Neat little lactic buffer...
Having used formic acid based descalers (kettles, coffee machines etc) before I came across a grade 'Oust All Purpose Descaler':
http://www.oustdescalers.co.uk/purposedescaler.html
These (3) sachets seemed to contain a rather thickish liquid. Intrigued, I Googled an MSDS that claimed '30 - 50 % Lactic Acid' (LA) for that product.
LA is of course 2-hydroxy-propanoic acid, MW = 90.08 g/mol and pKa = 3.86.
I bought a box (about £2) and quantitatively transferred the contents of the three sachets into a clean glass receptacle and titrated several
aliquots of accurately weighed 0.4 g of it against standardised 0.1 M NaOH, which revealed the syrupy liquid to be about 71.5 w% (!) LA.
I diluted it to about 250 ml and titrated again to find 21.8 w% LA, weighed up the solution and calculated the amount of NaOH to comply with:
1 LA + 1/2 NaOH ---> 1/2 NaLA + 1/2 LA + 1/2 H2O
That was 11.5 g of NaOH, which I dissolved in about 250 ml of water and combined it with the LA solution. Allowed to cool, the solution gave a pH of
3.9, as predicted by pH buffer = pKa, weak acid, for [LA]/[NaLA] = 1.
Considering the quantities, this particular pack of Oust seemed almost ready made to make up 500 ml of a 1.2 M (total molarity) buffer solution of pH
= 3.9...  And it does have the advantage over acetic acid/acetate of being
totally non-volatile...
[Edited on 6-9-2010 by blogfast25]
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