Primary standards // Standard solution (1 Viewer)

[homersimpson]

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?

Thanks

 

[FFC]

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?

Thanks

Sodium hydroxide is not suitable for use as a primary standard - it is hygroscopic (absorbs water molecules from the air) and as such is difficult to obtain and maintain in a very pure form. Also, it has a relatively low molar mass, and this is more likely to lead to errors in calculation down the line.

To titrate ethanoic acid (a weak acid), we need a strong base which has been standardised.

What we did was produce a standard sodium carbonate solution (which is suitable for use as a primary standard), then titrate to standardise an HCl solution, then titrate to standardise a sodium hydroxide solution, which can then be used against ethanoic acid. A lot more work, but far more accurate than using NaOH. Also, note it is unadvised to use the Na2CO3 against the ethanoic as it would be weak base-weak acid, which makes it difficult to determine and end point to the titration. As such, the above method is needed.

Hope that helps

 

[Pwnage101]

^^^ true. Excellent answer.

NaOH can, however, be used as a Secondary standard (i.e. dissolve an unkno0wn amount, perform a titration with ana cid of known concentration to determine the concentation of NaOH. Then this standardised NaOH can act as a secondary standard when titrating with ethanoic acid).

Similar question was asked in 2008 CSSA trial.

 

[mR sinister]

Just too add,
You wouldnt use Na2CO3 against ethanoic acid,
as Na2CO3 with any carboxylic acid would produce CO2 gas.

 

[brenton1987]

Just too add,
You wouldnt use Na2CO3 against ethanoic acid,
as Na2CO3 with any carboxylic acid would produce CO2 gas.

A carbonate and ANY acid produces CO₂.

Note that just before the end point you must boil the solution to expel the CO₂. Otherwise there will be carbonic acid present which will decrease the pH.

 

[sam5]

A carbonate and ANY acid produces CO₂.

Note that just before the end point you must boil the solution to expel the CO₂. Otherwise there will be carbonic acid present which will decrease the pH.

christ...you know EVERYTHING

jeez and i thought i was good at chem...

 

[sam5]

christ...you know EVERYTHING

jeez and i thought i was good at chem...

ohh oops...never noticed that ur past the hsc.

what did u do at uni?

 

[brenton1987]

ohh oops...never noticed that ur past the hsc.

what did u do at uni?

A double major in chemistry.

 

[sam5]

A double major in chemistry.

makes sense...

at newcastle uni?

 

[kaz1]

Someone correct me on this but I don't think you can use a Sodium Carbonate standard solution to titrate ethanoic acid as they are both weak and do not produce a distinct end point (point of inflexion) on the titration curve.

 

[brenton1987]

Someone correct me on this but I don't think you can use a Sodium Carbonate standard solution to titrate ethanoic acid as they are both weak and do not produce a distinct end point (point of inflexion) on the titration curve.

Noone was going to do a Na₂CO₃ --> CH₃COOH titration.

They were going to use Na₂CO₃ as a primary standard to produce a secondary standard of HCl. The HCl is then used to standardise a tertiary standard of NaOH. The NaOH is finally used to determine the concentration of CH₃COOH.
Na₂CO₃ --> HCl --> NaOH --> CH₃COOH

 

[MrMMMan]

You can also use hydrated oxalic acid as a suitable primary standard at school

 

[shinigami66]

Noone was going to do a Na₂CO₃ --> CH₃COOH titration.

They were going to use Na₂CO₃ as a primary standard to produce a secondary standard of HCl. The HCl is then used to standardise a tertiary standard of NaOH. The NaOH is finally used to determine the concentration of CH₃COOH.
Na₂CO₃ --> HCl --> NaOH --> CH₃COOH

Sorry, but I don't quite understand... I know NaOH is not suitable as a primary standard due to the fact that it's relatively unstable and stuff, but when you standardise it, wouldn't the mass of NaOH still be constantly changing and you can't really use it for titration??

Hah HSC on monday and I'm still so unsure about why you can do so =[

 

[brenton1987]

Sorry, but I don't quite understand... I know NaOH is not suitable as a primary standard due to the fact that it's relatively unstable and stuff, but when you standardise it, wouldn't the mass of NaOH still be constantly changing and you can't really use it for titration??

The mass of NaOH never changes. The percent composition of NaOH in a weighed sample will change due to absorbed water vapour. But since it is standardised as a weak solution it is surrounded by a whole heap of water so it loses its water absorbing properties.

The rough NaOH solution is made by weighing the required amount a powder assuming that is it 100% pure NaOH. Of course in reality it isnt because it absorbs water. So the solution made up has a concentration that is roughly known but not sufficiently accurate. So it needs to be standardised using a solution of known concentration which can only be made using a compound that does not absorb gases (Na₂CO₃). That way the weighed amount is 100% the required chemical. Of course you cant standardise a base with a base so that is where the secondary standard of HCl comes in.

So long as the volumetric flask stays sealed the concentration of NaOH will not change.