Redox Electrode for Determining Nitrous Acid and Nitrosyl Compounds

The present invention relates to a redox electode for determining nitrous acid and nitrosyl compounds in aqueous acidic solution.

For the measurement of nitrous acid and the nitrosyl compounds, the standard platinum redox electrode and redox electrodes of other platinum metals or gold have been tested to date. It has been found that electrodes of these materials are not selective because the potential is determined not only by the nitrous acid but also by oxygen, nitric acid and the diazo compounds formed during the diazotization. Furthermore, the materials described are not sufficiently chemically stable in electrolytes containing nitrosyl compounds and are poisoned by fairly highly concentrated solutions containing nitrous acid. Coating the stated electrode materials with silicone films resulted in a delay in reaching the potential and finally led to the formation of silica and hence to deactivation of the redox electrodes. Doped silicon carbide electrodes became sluggish and inactive due to silica formation, while electrodes containing soluble oxidation products were not selective and did not posses long-term stability.

In the diazotization of primary, aromatic amines or in the reaction of aliphatic amines with nitrous acid, the concentration of the nitrous acid, which also acts as an oxidizing agent in side reactions, must be controlled in order to prevent the formation of undesirable products. Depending on the mineral acid used, nitrous acid is present in the form of nitrosyl halide, nitrosyl sulfate or the nitrosonium ion. Some of the nitrous acid may furthermore be lost for the desired reaction as a result of disproportionation into nitric oxide and nitric acid.

In the purification of nitric acid, the nitrous acid and nitrosyl ions are reduced to concentrations of <10 ppm by degassing, making exact determination of the end point necessary. In sewage engineering, agriculture and food chemistry, nitrite as well as other oxidizing agents must be determined quantitatively. To date, these measurements have been very complicated and expensive.

It is an object of the present invention selectively to measure the nitrous acid and the nitrosyl compounds in addition to other oxidizing agents, eg. oxygen or nitric acid, without deactivation of the redox electrode occurring.

We have found that this object is achieved, according to the invention, if a redox electrode is used in which the electrode core consists of a metallic conductor on which an n-conducting metal oxide surface with a conductivity .kappa. of from 10.sup.-7 to 1.times.10.sup.2 .OMEGA..sup.-1 cm.sup.-1 is present.

The material used for the electrode core is a metallic conductor, eg. platinum, palladium, rhodium, ruthenium, iridium, osmium, copper, silver, gold, titanium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, tungsten or an alloy of these metals.

Tungsten, molybdenum, tantalum, niobium and titanium are preferred.

Metal oxides of the metals of subgroups IV to VI of the Periodic Table of elements are applied, as n-conducting oxides, to the electrode cores. Oxides or mixed oxides of tetravalent to hexavalent tungsten and/or molybdenum, trivalent or tetravalent titanium and trivalent to pentavalent vanadium and/or tantalum are preferably used.

The n-conducting metal oxide layers can be applied to the electrode cores, for example, by a vapor deposition method under greatly reduced pressure, chemical gas phase coating or anodic oxidation.

The redox electrodes are prepared using electrode cores in the form of sheets or rods having a smooth and non-porous surface. The purity of the metals is important, so that only metals with a purity of <99.9% by weight can be used.

Before the claimed oxide surface is produced on the electrode cores, the metal surface is freed from oxides and impurities by ion etching with a noble gas or by mechanical and chemical means. A mixture of hydrofluoric acid, nitric acid and phosphoric acid at temperatures of about +80.degree. C. is suitable for chemical pickling.

The n-conducting metal oxide surface on the electrode cores is preferably produced by anodic oxidation in an aqueous mineral acid. A particularly suitable aqueous mineral acid is 50% strength by weight aqueous sulfuric acid.

The potential of the redox electrode during formation of the oxide film by anodic oxidation should be about 5-10 V. To synthesize a selectively functioning oxide layer of <100 .ANG., an oxidation time of 5-30 minutes is required.

Oxide layers of <80 .ANG. do not ensure a sufficient selectivity.

The electrical conductivity .kappa. of the metal oxides or mixed oxides should be from 10.sup.-7 to 1.times.10.sup.2, preferably from 10.sup.-6 to 1.times.10.sup.1, .OMEGA..sup.-1.cm.sup.-1.

The redox electrode is suitable for the amperometric or potentiometric determination of the concentration of nitrous acid and nitrosyl compounds in aqueous, acidic electrolytes; a potentiometric measurement, polarovoltry, is preferably used. In this method of measurement, the redox electrode is cathodically polarized against an auxiliary electrode of platinum or a platinum metal, having a current density of from 0.1 to 5 .mu.A/cm.sup.2 surface area. The measurement of nitrite is carried out by acidification with a non-oxidizing mineral acid, via the nitrous acid liberated.

