A comparison of glass and quinhydrone electrodes for determining the pH of some Iowa soils. III. The change in pH of the soil-water mixture with time

Experiments were conducted to determine the influence on the hydrogen-ion concentration of five Iowa soils of length of time soil is in contact with water in suspension. Seventy-five cc of distilled water were added to 35 grams of soil and allowed to stand for 0, 1/4-, 1/2-, 1-, 6-, 12-, and 24- hour intervals, after which hydrogen-ion concentration was determined by the glass and quinhydrone electrodes on soil suspensions and on supernatant liquids. The data were analyzed statistically by the analysis of variance and the following conclusions have been drawn: 1. The variability in the pH of quadruplicate samples of different soils at any time was comparatively small when determinations were made in either the supernatant liquid or the soil suspension. 2. There was very little change in the pH of the supernatant liquids or the soil suspensions during the first 6 and 12 hours, respectively, after preparation. After that time, however, there was a significant increase in the pH of the acid soils and a decrease in the pH of the basic soil. This change in pH is presumably of no practical significance as it does not occur until a rather long time after preparation of the samples for pH determination. 3. The glass electrode method gave consistently lower results than the quinhydrone electrode method in natant liquid of Marshall silt loam where it gave slightly higher results. The differences obtained by the two methods were so small that they are considered of little significance in actual practice. 4. Repeated moving up and down of the glass electrode in the soil-water mixture resulted in lower pH values of the soils studied. It is recommended, therefore, the electrode be moved up and down only two or three times in the soil-water mixture immediately after immersion and that it then be left undisturbed until the pH determination is made.