Lake Water and Sediment II. Oxidation-Reduction Relations at The Mud-Water Interface

Rcclox potentials of mud cores from seven lakes, including acid bog, unproductive, and productive types, were measured with Mortimer’s multiple clect,rodc. A break existed below t,he surface, but variation in measurement was too great to characterize its depth within f30-50%. Accuracy of potential mcasuremcnt 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 f50 mv. In poorly poised systems the arca 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 H&3 causes a voltage drop, e.g., 20-min. bubbling of a reduced solution with HzS caused the voltage to fall from 0.63 to 0.17 v. Production of HzS in nature is abolished by 02. As a further dificulty the sediments of some lakes are able to deposit a sulphide plating on submerged platinum. ICvidence is presented that the real oxidized layer is one mm or less thick, and that the reportecl break in the clcctrode 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 prcscnce of sulphide.