give = 108/234 = 0.461 gram of MgSO4 Therefore if by mistake we make a standard out of salt B, actually we are taking 0.461 gram of MgSO4 and calculating it as 1 gram. So with this faulty standard estimation of MgSO4 in other unknown solution will give less result than the actual concentration. Hence it is important that primary standards must be anhydrous and of high molecular weight. (f) It can be weighed easily – It can be weighed easily because it is so pure that its weight is in fact a true representative of number of moles present in its actual weight. (g) It should be ready to use and available (h) It should be preferably non toxic (i) It should not be expensive