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An in situ technique has been developed for measuring the surface pH adjacent to a solid electrode/liquid interface during electrolysis. Measurements of the surface pH can be used to obtain insights regarding the electrodeposition of various transition metals and to obtain a better understanding of associated in situ surface chemistry effects. Many transition metals and alloys deposit with simultaneous hydrogen evolution and, as a result, are accompanied by a pH rise near the cathode, thereby affecting the reactivity of the nearby metal-ion species. Measurements of the surface pH of a solution containing simple salts during hydrogen evolution from a cathode were performed. The surface pH of a cathode during Ni and NiFe electrodeposition was also measured. The experiments demonstrated that, in the absence of buffers or metal ions, the surface pH rises many pH units above the bulk value. During Ni and NiFe electrodeposition, however, the surface pH of solutions consisting of simple salts and starting from a bulk pH level of 2 does not increase more than 3 pH units from the bulk value. In the case of Ni and NiFe electrodeposition, surface buffering occurs because of the hydrolysis of the metal-ion species present. Additionally, it is found that during the anomalous codeposition of NiFe, the surface pH is much lower than that required by the Dahms-Croll hypothesis.
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