Why is sodium thiosulphate not suitable for use as a primary standard?

Answer 1

Sodium hydroxide cannot be used as a primary standard because of its hygroscopic properties as a solid. Because it is so prone to absorbing water, it is impossible to accurately measure the mass of a solid sample, so instead it must be put into solution and titrated with a known acidic solution. This makes it a secondary standard as opposed to a primary standard. The same logic holds true for hydrated sodium carbonate. Any solution that MUST be titrated in order to know it true molarity is considered a secondary standard.

It also reacts with atmospheric carbon dioxide to form sodium carbonate, so it is unstable.

Answer 2

Industrially, NaOH is commonly used to remove HCl and Phosphoric acid from streams. However, most industrial practices avoid processes where HCl and phosphoric acid are formed or used. Also, in high enough concentrations a slurry is created which coats equipment leading to undesired results.

Answer 3

A primary standard is a substance which can be weighed out and made into a solution of exact, known concentration. The solution can then be used in a titration to find, accurately, the concentration of another solution. So it must be available in a reliably pure state. It will not, ideally, contain water of crystallisation because this invariably throws doubt on its absolute composition; it might absorb or release unknown amounts of this water. Sodium thiosulfate contains water of crystallisation.

-Rahul Jain

IIT Kanpur

Answer 4

Sodium hydroxide is so hygroscopic that it is almost impossible to use it as a primary standard, since even a perfectly pure sample is likely to absorb water by even a very short period of contact with air. If this happens the expected reactivity of the sample in some reaction that tries to use sodium hydroxide will be reduced.

Answer 5

Because NaOH is not stable when contact with CO2

Answer 6

Because it will pick up CO2 from the air forming OH and therefore will turn into a base.

Answer 7

A primary standard needs to have low hygroscopity--iow, it needs to not absorb moisture from the air. Both of the chemicals you named are very hygroscopic.

Answer 8

It has lower molecular wt, it is hygroscopic and has lower purity so it is not used as the primary standard

Answer 9

It is used as a standard but not as a primary standard. It tends over time to precipitate sulfur.

Na2S2O3 = Na2SO3 + S

Answer 10

NaOH reacts with CO2 from the atmosphere and thus minute but measurable variation is introduced.