I'd lean towards the carbonate myself. It would change the pH more slowly.
IIRC, K2CO3 is a weak base and only partially dissociates into K2+ and CO3- and that dissociation is based on several factors including the current pH of the solution and only the CO3 can accept an H+ ion raising the pH (lowering the acidity) while KOH is a strong base that fully ionizes in solution contributing 1M of OH- per 1M of KOH solution raising the pH (lowering the acidity) because of the hydroxide contribution.... I think....
Please correct this if I'm mistaken - I'm no scientist and the information above was pieced together from Wikipedia and Google.
Based on my strong Google skill it seems it would take more K2CO3 than KOH to achieve the same effect on pH.
The only reason I mention any of this is that the BJCP mead guide that I linked to above says:
Excess carbonate can also impart a metallic or soapy note in the flavor. Don’t use more than 5 grams of potassium carbonate when adjusting pH of the must.
Anyone have any experience with this or nuggets of chemistry knowledge to drop? Is 5g of K2CO3 enough to achieve the desired effect? Also, why does the BJCP mead guide have the above quote and also list:
6g K2CO3 or 150ppm KOH (30ml of 2M solution)
as one of the ingredients of the basic mead making process on page 86 of the pdf? Did it actually mean, "Up to 6g K2CO3 based on pH shift during fermentation"?
Edit: 8/31/10 - from lowering pH to raising pH - cause of my dumbness...