Electrodes quinhydrone

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. 

Quinhydrone electrode. If the equimolar compound formed by (benzo)quin-one (Q) and (benzo)hydroquinone (QH2), the so-called quinhydrone (Q2H2), is dissolved in water, it dissociates to the extent of about 90% into the two components. In conjunction with H+ ions and electrons an equilibrium is established on the basis of a completely reversible redox reaction ... 

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. 

Quinhydrone electrode — introduced by Biilmann in 1921 [i-vi] is a redox electrode for pH measurements [vii, viii]. Quinhydrone is a sparingly soluble charge-transfer complex consisting of quinone and hy-droquinone in 1 1 ratio. At an inert electron conductor (e.g., - platinum wire or modified -> graphite) the following electrochemical equilibrium is established ... 

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. ... 

A quinhydrone electrode in a solution of unknown pH was combined with a KC1(0.1 n), Hg2Cl2(s), Hg electrode through a saturated potassium chloride salt bridge the e.m.f. of the resulting cell was — 0.3394 volt at 30°. Calculate the pH of the solution. 

If the oxygen electrode were reversible, what change of e.m.f. would be expected when an oxygen gas electrode at 1 atm. pressure replaced (i) a hydrogen electrode at 1 atm., and (ii) a quinhydrone electrode, in a given cell at 25° ... 

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). 

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