You could dissolve each of the salts at 200 mM separately, then adjust the pH of one of them using the other, while stirring and measuring the pH with a pH meter. I would put the dibasic (Na2HPO4) in the beaker and add the monobasic (NaH2PO4) little by little to lower the pH.

You could dissolve each of the salts at 200 mM separately, then adjust the pH of one of them using the other, while stirring and measuring the pH with a pH meter. I would put the dibasic (Na2HPO4) in the beaker and add the monobasic (NaH2PO4) little by little to lower the pH.

Thanks Adam B Shapiro and Aliakbar Khalili Yazdi for your kind suggestions.

Dear Adam B Shapiro,

Is it fine, if i prepare the total solution upto 1L by Distilled water after measuring individually or i have to prepare them in 1L separately and have to mix in particular amount (in ml) of each?

Best Regards,

Once the solution has reached the desired pH, there is no further addition of water. Both solutions started at 200 mM phosphate concentration, so the pH-adjusted solution is also at 200 mM phosphate (the sodium concentration depends on the pH). You will have some of one of the solutions left over if not all of it was used to adjust the pH of the other solution.

Respected Adam B Shapiro,

Thanks a lot for providing this valuable information. As much as i understood, after measuring the amount for 200 mM from both one, i have to mix those and have to prepare up to 1 L by adding distilled water and then have to adjust our required pH by adding any of these. Is it ok?

Best Regards

I think what you said is correct. Let me state it another way just to be sure. You plan to make a 1-liter solution of 200 mM monobasic sodium phosphate and another 1-liter solution of 200 mM dibasic sodium phosphate. Then you will adjust the pH of one of the solutions by adding some of the other solution to it. For pH 7.4, add the monobasic solution to the dibasic solution. You will need about 2 parts of the monobasic solution to about 8 parts of the dibasic solution.

Respected Adam B Shapiro,

Thanks for your answer. This is another possibilities, right? According to your first answer, i can prepare 1 L sodium phosphate buffer by mixing both together, then adding distilled water up to 1 L. Is this also right way?

Sorry for annoying. I am just want to be clear about all possibilities of Sodium phosphate buffer preparation.

Best Regards,

200 mM solution of Na2HPO4 + 200 mM solution of NaH2PO4 = 200 mM buffer. You either need to know exact ratio of two solutions for exact pH (by HH equation) or have to add one to the other and track pH till it reaches the required pH (no HH equation required but the ratio at the pH is same as calc from HH equation). In this case as Adam suggested take 200 mM solution of Na2HPO4 in a beaker and keep adding 200 mM solution NaH2PO4 till the point pH is 7.4. No dilution.