The open-circuit potential of the redox electrode in the case of cathodic polarization and in the absence of nitrous acid and nitrosyl compounds should be about +20 mV relative to the pH-dependent hydrogen evolution potential in the measuring electrolyte.

Calomel, thalamide or silver/silver chloride electrodes are used as reference electrodes for determining the potential, which serves as a parameter for the nitrous acid and the nitrosyl compounds.

If the redox electrode becomes contaminated with coatings, for example with tar-like substances which may occur as byproducts in diazotization, cleaning is possible by means of brief cathodic or anodic polarization for from 1 to 3 minutes using from 10 to 500, preferably from 50 to 200, mA/cm.sup.2 surface area. Cleaning is preferably carried out by cathodic polarization. Instead of electrochemical cleaning, chemical cleaning is also possible, for example with about 70% strength by weight sulfuric acid or with organic solvents, such as acetone, dioxane and tetrahydrofuran.

A tungsten wire consisting of 99.9% of tungsten and having a diameter of 2 mm and a length of 150 mm is surrounded with glass fiber-reinforced teflon, 10 mm of wire at both ends being left exposed for connection of contacts and as a measuring surface. The free measuring surface is freed from impurities and oxide residues by grinding with fine corundum powder and water. After this treatment, the surface is cleaned with acetone and water. To produce the oxide surface, the tungsten electrode is anodically polarized at a potential .epsilon..sub.h of about +5 V against a platinum electrode for about 15 minutes at +25.degree. C. in 50% strength by weight aqueous sulfuric acid. After this treatment, the metallic tungsten core has an oxide surface which possesses a conductivity .kappa. of 0.3.times.10.sup.-1 .OMEGA..sup.-1. The measuring electrode, which is ready for use, is cathodically polarized against an auxiliary platinum electrode in 5% strength by weight aqueous hydrochloric acid using a current of 2.3 .mu. A. A 9 V d.c. voltage source with a 3.88M .OMEGA. series resistor is used for polarization. The potential of the redox electrode is measured at high resistance against a silver/silver chloride reference electrode. After the addition of NaNO.sub.2 solution containing hydrochloric acid, the following potentials, based on the HNO.sub.2 content, are obtained:

If nitric acid is added to the electrolyte containing hydrochloric acid and 1 ppm of HNO.sub.2, the potential does not change significantly.

Under the above measuring conditions, a platinum redox electrode indicates the following dependence of the potential on the HNO.sub.2 concentration in the HNO.sub.2 -containing hydrochloric acid.

The tungsten electrode is used for the measurement of the HNO.sub.2 concentration in the diazotization of aniline in hydrochloric acid-containing solution with sodium nitrite to give benzenediazonium chloride. Aniline and benzenediazonium chloride have no effect on the potential of the redox electrode.

A tungsten/molybdenum alloy consisting of 95% by weight of tungsten and 5% by weight of molybdenum is pretreated and activated as described in Example 1. the electrical conductivity .kappa. of the mixed oxide produced is 5.times.10.sup.-3 .OMEGA..sup.-1 cm.sup.-1. The measuring electrode, which is ready for use, is cathodically polarized against an auxiliary iridium electrode using a current of 1.73 .mu.A. A 9 V d.c. voltage source with a 5.2M .OMEGA. series resistor is used for polarization. The redox electrode is used together with a silver/silver chloride reference electrode for measuring nitrous acid and nitrosyl compounds in wastewaters containing sulfuric acid.

A tungsten electrode is prepared and cleaned as described in Example 1. To produce the oxide surface, the electrode is anodically polarized at a potential .epsilon..sub.h of about +7 V against a platinum electrode for about 25 minutes at +25.degree. C. to 50% strength by weight aqueous sulfuric acid. The electrical conductivity .kappa. of the metal oxide produced is 5.times.10.sup.-2 .OMEGA..sup.-1 cm.sup.-1. The prepared redox electrode is cathodically polarized against an auxiliary platinum electrode using a current of 3.2 .mu.A. A 9 V d.c. voltage source with a 2.8M .OMEGA. series resistor is used for polarization. The redox electrode is used together with a silver/silver chloride reference electrode for measuring nitrous acid in nitric acid and acidic ammonium nitrate solutions.

In aqueous 10% strength by weight nitric acid, the following potentials, based on the HNO.sub.2 content, are obtained:

In aqueous 10% strength by weight ammonium nitrate solution having a pH of 1, the following potentials, based on the HNO.sub.2 or nitrite content, are measured: