Question

I want to make up 1L of 50mM phosphate buffer, pH 6.8. I have 1M phosphate buffer, pH 12 and 2M HCl. How will I make up this buffer. Please explain! Thank you.

Answer 1

Usually to buffer solutions are prepared by mixing either equal or different volume of equimolar solutions of a weak acid and its salts or equimolar solutions of a weak base and its salt.

Since you have only one buffer solution 1 M phosphate buffer pH 12, and a strong acid HCl (2M), you just need to dilute to the buffer solution to the required concentration and adjust the pH using HCl.

By using M1V1 = M2V2, you can find out how many mL of 1M phosphate buffer needed to make 1 L of 50 mM buffer solution.

1000mL x 50 mM = V2 x 1000 mM

V2 = 1000mL x 50 mM/ 1000 mM = 50 mL

You need to take 50 mL of 1M phosphate buffer and dilute to the volume approximately 900 mL using deionized water. With pH meter, adjust the pH to 6.8 using 2 M HCl solution. Add HCl solution slowly while adjusting the pH. Sometimes, it will overshoot. Finally, you need to adjust the total volume to 1L. This will give you the desired solution 1L of 50 mM phosphate buffer, pH 6.8.