
According to "The Theory of the Photographic Process", Third edition), the temperature dependence of the rate of development can be expressed roughly in terms of a coefficient, called k, which is the ratio of the rate at a particular temperature to the rate at a temperature ten degrees higher on the Kelvin scale or 18 degrees higher on the Fahrenheit scale. Among the factors affecting k is pH. It was not noted in the reference, but a plot of log(k) vs pH is for practical purposes a straight line between pH values of 7.8 and 10 and values of log(k) from .477 to .362. This region of linearity includes various compositions of Metol, Metol and hydroquinone, and catechol.
Here is a table of the data presented in the reference.
Developer Kbr pH k log(k)
Sease III 1.0 7.8 3.0 0.477
Metol without alkali 0 8.0 2.9 0.462
D76 0 8.4 2.8 0.447
Gevaert 206 (MQ) 0 8.9 2.6 0.415
Agfa 14a 0.5 9.75 2.4 0.380
MetolNa2CO3 0.5 9.85 2.3 0.352
The attached graph shows k vs. pH on semilog paper.
We can calculate the k for line C on the graph PE sent to be 2.2. D19 is on that line. If the log(k) vs pH relationship stands, the pH of D19 should be about 10.4.