A novel method and apparatus for the measurement of acid concentrations using semiconductor electrodes is described. Prior chemical or instrumental approaches required the use of a variety of chemicals and or separation techniques to minimize metal ion interference. Germanium, indium antimonide, and silicon have proved satisfactory for acid measurements. The semiconductor serves as the anode of an electrochemical cell operated at a positive potential with respect to a cathode of material inert to the electrolyte (stainless steel, titanium, or platinum). However, silicon was the best all around semiconductor examined, as it does not appear to be influenced by high metal ion concentrations as are germanium and indium antimonide. Ammonium fluoride is a key component of the electrolyte both for preventing film formation at the electrodes as well as increasing electrode sensitivity. Calibration curves have been drawn for hydrochloric, hydrofluoric, sulfuric, nitric, acetic, perchloric, phosphoric, sulfurous, oxalic, citric, tartaric, and sulfanilic acids. The approach was developed to monitor acid concentrations in stainless steel and titanium pickling baths (H2S04, HN03, HF-HN03). No discernible metal ion interference has been met when using the silicon electrode.Measurement of acid concentrations in the presence of hydrolyzable cations has always been difficult. Efforts to use complexing or chelating agents have not produced accurate data as the agents themselves often cause an alkaline or acid reaction following hydrolysis or chelation. Physical methods such as conductivity or specific gravity are usually indirect and are only applicable to single acids such as hydrochloric or sulfuric. In the manufacture of stainless steels, it is not uncommon to use consecutive baths of sulfuric, mixed nitrichydrofluoric, and finally nitric acid alone. An earlier communication (1) briefly described the investigation which resulted in the development of a method and apparatus for the measurement of acid concentrations. This paper gives a more detailed account of the experimental work which occurred during the development.The initial program was aimed at the development of a monitor for hydrofluoric acid (HF) after it was found that Turner (2) had published a method for fluoride ion using concentrated nitric acid as an electrolyte. Turner had used A-type silicon as the anode of an electrochemical cell with platinum as the cathode separated by a potential of 1.56 volts. He indicated that the limiting current produced was proportional to the amount of HF. L. Steinbrecher and coworkers (3) extended Turner's approach to include a constant source of light visible to the silicon electrode in order to make it more sensitive to lower concentrations of fluoride ion. Both Turner and Steinbrecher reportedly used A-type silicon of 0.7 ohm-cm resistivity for the measurement of fluoride ions.

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