3 replies to this topic

[BMandelsonn]

I decided to create this buffer based on a few different protocols I'd read. There seem to be so many different ways of making this buffer - so I decided to try work out how to make it using the good old classic equations. Does anyone have a better recipe? Please point out any flaws in the method I used (below).

0.15M Borate Buffer pH 8.3

pH = pKa + log [salt]/[acid]
[buffer] = [weak acid] + [weak base]
In this case: Borax (MW = 381.37) is the salt or weak base, Boric Acid (MW = 61.83) is the weak acid.

pKa of Boric Acid = 9.14
Desired pH = 8.3
Desired [buffer] = 0.15M

8.3 = 9.14 + log [salt]/[acid]
antilog(8.3 - 9.14) = [salt]/[acid]
0.145 = [salt]/[acid]

To obtain ratio of salt to acid...
1/0.145 = 6.9
Therefore...
1/6.9 = 0.145
Apply these values to: [buffer] = [weak acid] + [weak base]
0.15M = 1x + 6.9x
1 + 6.9 = 7.9
x = 0.15/7.9 = 0.0189
0.015M = 0.019M + 0.131M

Determine the weight of Borax and Boric Acid required using:
n = C V/1000
n x MW = mass
(n, number of moles; mass, mass in grams; MW, molecular weight; C, molar concentration; V, volume in millilitres)

For example if you want to create 100ml of 0.15M Borate Buffer pH 8.3:
Borax (MW = 381.37)
0.019 x 100 /1000 = 0.0019
0.0019 x 381.37 = 0.72g

Boric Acid (MW = 61.83)
0.131 x 100 /1000 = 0.0131
0.0131 x 61.83 = 0.81g

i.e. dissolve 0.72g Borax and 0.81g Boric Acid in 100ml of water.

[mdfenko]

i would just take boric acid and adjust to the desired pH with base (naoh or koh, whichever i want).

[BMandelsonn]

mdfenko, on Nov 4 2009, 09:41 PM, said:

i would just take boric acid and adjust to the desired pH with base (naoh or koh, whichever i want).

Thanks for your reply - your method is certainly much easier - however would this method affect the properties of the buffer e.g. buffer capacity etc.?

[mdfenko]

BMandelsonn, on Nov 6 2009, 07:14 AM, said:

Thanks for your reply - your method is certainly much easier - however would this method affect the properties of the buffer e.g. buffer capacity etc.?

not at all. you are just converting some of the "acidic component" into the "basic component". the final concentration of the buffering species will determine capacity.