NEW APPARATUS FOR ELECTROMETRIC DETERMINATION OF HYDROGEN-ION CONCENTRATION AND FOR GAS ANALYSIS.
The hydrogen electrodes were each made of a gold disk (later covered with a thin layer of palladium black) welded to a platinum wire sealed through the glass. There seems to be only one way to do this. A bit of gold is wound or folded over the end of the platinum wire and held in the flame until it just melts; then it is instantly removed. The congealed gold drop is then hammered on to the edge of the gold disk. The electrode is cleaned by filling the glass parts with a saturated solution of potassium bichromate in concentrated sulphuric acid (cleaning fluid) and rinsed thoroughly.
The palladizing solution is made by dissolving about 1 per cent of palladium chloride in distilled water and filtering. No reducing agent, such as basic lead acetate, is necessary. In fact, our solutions were weakened by spontaneous reduction, becoming dark with colloidal palladium; but this finally settled out, by further redttotion, and the necessity of refiltering in order to watch the pailadization was avoided.
The apparatus was filled with the palladium solution, a platinum wire was inserted for an anode, and a current of 4 volts passed until the gold became black. When necessary to repalladize, the apparatus was left full of nitric acid over night, which completely removed the palladium; it was then rinsed and the process was repeated. Palladium black does not seem to have the effective life of platinum black, but it was necessary to use it, as it can be removed without destroying the glass parts or the use of aqua regia. There is no necessity of electrolyzing in a hydrogen electrode, since any free chlorine is removed with the anode when it is taken out unless some aqua regia has been formed in it. If the palladium black is deposited too rapidly it will not stick.
We do not understand why a freshly palladized electrode behaves in a more reliable manner than one which has soaked in distilled water for many days. The electrode is thoroughly charged with hydrogen when palladized, but much of this is probably lost during the rinsing.
Nitric and, to a smaller extent, other mineral acids attack palladium black and oxidizing agents probably affect it; hence we never cleaned palladized electrodes with cleaning fluid. It seems probable that the alcohol and indicator were slowly reduced by hydrogen at the surface of the palladium black, although we did not detect any decrease in the indicator concentration. It is probable that most of the alcohol had been removed from the solution by the current of gas before our final readings were taken. Such suppositions as to the effect of alcohol were based on variations of the potential of the electrode, and it is well known that such occur whenever free oxygen or oxyhemoglobin, that are reduced at the electrode, are present. The phenomenon is probably general, the ease with which the substance is reduced probably deter mining merely the rate at which reduction occurs. Even with a small amount of free oxygen in the solution, we found it possible to obtain the correct result by tilting the apparatus so that the platinized or palladized disk communicated with the main mass of fluid by only a thin film.
The oxygen is apparently used up in the immediate vicinity of the electrode in a relatively short time and the proximity to a large store of hydrogen hastens the resaturation of the electrode. But because of our ignorance of some of the factors involved, we did not wish to rely on any such assumptions in this set of experiments, and consequently we made about half of the determinations soon after immersion and half soon after removal of the electrode by tilting, and took the mean value. In some cases it was necessary to apply corrections for pressure of and H2 when they were not exactly the same in the gas and water phases, but these differences never exceeded 1 per cent of the partial pressure of the gas. We were very much impressed with the slowness with which equilibrium is established between the solution and the gas above it.