Suppose you have a solution of known substance X but unknown concentration. You want to learn the concentration by fully reacting it with substance Y. Knowing how much of Y was used up tells you the amount in the sample of X, and therefore solution X's concentration. Such chemical analysis is called "titration."
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The concentration of solution Y may not, however, be very constant, due to one sort of deterioration or another. Therefore, to determine its concentration just before the titration against X, it may be titrated against a more stable solution--say, solution Z. Y is therefore called the "standard solution," while Z is the "primary standard."
How Concentration Changes
To understand why there is a need for a separate standard (and therefore what characteristics are desirable in it), it helps to look at what can go wrong with the standard solution. It can absorb moisture from the air (hygroscopic behaviour), which would reduce its concentration. It could react with gases in the air or contaminants in the equipment used for titration. It could evaporate, even while in a container, again reducing its concentration. And the solvent could evaporate, leaving crystals of the standard on the container wall (efflorescence). A good primary standard, therefore, must not be prone to these pitfalls.
Aside from not being hygroscopic, reactive, or efflorescent, a primary standard should be 100 per cent pure. But if it's not pure, then its impurities at least should be inert. It should have a high molecular weight, to minimise the effect of the inevitable weighing error. It should have no waters of hydration, which are water molecules that are part of certain crystals but that aren't covalently (strongly) chemically bonded.
The primary standard should be in the solid state, for accuracy of weighing. When it is oven-baked to remove any liquid retained, it should remain stable (non-reactive) in spite of the heat. It should dissolve easily and react instantaneously with the standard solution when titrated against it. (Slow reactions can lead a lab technician to overshoot the point when the standard solution has been fully reacted.) This reaction should be stoichiometric; that is, the two compounds' molecules should react only in a certain ratio---a definite number of molecules of one reacting with a definite number of molecules of the other.
High Molecular Weight
Primary standards are chosen to be of high molecular weight. The point of a high molecular weight is to have as few molecules per unit weight in the primary standard as possible. For example, your scale may be able to weigh with an accuracy of plus or minus 1/10 grams. Suppose you expect to use about 1 mole (6.022 x 10^23 molecules) per titration. A primary standard of molecular weight 5g/mole may therefore be off in its measurement of 1 mole by as much as 2 per cent. A primary standard of molecular weight 200g/mole would be off by no more than 0.05 per cent in its mole measurement.
Common Primary Standards
Compounds commonly used for primary standard solutions are sodium chloride, potassium dichromate, sodium oxalate, calcium carbonate, oxalic acid, sodium borate and sodium carbonate.
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