Lake Water and Sediment1

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Abstract
Redox potentials of mud cores from seven lakes, including acid bog, unproductive, and productive types, were measured with Mortimer’s multiple electrode. A break existed below the surface, but variation in measurement was too great to characterize its depth within ±30–50%. Accuracy of potential measurement is limited by lack of poise, so that the scatter of units of the multiple electrode increases as a standard solution or a natural mud is diluted with distilled water. With the muds studied the scatter was ±50 mv. In poorly poised systems the area of platinum is also important, so that potentials of 0.01 to 0.42 volts were obtained on the same sample with electrodes of different sizes. The presence of H2S causes a voltage drop, e.g., 20‐min. bubbling of a reduced solution with H2S caused the voltage to fall from 0.63 to 0.17 v. Production of H2S in nature is abolishedby O2. As a further difficulty the sediments of some lakes are able to deposit a sulphide plating on submerged platinum. Evidence is presented that the real oxidized layer is one mm or less thick, and that the reported break in the electrode curve at about one cm is caused by factors other than oxidation of iron salts, e.g., turbulent stirring of the sediment by wind‐induced currents, and presence of sulphide.

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