The quinhydrone electrode

This electrode system is now little used as a means of pH determination but it is a good example of an organic redox system which behaves reversibly. For this reason we shall consider its mode of operation. 

Quinhydrone is in fact the name given to the molecular crystal formed between quinone and hydroquinone. When dissolved in water the crystal decomposes into its constituent compounds. The redox reaction upon which the electrode operates is 

The potential of an inert wire electrode placed in a mixture of Q x and Qred is given approximately, in terms of concentrations, by 

The total concentration of quinhydrone (present in all forms) is given by  

Cred = [H2Qred] + [HQred ] + [Qred l or, in terms of [Qred l via the dissociation constants, Ki and K2, 

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Dissolve 100 g. of iron alum (ferric ammonium sulphate) in 300 ml. of water at 65°, Pour the solution, with stirring, into a solution of 25 g. of hydroquinone in 100 ml, of water contained in a 600 ml. beaker. The quinhydrone is precipitated in fine needles. Cool the mixture in ice, filter with suction, and wash three or four times with cold water. Dry in the air between filter paper. The yield of quinhydrone, m.p, 172°, is 15 g. It contains a trace of iron, but this has no influence upon the e.m.f, of the quinhydrone electrode provided that the washing of the crude material has been thorough. The quinhydrone should be stored in a tightly-Btoppered bottle. 

Some special redox electrodes. Within the group of redox electrodes, attention should be paid to the hydrogen and oxygen electrodes, and also to the quinhydrone electrode and its tetrachloro version. 

The quinhydrone electrode (Section 3.2.5) is an example for such a more complicated redox reaction. 

For the quinhydrone electrode, the equation for the electrode potential has the form... 

The measurement of pH in cheese making is extremely important to control fermentation/acid production and hence the final quality. While there are no standard methods available for measuring cheese pH, there have been few methods reported in the literature. One method involves preparing a slurry of 10 g of grated cheese in water and measuring the pH potentiometrically (Fox et al., 2004a). However, this method may alter the balance between colloidal and soluble calcium phosphate and hence it is preferable to measure the pH of the cheese directly. The quinhydrone electrode method (Marshall, 1992) measures the pH directly. The potential (mV) created by a paste of cheese and quinhydrone in saturated KC1 is measured and used to determine the pH at a particular temperature. 

The dissociation of acetic acid in aqueous solution is an example of the simplest type of protolytic process. The dissociation constant was one of the first chemical parameters to be studied as a function of isotopic composition of the solvent (La Mer and Chittum, 1936 Homel and Butler, 1936), and the determinations have been repeated by several groups of workers. Conductivity measurements (La Mer and Chittum, 1936), potentiometry using the quinhydrone electrode (Korman and La Mer, 1936) or glass electrodes (Salomaa et al., 1964a Gold and Lowe, 1968), and measurements of the rate of a hydrogen-ion... 

Formal potentials can be defined on different levels of conditions Thus the formal potential of the -> quinhydrone electrode may be defined (I) as including (a) the standard potential of the hydroquinone di-anion/quinone system, (b) the two acidity constants of the hydroquinone, and (c) the activity coefficients of the hydroquinone dianion and quinone, or, (II), it may also include (c) the pH value. In the latter case, for each pH value there is one formal potential, whereas in the first case one has one formal potential for all pH values, and an equation describing the dependence of the electrode potential as a function of that formal potential and the individual pH values. Formal potentials are strictly thermodynamic quantities, and no kinetic effects (e.g., by electrochemical -> irreversibility) are considered. 

This is the potential of the quinhydrone electrode against the Hg, Hg2Cl2,... 

The quinhydrone electrode has been adapted for pH measurements in non-aqueous media, such as alcohols, acetone, formic acid, benzene and liquid ammonia. For the determination of hydrogen ion activities in solutions in pure acetic acid a form of quinhydrone electrode involving tetrachloroquinone (chloranil) and its hydroquinone has been used. ... 

Similar calibrations can also be performed conveniently when any pH electrode, such as the hydrogen or the quinhydrone electrode, is substituted for the glass electrode in Cell (3-9). 

It if necessary to use pure water, as used for electric conductivity work, and to adhere to constant conditions of working. The method is frequently combined with a hearing trial. Thus liansen heated smokeless powders in the ground condition at no and tested samples at intervals pf an hour by means of the quinhydrone electrode to ascertain the rate of formarimodified method the powder is heated under water at 100 to bring about a hydrolytic decomposition. Grottanelli passes a slow current of air over a large sample (500 g.) of powder, which is maintained at 80 . and then into an absorption vessel in which the /H value is measured. Pariik also passes the (pues into water under modified ccalcium carbonate in powders has been examined by Mete. 

F. M. Cray and G. M. Westrip have determined the influence of acetone upon the transformation interval of a number of indicators. They worked with acetone containing 10% by volume of water. They prepared various buffer mixtures, the pH s of which were measured potentiometrically by means of the quinhydrone electrode. The following table shows how large an effect acetone has upon the magnitude of the dissociation constant. 

Preparation of suitable indicator solutions. The prepar on of indicator solutions for use in the colorimetric pH determination has already been described in detail in Chapter Five ( 3). These solutions, however, were suited only for measurements of buffered solutions. Isohydric indicator solutions have to e prepared in another manner. H. T. Stern titrates the indicator with sodium hydroxide and measures the pH during neutralization with the quinhydrone electrode. A similar procedure has been described by Pierre and Fxjdge. Fawcett and Acree keep in stock a large series of neutralized indicator solutions and determine their pH approximately by colorimetric means. 

The Standard Potential of the Quinhydrone Electrode. The quin-hydrone electrode is of interest and importance as a method for the determination of pH values and because the oxidation-reduction relations of quinone and hydroquinone have been extensively studied. It will however receive consideration here because it is an excellent example of the use of cells without40 liquid junctions for the determination of the standard potential of a galvanic cell of a somewhat more complex type than those so far considered. 

The Practical Determination of pH by Potentiometric Methods. There are four potentiometric methods in general use for determining pH values. These depend upon (a) the hydrogen electrode, (b) the quinhydrone electrode, (c) the antimony-antimony trioxide electrode, and (d) the glass "electrode. These will be discussed in the order... 

Table I. Constants, Bt for Computing the "Salt Error of the Quinhydrone Electrode at 25°...

In making pH measurements with the quinhydrone electrode the solution should be saturated with quinhydrone, i. e.a the solid substance should be present. Biilniann and Jensen 7 have found that errors may arise if an unsaturated solution is used. Such errors will be about 0.01 pH unit if the quinhydrone concentration is one-tenth of that of the saturated solution, and the error increases if still less of the substance is present. 

Table III. Values of the Constant, eU for the Hydrogen Electrode and of Eg for the Quinhydrone Electrode for use in pH,Determinations at Various Temperatures with the 0.1 Normal Calomel Electrode...

The quinhydrone electrode can be used for the potentiometric determination of pH. The solution to be measured is saturated with quinhydrone, an equimolar mixture of quinone (Q) and hydroquinone (HQ), and the potential of the solution is measured with a platinum electrode. The half-reaction and its standard potential are as follows ... 

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