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Electrochemical measurement of pH on paper artifacts is a very important tool to assess the state of paper conservation. Different methods using different types of electrodes have been standardized to provide reproducible pH values of paper. One of these methodologies is not very invasive since it is made on the paper surface by means of a surface electrode in contact with the paper through a small layer of water previously added at the measurement point. This method is very convenient but suffers from the fact that the pH value given by the instrument is continuously changing during the measurement with a very fast rate in the first seconds and tending to reach a plateau value after very long time. Usually, the measure is taken at a time after which the rate of pH change is below a given value, i.e., when the value can be considered constant. This condition is usually signaled by an automatic instrumental alert provided to the operator. This pH value, however, is not the asymptotic one, which can be reached only at infinite time. Here we have investigated this behavior showing that it is simply due to the gradual diffusion of water away from the measurement region under the electrode. A theoretical model, which reproduces very well the time dependence discussed above, has been developed. The validity of the model has been completely proven by taking in consideration experiments where water diffusion is prevented in a simple confining apparatus. This work shows that it is possible to eliminate any factor affecting accuracy and reproducibility of the pH data for paper measured by the surface electrode method.