[ugotmyc]

to do my experiment, i need to make some sodium phosphate buffer at two different pH's. according to the textbook, i first made the 1M sodium phosphate dibasic and 1M sodium phosphate monobasic. this is NOT the first time i made those solutions, but before i used to make a smaller volume (50ml) in a polypropylene conical tube (and encountered NO problems). this time i tried
to make a larger volume (400 ml) but kept the solution in a glass bottle (Pyrex). Surprisingly, while 1M sodium phosphate monobasic was made with no trouble, 1M sodium phosphate dibasic solution kept forming crystals even after an initial dissolution (crystal forms after a couple of hours of standing alone at room temp) i ended up making 0.5 M sodium phosphate dibasic and the problem of crystal formation was then solved. i have no problem using 0.5 M sodium phosphate dibasic and mix it with 1 M sodium phosphate monobasic to get the right sodium phosphate buffer i need. but i am curious why i can not make 1 M sodium phosphate dibasic . any clue ? thanks for reading this lengthy post

[Borek]

Have you checked in CRC handbook what is a solubility of the salt? I don't have any handbook here right now, so I can use only CASC density tables - for Na₂HPO₄ they end at 6% (which is 0.451M) - perhaps 1M is above the salt solubility? Hmmm.... but it will not explain why it precipitated after dissolution.

[ugotmyc]

i'll check w/ the CRC, but according to my protocol book (molecular cloning,
by Tom Maniatis), to make phosphate buffers of different pH, we have to make
1M Na2HPO4 and 1M NaH2PO4 stock and mix them with different amounts to get the desired final pH - for example,
with 25.5 ml 1M Na2HPO4 + 74.5 ml 1M NaH2PO4 into a final 1 L, u get a
0.1 M sodium phosphate buffer of pH 6.4
whereas if u use 93.2 ml 1M Na2HPO4 + 6.8 ml 1M NaH2PO4 into a final 1 L,
u get a 0.1 M sodium phosphate buffer of pH 8.0
so from this, i assume we CAN make a 1M Na2HPO4 stock solution.

[AWK]

Na2HPO4 dramatically increases its solubility  between 20 and 30 C. About 30 C you can prepare 1M solution without any problem.

[salman94]

Hi to all,

I am facing the same problem. I have prepared IM Na2HPO4.7H2O by dissolving 268g in 1liter. At first it was not dissolving and remained in powder form. But after stirring 24 hrs continuously, it got dissolved. But after few hours crystals were formed and settled on the bottom of the bottle. I have tried to dissolve it again by stirring and heating but it is not dissolving. So, could you plz recommend any solution.