[buffer maker]

Hi Everyone,

I have been using powdered sodium nitrite (>99.0% pure) for over a year now. I use between 0.2-1 M in 2 M guanidine thiocyanate. Until recently the pH has been between pH 7.3-8.5 (I buffer the guanidine with phosphate). Now the pH is 9.0 and higher, depending on the amount I add. The sodium nitrite appears to have absorbed some water (it is developing clumps) but has not changed color. My question is: why would this effect the pH?

I appreciate any help you guys can give me.
Thanks!

[Arkcon]

Sodium nitrate should be stable, even when wet.  That is, I don't expect it's decomposed into something that is now alkaline.  More likely there is another source for your pH difference.

[agrobert]

I think the explanation is simple.  You are weighing wet sodium nitrite and assuming it is dry.  Therefore your sodium nitrite mass is really less than what you assume you are putting in.  If you find a way to quantify the percentage of sodium nitrite per gram of your wet sodium nitrite then it should still be fine to use.  Basically your buffer is off because you are putting in an unknown lesser amount of sodium nitrite.

[Borek]

Basically your buffer is off because you are putting in an unknown lesser amount of sodium nitrite.

Nitrite is a base, so lesser aomunt should lower pH, not push it up. Or am I missing something?

[agrobert]

Right, but by adding water to the buffer the concentration of [H+] should decrease resulting in higher pH.  I really don't know, just trying to make sense of it all.  It would be nice if Buffer Maker posted a solution prep.

[buffer maker]

Formula I use:

4 M guanidine thiocyanate
0.4 M sodium nitrite
20 mM sodium phosphate
DI water

Thanks again for your *delete me*

[Arkcon]

Do you adjust the pH with conc. acid or base?  Or has the formula you provided always produced the correct pH? 

None of your components is the conjugate acid or base of the other, so it isn't really a buffer. 

Furthermore, any one of your components -- including and especially your DI water, may be contaminated, and be the source of your pH problem.