How can I measure the pH of high-viscosity fluids like dough?

Question

I would like to measure the pH value of dough with a pH-Meter, to ensure a pH value of 4.1 (max). Because of the high viscosity I would thin down the dough with demineralized water for two reasons:

1. I expect more accurate measurements
2. This reduces pollution of the measurement device.

As far as I understand this will affect the measured value, so I would thin down the dough in a defined ratio of 1:10.

Questions:

1. Is it correct that the thinning with demineralized water changes the pH value?

2. The ratio 1:10 should cause an offset of 1 in ph value, right? In which direction? Do I have to add 1.0 to or subtract 1 from the measurement to estimate the real value?

Edit #1

As it seems to be important: We are talking about weak acids produced by fermentation of rye flour (or wheat flour) using sourdough (lactose bacteria + yeast). Some samplings are taken during the fermentation process to check that the pH value is low enough (value of 4.1 or lower). I'm less interested in the -details- of the theoretical background, more in a practical solution for the issue. The actual real value isn't that important (I need to check, if the pH limit is reached), but it would be interesting how to measure it as there might be future experiments where the value matters...

The use of additional chemicals except (demineralized) water should be avoided for simplicity. Of course the use of tab water would be preferable, but I guess this becomes more complicated because of unknown water hardness and varying ion concentrations.

Answer 1

In addition to buckminst’s answer, for weak acids, which are not fully ionized, the pH of the solution depends on the ratio of the concentrations of the conjugate base $\ce{A-}$ to the neutral acid $\ce{HA}$, especially in the pH regime close to the $\mathrm{p}K_\text{a}$ of the acid (the buffer region). Diluting the solution will change the concentrations, but in the same way. The ratio will not change. The Henderson-Hasselbalch equation can be derived from the mass action expression for $K_\text{a}$ of a weak acid:

$$\ce{HA <=>H+ + A-}$$ $$K_\text{a}=\dfrac{\ce{[H+][A^{-}]}}{\ce{[HA]}}$$ $$\text{p}K_\text{a}=-\log{K_\text{a}}=-\log \left(\dfrac{\ce{[H+][A^{-}]}}{\ce{[HA]}}\right)=-\log[\ce{H+}]-\log\left(\dfrac{\ce{[A^{-}]}}{\ce{[HA]}}\right)$$ $$\text{p}K_\text{a}=\text{pH}-\log\left(\dfrac{\ce{[A^{-}]}}{\ce{[HA]}}\right)$$ $$\text{pH}=\text{p}K_\text{a}+\log\left(\dfrac{\ce{[A^{-}]}}{\ce{[HA]}}\right)$$

If the volume increases by a factor of 10, then $[\ce{A-}]$ and $[\ce{HA}]$ both decrease by a factor of 10, and $\dfrac{\ce{[A^{-}]}}{\ce{[HA]}}$ remains constant.

Since the acids in dough are likely acetic or lactic from fermentation; carbonic or tartaric from leavening; or citric or ascorbic from preservatives – all of which are weak acids – diluting the dough might not change the pH.

Answer 2

1. Yes, dilution changes pH. pH is defined as the negative log of $\ce{H+}$ concentration, so changing the concentration will change pH.

2. Not necessarily. For dilute acids and bases, the auto-ionization of water becomes a factor in calculating pH, and will change by less than one pH unit. For more concentrated acids (pH<5 or pH>9 for initial solution), one pH unit is correct. The pH will change toward neutral (pH=7) upon dilution, so it depends on initial pH whether you must subtract or add to make a correction.

Answer 3

If all you want is a practical answer, just try taking a constant mass of dough, dissolve in a constant amount of deionized water, and measure the pH with a standard pH meter. The pH of the measured solution should correlate with the pH of the dough, even if they are not an exact match. The key here is to perform the assay the same way every time, using slightly different amounts of dough or water can change the pH you measure.

You might also consider using a buffered solution of water instead of deionized water. I've seen "pure" water in our lab with pH anywhere between 6 and 7, so a dilute buffer at pH 7 should help get more consistent results. Just keep it dilute so that it doesn't buffer out the pH of the dough.