2 100ml samples of each mix were prepared in 5.5 oz plastic portion pots and pH was recorded while stirring as the mix was prepared, after 12 hours, and then daily up to 10 days. The graphs show the pH for each of the 2 samples of each mix.
I looked at 4 different types of mix:
1) No pH adjustment.
I thought that pH changes were probably due to carbon dioxide (CO2) dissolving in the mix, so one pair of samples was left uncovered and the other were stored with lids on. In both cases the pH decreased steadily, but more quickly for the uncovered samples, which supports the hypothesis.
2) pH adjusted to 7.6 with double acting baking powder.
I tried two different brands of baking powder: 'Clabber Girl' and 'Western Family'. In both cases the pH decreased for the first 12 hours or so and then increased again. Double acting baking powders have a room temperature component and a high temperature component (which kicks in when you put your cake or whatever in the oven). I think that the decrease over the first 12 hours is probably due to that second component reacting slowly at room temperature. Most of the CO2 produced by the baking powder then gases off over a period of days resulting in a steady increase in pH. The final pH will depend on what is left behind as the CO2 leaves.
3) pH adjusted to 7.6 with single acting baking powder.
I only used one brand: 'Rumford'. No initial decrease in pH as there is no second component, so the pH starts increasing immediately as the CO2 gases off.
4) pH adjusted to 7.6 with acid
I tried 3 different acids: tartaric (cream of tartar), citric (lemon juice), and hydrochloric. There was no significant change in pH over the 10 day period.