Partial sodium metaborate convertion buffer

[elerion]

Hi everyone.
I hope this hasn't been treated before in other threads. I searched, but didn't find any.
This is not a question, rather than sharing some thoughts and maybe asking for "am I wrong?" and "what do you think?".

Here, there's information about making a borax+sodium hydroxide buffer:
http://delloyd.50megs.com/moreinfo/buffers2.html (scroll mid page)
For instance, to get a pH 10 buffer, we need 2.5 mmol of borax + 3.66 mmol of NaOH (36.6 ml, 0.1 M).

We know that sodium metaborate can be substituted by borax+sodium hydroxide in the correct proportions. One mole of borax and two moles of NaOH. So, ratio is 1:2 molar.

In that situation, we are outside the table on the website, whose last row is for pH of 10.80 (for a mix of 48.5 ml of 0.1 M NaOH, or 4.85 mmol, and 2.5 mmol of borax).

So, if we try to get sodium metaborate from borax and NaOH, and we use less NaOH than required, we are getting some buffering action. Borax by itself is a buffer too, but sodium metaborate is not (pH varies with concentration in a similar manner as sodium carbonate).

So, while we are adding NaOH, buffer capacity declines, while pH raises. A nice tradeoff to have in mind, I think.

 

[Alan Johnson]

https://www.borax.com/BoraxCorp/med...letin/pH-values-borate-solutions.pdf?ext=.pdf
Borax or a stronger solution of the metaborate show less variation of pH and are better for calibrating pH meters.

 

[elerion]

...a stronger solution of the metaborate show less variation of pH...

Do you mean, less than sodium carbonate?
At any rate, metaborate is (at least theoretically) not a buffer.
AFAIK it is not just a matter of pH variation with weight, but the ability to keep pH stable in presence of other products, and also when diluting the solution.
The graph does show clearly how borax is a buffer on its own, which is nice.

 

[Alan Johnson]

There is published data on all of them, your link table G,and metaborate here:
https://www.borax.com/BoraxCorp/med...a-Sheets/sodium-metaborate-8-mol.pdf?ext=.pdf
Sodium Carbonate is another where there is data for calibrating a pH meter sufficiently accurately for photographic developer measurements.