citrate/phosphate buffer- pH 3.0 (McIlvaine's Buffer)

[Richard987x]

(1) Anybody has ever heard about this buffer?
(2) I prepared this buffer recently using a recipe from a web site and noticed final pH was 3.1 after combining corresponding salt of citric acid and dibasic sodium salt. Based on chemistry 1401, you are supposed to add concentrated acid or base to adjust final pH, I am a bit hesitant to do with phosphoric acid as I would be introducing the phosphate ion, more weak base thus potentially changing the ionic strength. (3) Adding con. HCl may be an option but require further evaluation to establish if it the presence of the chloride ion is going to affect the analyte of interest.

Ideas area welcome.

[Uwe Neue]

A citrate buffer prepared from a 1:1 mix of citric acid and citrate is supposed to have a pH of 3.1, not 3.0. If you mixed monobasic and dibasic citrate, your pH would have been 4.75. So what did you do?

[Richard987x]

Based on the recipe- 4.6 grams citric acid (MW 192, pKa 3.15) and 1.6 grams of K2HPO4 (MW 142) in 1 Liter, pH 3.1 adding weak base does not lower pH and it is changing ionic strength. My target pH is 3.0. Conc. H3PO4 seems to be a reasonable choice to adjust hydrogen concentration but I'm concern that while I'll be increasing the concentration of the weak base, phosphate ion.

Asssuming no salt effects (salting out analyte of interest), adding a few drops of HCl may be the way to go.

[Uwe Neue]

Allright, this makes more sense: you are combining a dibasic phosphate salt with citric acid. The pH is what you should get from an equimolar mixture. If you want to get to pH 3.0, you can get there by adding a dash of phosphoric acid without changing the nature of the buffer. However, what you do loose is the precision with which a McIlvaine buffer is made (by weighing out stuff, instead of measuring pH).

See if there is a recipe for a McIlvaine buffer for pH 3.0.

(In case that you followed a recipe for a pH 3.0 buffer and measured 3.1, you either need to recalibrate your pH meter, or you missed something in the recipe, such as the level of hydration of an ingredient. I am too lazy at this moment to check the recipe.)