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Khan Joy

Why is Na2CO3 used as a primary standard?

Andrew Ng  Follow

Although others have answered that it is a primary standard. (it is !)

Most people would not use it.

For acid titration (its main use) e.g. TRIS is better.

  • sodium carbonate: Na2CO3, MM. = 105.988 g/mol
  • tris-(hydroxymethyl)aminomethane (short TRIS): (CH2OH)3CNH2, MM. = 121.136 g/mol

TRIS has a greater mass per mole of protons consumed (121 in comparison with 53)

The weighing error is smaller using tris.

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Dan Zauner  Follow

Only those acids or bases are considered primary standard which are stable and hence their strength do not change with time.Strength of Na2CO3 also do not change hence it is considered as a primary standard.

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Angel Vasquez  Follow

All of the answers have partially addressed the question. To completely answer the question one have to consider what are the desired properties for a primary standard. There six:

1. Obtain in high purity - and without too much effort.
2. High stability (low reactivity).
3. Low hygroscopicity (don’t want it soaking up water)
4. High equivalent weight (to minimize weighing errors)
5. Non-toxic
6. Cheap and readily available

1–4 are more important than 5–6. With respect to these criteria, sodium carbonate satisfies all but 4. It has a low equivalent weight. The molecular weight of sodium carbonate is 105.9885 g/mole, but the equivalent weight is only 52.9943 g/mole. As recommended by Kurt, Tris is a better choice because of its molecular weight (121.14 g/mole) and this is also its equivalent weight.

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Chandrakala Chaulagain  Follow

Is there any substance which is a better primary standard than sodium hydroxide and can be used to make a standard solution which can then be used to titrate ethanoic acid?

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Barbara Forbes-Lyons  Follow

Na2CO3 is used as primary standard because it's solution's molarity remains constant for a very long period.
Hope that helps!

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Arno Vigen  Follow

Only those acids or bases are considered primary standard which are stable and hence their strength do not change with time. Strength of Na2CO3 also do not change hence it is considered as a primary standard. ... Na2CO3 is used as primary standard because it's solution's molarity remains constant for a very long period.

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Carolina Kishwaukee  Follow

Normally used to standardise HCl solution.

Na2CO3 is a solid,can be dried easily & remains in a pure form, so can be used to prepare standard solution i.e can be accurately weighed & dissolved in definite volume.

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Debra Barnett  Follow

Na2CO3 is primary standard

because,

molarity remains same with time, and it is of high molecular weight.

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David Borgstrom  Follow

That depends on the concentration. The normality is defined as concentration divided by the equivalence factor. For sodium carbonate, the equivalence factor is 2, so the normality is half the concentration (or half the molarity).

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