Apparatus using semiconductor electrodes for the measurement of acid concentrations

Abstract

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.

Instability of current followers in potentiostat circuits

“…The input impedance of the current follower is calculated by means of Laplace Transforms (3) and involves the following assumptions: 1) the system is linear, 2) the output impedance of the operational amplifier is zero, 3) the input impedance of the operational amplifier is infinite, and 4) the transfer function of the operational amplifier is adequately described as G = K (1 + ts)-1-i.e., a simple rolloff in which K is the dc gain and l/ is the break point frequency, f0. Employing the convention that Laplace 1 Author to whom reprint requests should be addressed.…”

“…The electrode is selective for HF and, when measuring other acids, depends on the use of an excess of NH4F electrolyte for three reasons. First, to produce a weakly ionized molecular species HF by forcing the equilibrium in that direction, e.g., with hydrochloric acid: NH4+ + F~+ H+ + Cl™ =* HF + NH4+ + Cl™ (1) As such, the electrode responds only to HF or fluoride ions in acid solution and is not influenced by changing ionic strength of salts such as NaCl up to 3.0 molar. Second, as the electrode responds only to HF or free fluoride ion in acid solution, excess fluoride is required to produce current saturation.…”

Determination of acid concentrations in specialty alloy pickling baths by ion chromatography

“…Several investigators have applied semiconductor (3)(4)(5) ion-exchange membrane and anion-selective (6,7) electrodes to the determination of the major mineral acid components of pickling baths, and commercial instruments based on some of this work have been marketed in the United States. The contaminant survey of fresh acids is not possible with these instruments, however.…”

Analytical monitors using electrodes of semiconductors (germanium, indium-antimonide, and silicon)

“…Arthur and Leonard (5) also combined silicon electrodes with a reference electrode to measure hydrogen ion as well as fluoride ion concentration. In using a wafer of n-p-n silicon, the sensor measured hydrogen ion, and with a p-n-p silicon wafer, the sensor was sensitive to fluoride ions in a pH 7 buffer solution. The patent claims application to cations as well as anions but indicates only application to hydrogen and fluoride ions and does not give any indication of crystal orientation or resistivity.…”

“…McKaveney and Byrnes (7,8) applied semiconductors of germanium, indium-antimonide, and silicon to the measurement of acid concentrations. The semiconductor formed the anode of an electrolytic cell while stainless steel, titanium, or platinum was used as the cathode.…”