NaOH or NA2CO3 as a primary standard?

[luken93]

So I have a question, which substance out of Sodium Hydroxide or Anhydrous Sodium Carbonate would be better to use as a primary standard for an Acid-Base Titration?

The only thing is,
 - Both are hygroscopic
 - Both are strong bases

The only thing that differs is their molar mass, therefore is Sodium Carbonate more suitable?
And secondly, what difference does the "anhydrous" make to my decision? Does it mean that it WILL NOT absorb moisture in the air?

[pi]

I'm not sure of the answer, but in a prac (last week), we used anhydrous sodium carbonate as our primary standard. Not sure why though...

[luken93]

^ Yeah same, although I've also used NaOH as well...

[pi]

Well, our teacher told us that a good primary standard should have a high molar mass, so that natural errors due to equipment will be negated (due to percentage of mass of the error compared to the total molar mass). So that would make anhydrous sodium carbonate better.

[m@tty]

Yeah that's true, a higher molar mass is better as it reduces errors because a given mass of the standard corresponds to a smaller differeence in the result.

[luken93]

Well, our teacher told us that a good primary standard should have a high molar mass, so that natural errors due to equipment will be negated (due to percentage of mass of the error compared to the total molar mass). So that would make anhydrous sodium carbonate better.

Yeah that's the only thing that basically differentiates the two, just wanted some clarification

Moderator action: removed real name, sorry for the inconvenience

[Water]

Anhydrous Sodium Carbonate Would be the correct answer, because NaOH, when exposed to the air, will react with moisture and carbon dioxide. Because of this, it is no longer pure and contaminated. Hence Sodium Carbonate is the preferred primary standard. High Molar Mass, would also be a legit answer as well

[luken93]

Anhydrous Sodium Carbonate Would be the correct answer, because NaOH, when exposed to the air, will react with moisture and carbon dioxide. Because of this, it is no longer pure and contaminated. Hence Sodium Carbonate is the preferred primary standard. High Molar Mass, would also be a legit answer as well

both are hygroscopic, that's why I just wanted to check that the higher molar mass was the only factor

Yeah that's true, a higher molar mass is better as it reduces errors because a given mass of the standard corresponds to a smaller differeence in the result.

I hope this hasn't further reduced your expectations of me :p

[Water]

both are hygroscopic, that's why I just wanted to check that the higher molar mass was the only factor

Damn, my chem teacher, said NaOH is the one that absorbs moisture readily, whilst the latter didn't =\



PS: But when you look at it in real life perspective, NaOH would attract moisture far faster than Anhydrous Sodium Carbonate? Having said that, you guys just proved my chem teacher wrong ):, actually, I'm quite unsure now. Unless Molar mass, is the only answer?


Just checked on internet. If this is any valid




PS No .2: Anhydrous sodium carbonate is not deliquescent, but it does absorb carbon dioxide and water from the air to give sodium hydrogencarbonate. When heated, this sodium hydrogencarbonate decomposes back to sodium carbonate again <<<<Reason for Primary Standard

contrary to

Sodium hydroxide suffers from two problems: it is deliquescent (absorbs water from the air); it also absorbs carbon dioxide from the air to give a surface layer of sodium carbonate and sodium hydrogencarbonate.

[m@tty]

"Anhydrous" is a very important word.. It means "without water".

[luken93]

Thanks water, hopefully it expanded your knowledge as well :p

Oh well, only needed to know which one would be better.
, I think your post satisfies my curiosity..

[Mao]

The solid pellets of NaOH decomposes if exposed to air. Thus you cannot trust the measured weight of sodium hydroxide. It cannot be a primary standard.

[luken93]

The solid pellets of NaOH decomposes if exposed to air. Thus you cannot trust the measured weight of sodium hydroxide. It cannot be a primary standard.

Thanks a lot

[schnappy]

An NaOH solution can also not be a primary standard because of evaporation.