Describe the use of \"primary standards\" in chemical analyses: a) first briefly

Question

Describe the use of "primary standards" in chemical analyses: a) first briefly define the role of the standard in making the analysis quantitative; and b) List any special Describe the use characteristics of primary standards or required steps in preparing it for analysis. In your response, it may be helpful to consider one or more example from lab: for instance, the EDTA titration of Ca^2+ and Mg^2, the NaOH titration of KHP or the s_2O_3^2- titration of lO_3^-. I am NOT asking you to list procedures specific to any one of these labs, but instead general principles common to them.

Explanation / Answer

Primary standard is a compound of high purity from which standard solutions of known normality’s can be prepared by direct weighing of it and diluting to a defined volume of solution.

A primary standard is a chemical or reagent which has certain properties such as

(a) It is extremely pure,

(b) Highly stable

(c) It is anhydrous

(d) It is less hygroscopic

(e) Has very high molecular weight

(f) Can be weighed easily

(g) Should be ready to use and available

(h) Should be preferably non toxic

(i) Should not be expensive

Some examples of primary standards are

EDTA titration for the determination of Calcium and magnesium ions

This method, called a complexometric titration, is used to find the total calcium and magnesium content of milk, sea water and various solid materials. It can also be used to determine the total hardness of fresh water provided the solutions used are diluted. The combined concentration of calcium and magnesium ions is considered to be the measure of water hardness.The method uses a very large molecule called EDTA which forms a complex with calcium and magnesium ions. EDTA is short for ethylenediaminetetraacetic acid. A blue dye called Eriochrome Black (ErioT) is used as the indicator. This blue dye also forms a complex with the calcium and magnesium ions, changing colour from blue to pink in the process. The dye–metal ion complex is less stable than the EDTA–metal ion complex. For the titration, the sample solution containing the calcium and magnesium ions is reacted with an excess of EDTA. The indicator is added and remains blue as all the Ca2+and Mg2+ions present are complexed with the EDTA. A back titration is carried out using a solution of magnesium chloride. This forms a complex with the excess EDTA molecules until the end-point, when all the excess EDTA has been complexed. The remaining magnesium ions of the magnesium chloride solution then start to complex with ErioT indicator, immediately changing its colour from blue to pink

The main reaction is     Ca2+ + EDTA4 [Ca-EDTA]2

Back titration               EDTA4+ Mg2+ [Mg-EDTA]2

Indicator reaction: note, ErioT is blue and ErioT-Mg is pink           ErioT + Mg2+ ErioT-Mg

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