for you to prepare citrate buffer of pH 6.6, you need the pKa nearest to the pH. In this case, the pKa to use is 6.4. The equation for calculating the grammes of salt and acid to weigh per litre is the HENDERSON HASSELBACH EQUATION:

pH = pKa + log([salt]/[acid])

6.6 = 6.4 + log ([salt]/[acid])

0.2 = log([salt]/[acid])

antilog(0.2) = [salt]/[acid]

1.585 = [salt]/[acid]

1.585[acid] = [salt] …………………equation 1

[salt] + [acid] = 0.05 ……………… equation 2

substitute equation 1 in equation 2,

1.585[acid] + [acid] = 0.05

2.585[acid] = 0.05

Therefore, concentration of acid = 0.05/2.585

                  = 0.0193M

Also, from equation 2, [salt] = 0.05 - 0.0193

                                               = 0.0306M

You will then calculate the mass concentrations of the salt and acid from their respective molar concentration.

i.e., mass concentration (acid) = molar concentration x molar mass

        mass concentration (salt) = molar concentration x molar mass

I DO NOT KNOW THE KIND OF SALT AND ACID YOU WANT TO USE, SO I WILL NOT BE ABLE TO CONTINUE THE CALCULATIONS.

WHAT YOU WILL DO IS TO CALCULATE THE MOLAR MASSES OF THE SALT, AND FOR ACID, OR YOU CHECK THE LABEL ON THEIR CONTAINERS, THEN USE IT TO CALCULATE THE MASS CONCENTRATION IN g/L

After all these, you will weigh in grammes the mass of salt, and mass of acid, add together and dissolve in 1L of distilled water. check for the pH using a pH meter. if the pH is not 6.6, adjust it to 6.6, using drop(s) of conc HCl (if pH is greater than 6.6, or you use drop(s) of conc NaOH if pH is lesser than 6.6

I HOPE YOU WILL UNDERSTANT THESE, AND FIND IT USEFUL

If you find the tables and calculations too tedious try the following online resources:

http://home.fuse.net/clymer/buffers/cit.html

http://www.biomol.net/en/tools/buffercalculator.htm

I am very grateful to all respondents who tried their best to explain the procedure of making citrate buffer. 

Thanks for all researchers

Greetings,

This article on "Protein Buffers by G Gomori" is quite informative and would be helpful for future references.

Thank you so much

Please send reference details and oblige 

You can use

Buffer Calculator - Cusabio,

https://www.cusabio.com

pH = pka+log [salt]/[acid]

About predicting the pH of citric acid ― sodium citrate solutions and buffers for approx. pH < 4.0 ― cf. my post at:

https://www.researchgate.net/post/How-to-prepare-a-citric-acid-solution-of-pH-25-and-3-from-100-citric-acid-powder

About predicting the pH of citric acid ― sodium citrate solutions and buffers for approx. 4.0 < pH < 5.5 ― cf. my posts at:

https://www.researchgate.net/post/Why-the-pH-of-Citrate-buffer-increases-when-diluted-with-water

About predicting the pH of citric acid ― sodium citrate solutions and buffers for approx. 5.5 < pH < 8.0 ― cf. my posts at:

https://www.researchgate.net/post/Why-the-pH-of-Citrate-buffer-increases-when-diluted-with-water

About predicting the pH of citric acid ― sodium citrate solutions and buffers for approx. pH > 8.0 ― and for approx. pH > 9.0 ― cf. my posts at:

https://www.researchgate.net/post/Why-the-pH-of-Citrate-buffer-increases-when-diluted-with-water