Hi, I have to prepare a solution reported like this in the protocol I am reading:
0.1 M Maleic acid, 0.15 M NaCl; pH 7.5 (20° C); 0.3% (v/v) Tween 20
How do I interpret these instructions in your opinion? I think I have to prepare first a solution with final concentrations of Maleic acid and NaCl respectively of 0.1M and 0.15 M and adjust it up to pH 7.5 with NaOH. Then , I think I should take 99.7 ml of this solution and add it to 0.3 ml of Tween20 to arrive at a final volume of 100 ml. Is it right for you?
That is a very interesting solution. It has almost no buffering capacity at all. The pKa1 of maleic acid is 1.9 and pKa2 is 6.07. At pH 7.5, the Henderson-Hasselbalch equation says that 96.4% of the maleic acid is in the dianion form and about 3.6% is in the monovalent anionic form. In other words, if you start with 0.10 moles of maleic acid, you will need to add 0.196 moles of sodium hydroxide to the solution to get the pH up to 7.5. Far easier is to start with 0.1 moles of disodium maleate (which should have a pH of ~10.4) and titrate down to 7.5 with HCl. In the case of adding NaOH, you know have increased the Na concentration from 0.15 M(due to the NaCl) to 0.35 M. In the case of using acid to bring the pH down, the chloride concentration will increase from 0.15 M to 0.154M. You could also mix acid sodium maleate (NaHC4H2O4) with disodium maleate solution.
But really, this will not work either. And that is because there is a lot of carbon dioxide in the air. When you make your solution of disodium maleate, you will find that the pH will be significantly lower. That is because we have almost 400 ppm carbon dioxide in the air and your real solution will be buffered by carbonate (see the attachment).
So, I would add enough NaCl and disodium maleate to a flask so that the final concentration will be 0.15 and 0.1 M, respectively. I would add water to about 90% of my calculated final volume. I would titrate down with HCl, probably 1 M HCl. Then I would add the Tween20 so that with the final water added to the mark on the graduated flask, will provide the correct concentrations of all components. NOTE: The pH will not be very stable because you are too far from the pKa. Maybe that is OK for your experiment or maybe you should choose a solution with a better buffering capacity.
Silvia,
As I thought about my answer over the weekend, I realized that my answer may have suggested that titrating disodium maleate with HCl would reduce the amount of sodium in the final assay. That is not true. Under any circumstance, the concentration of cations will be ~0.35 N. 0.15 of that is from the NaCl and the rest from the maleate which needs to be almost completely in the dianion form.
What are you using the solution for? If it is for capillary electrophoresis, you might get better results with a less conductive solution. At least you will be able to run at higher voltages without overheating.
If the use is for biochemical assays, you might want to choose a buffer that has a pKa closer to the pH 7.5 you plan on running at.