Hydrogen gas electrode

The primary method for pH is based on the measurement of the potential difference of the electrochemical cell without a liquid junction involving a selected buffer solution, a platinum hydrogen gas electrode and a silver/silver chloride reference electrode, often also referred to as a Harned cell. 

The pH may be measured by means of a hydrogen gas electrode in the solution of interest (X), connected by a salt bridge to a reversible electrode, such as the AgCl/Ag electrode or the calomel electrode ... 

Usually, rather than using a hydrogen gas electrode, a glass membrane electrode is used for the measurement. As discussed in Sec. 8, the potential across such a membrane can be proportional to the difference in pH s of the solutions on each side of the membrane. One design for a membrane-type pH electrode, which incorporates a Ag/AgCl reference electrode in a tube concentric to the membrane electrode, is shown in Fig. 6. The electrode is immersed in the solution whose pH is to be measured, with the solution level above the porous plug. 

Another type of cell in which the two electrodes are constituted of the same material is one involving two hydrogen gas electrodes, viz.,... 

A hydrogen gas electrode in 0.05 molar potassium acid phthalate, when combined with a calomel electrode tontaining saturated potassium chloride, gave a cell with an b.m.p. of 0.4765 volt at 38°. Calculate the pH of the solution which gave an e.m.f. of 0.7243 volt in a similar cell. 

In view of the importance of buffer mixtures in various aspects of scientific work a number of such solutions have been made up and their pH values carefully checked by direct experiment with the hydrogen gas electrode. By following the directions given in each case a solution of any desired pH can be prepared with rapidity and precision. A few of the mixtures studied, and their effective ranges, are recorded in Table LXXIII for further details the original literature or special monographs should be consulted. 

Bates indicated that the soundest procedure for experimentally establishing a practical scale for pH in a given solvent requires that the hydrogen gas electrode and the silver-silver chloride electrodes be thermodynamically reversible and stable in the solvent system, the glass (or other) electrode respond in a Nernstian way, and the liquid-junction potential be little affected by change in acidity of the solution. A reference value for pH should be selected that is close to that of the solution to be measured and that gives rational meaning to pH values for the solutions examined. 

Most pH determinations are made by electrometric methods, the pH of the unkown solution (X) being calculated from that of a known standard (S) and the emf ( x and s) of a cell composed of a hydrogen ion-responsive electrode (for example, a glass electrode or a hydrogen gas electrode) coupled with a reference electrode (calomel, silver-silver chloride). This cell is filled successively with the standard solution S and with the unknown solution X. A liquid junction potential j exists where these solutions make contact with the concentrated KCl solution of the reference electrode. From the Nernst equation for the cell reactions and assuming an ideal hydrogen ion response ... 

The hydrogen gas electrode comprises a platinized platinum foil with pure hydrogen bubbling over it. If the gas and the acid are at unit activity (1.18 M solution), then it is known as the standard hydrogen electrode (SHE), the... 

Since hydrogen must be continually supplied from outside the cell, an electrochemical device using a hydrogen gas electrode would be considered a fuel cell. —... 

We see from this equation that the actual potential produced by an electrochemical cell involving a hydrogen (or hydronium) ion depends linearly on the pH of the solution. The total cell potential also depends on the activity pf the metal ion in the other halfcell, which usually would be approximately constant, and the activity of molecular hydrogen, which can be controlled by its partiarpressure. Consequently, the primary variation of the cell potential is with pH.-This result suggests that an electrochemical cell can be used to measure the pH of a solution. This is actually done in the laboratory, but by using specially chosen liquid electrodes rather than a hydrogen gas electrode, which is not convenient to use.. ... 

The physical setup that defines E° for a half-reaction is a hydrogen gas electrode (Standard Hydrogen Electrode) connected to an electrode where the half-reaction of interest takes place (let us say the l2 aq) + 2e 21" half-reaction see Fig. 7-2). The cell reaction for standard conditions, 25 , unit activities, and 1 atm Haoi pressure, is... 

Slant lines, vertical lines, or sometimes a semicolon, indicate phase boundaries across which there arise potential differences that are included in the measured potential of the entire cell. Conventionally, a double slant or vertical line signifies a liquid junction— the zone of contact between two electrolyte solutions. Physically, this may be a porous membrane as in Figure 2.1, or a salt bridge of some sort. The anode is written to the left, the cathode on the right. If there are several components in one electrolyte solution, the components are separated by a comma. For example, for a cell (without liquid junction) composed of a silver/silver-chloride half-cell and a hydrogen gas electrode, one could write for one set of conditions... 

The hydrogen electrode behaves reversibly in many of the protic solvents. Some problems are encountered in the amides where the presence of platinum may lead to catalytic decomposition of the solvent, especially in the presence of HCl. Feakins and French report that the platinum-hydrogen gas electrode in acetone-water mixtures functions well on a mechanically roughened platinum surface. The use of platinum-black was found to be deleterious. 

D7.9 The pH of an aqueous solution can in principle be measured with any electrode having an emf that is sensitive to H (aq) concentration (activity). In principle, the hydrogen gas electrode is the simplest... 

The value of pH is one of the most frequently measured chemical quantities. There are many ways to measure the pH, but most often it is measured by potentiometry. The classical potentiometric tool for pH measurement has been the hydrogen gas electrode. This is not a very convenient tool in an analytical laboratory and has been replaced in this application by more convenient, albeit somewhat less perfect, electrodes—first of all by glass electrodes and occasionally by the quinhydrone electrode or various metal oxide-type electrodes. 

Hydrogen (gas) electrode. There are different versions but usually it consists of a platinum foil electrolytically covered by finely divided platinum ( platinum black ) immersed in the analyte which is saturated with hydrogen gas. 

Hydrogen gas electrodes were widely used in early electrochemical studies not only as reference electrodes but also as indicator electrodes for determining pH. The composition of this type of eleetrode can be represented as... 

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