Concentrations of volumetric solutions

All the volumetric methods described in later chapters include an instruction such as titrate with 0.1 M hydrochloric acid . The subsequent calculation then includes a term C, the exact concentration of titrant in mol/1. Why not use the known value, 0.1 Because a reagent which is nominally 0.1 M or 1.0 M may actually be slightly weaker or stronger than the nominal concentration. In some laboratories it is the practice to prepare solutions that are exactly 0.1 M, 1.0M, etc., but this seems unnecessarily laborious. In order to prepare such a solution it is necessary first to prepare a solution of approximately the desired concentration, standardise it very accurately, then dilute or strengthen it, and finally restandardise it to make sure the adjusted solution is correct. The adjustment is unnecessary when insertion into the equation of the concentration found by the first standardisation is all that is needed. 

Solutions made by the careful use of the vials of concentrated reagents offered by most laboratory suppliers may generally be assumed to be of the nominal concentration, but for very accurate work it is advisable to standardise them against a primary standard. 

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