Phosphate buffer calculation help

In summary, the author is trying to make a buffer solution with phosphate, potassium, and sodium ions, but their calculations were off because of the increased ionic strength.
  • #1
newlabguy
10
0
This isn't homework. It's for my job. I'm supposed to be making a 50mM phosphate buffer to pH 7.2, 0.5 NaOH. I have access to salts, K2HPO4 (dibasic), KH2PO4 (monobasic), and NaCl. I've determined that I need two liters of buffer solution. I did the calculations but when I measured the pH after making the solution, it was more acidic than it should be. I adjusted the pH with strong base, so it's alright now. I'm just wondering if someone can verify my calculations.

FW K2HPO4 = 174.18 g/mol
FW KH2PO4 = 136.09 g/mol
FW NaCl = 58.44 g/mol

pH = pKa + log ([K2HPO4]/[KH2PO4])

Since pKa = 7.2 and pH =7.2, the HH equation reduces to [KH2PO4]=[K2HPO4]

For two liters, I need 0.1 mol phosphate buffer
0.1 mol= [KH2PO4]+[K2HPO4]
then
0.05 mol = [KH2PO4]
0.05 mol = [K2HPO4]

then I weigh out 136.09 g/mol * 0.05 mol = 6.8045 g [KH2PO4]
and 174.18 g/mol * 0.05 mol = 8.709 g [K2HPO4]
and 58.44 g NaCl

and fill the volume to 95% and adjust pH if necessary then fill to mark. Did I do something wrong? Some buffer calculators are showing something like 3 times the amount of base as acid and I don't know why. For example, http://www.biomol.net/en/tools/buffercalculator.htm
 
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  • #2
Without actually trying to go through your plan and calculations (which will require studying for a few days or more), just realize that your models will let you predict what quantities will give you the desired measured pH result, but the models are not the perfect reality for what you expect. Loosely speaking, if your result was close, then you did well. If your adjustment was small, then you did well.
 
  • #3
Your calculations don't take ionic strength and activity coefficients into account. That's why you are a way off. See ionic strength & activity coefficients primer.

Blatant ad, this is Buffer Maker at work:

phosphate_buffer_72.png


You wrote that your buffer should be 0.5M NaOH, yet you added NaCl. That further increased ionic strength - 0.5M solution has ionic strength around 0.5. That's too much for a reasonable calculations (that is, we are outside of the application range of the Debye-Huckel theory and Davies equation).

I am not sure what the buffer recipe should be, as changing amounts of ions present you change ionic strength and osmolality of the solution, these can be important for the application. Using potassium phosphates AND sodium chloride you may be wrong.

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FAQ: Phosphate buffer calculation help

How do I calculate the amount of phosphate buffer needed for my experiment?

The amount of phosphate buffer needed depends on the desired concentration, volume, and pH of the buffer. To calculate the amount, you can use the Henderson-Hasselbalch equation, which takes into account the pKa of the phosphate buffer and the desired pH. Alternatively, you can use online buffer calculators to simplify the calculation process.

What is the pKa of a phosphate buffer?

The pKa of a phosphate buffer depends on the specific type of phosphate used. For example, the pKa of monobasic sodium phosphate is 7.2, while the pKa of dibasic sodium phosphate is 6.8. It is important to know the pKa when calculating the amount of buffer needed to achieve a specific pH.

Can I use any phosphate salt for my buffer?

No, you cannot use any phosphate salt for your buffer. It is important to use a phosphate salt that has the appropriate pKa for your desired pH. In addition, make sure to use high-quality salts to avoid impurities that may affect the accuracy of your buffer.

How do I adjust the pH of a phosphate buffer?

To adjust the pH of a phosphate buffer, you can add either acid or base to the solution. The amount of acid or base needed depends on the initial pH and the desired pH. It is important to add small increments and measure the pH after each addition to avoid overshooting the desired pH.

How do I prepare a phosphate buffer?

To prepare a phosphate buffer, you will need to mix the appropriate amounts of monobasic and dibasic phosphate salts in the desired volume of water. You can then adjust the pH if needed. It is important to mix the salts thoroughly to ensure an accurate buffer. You can also use pre-made phosphate buffer solutions, which are commercially available.

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