Voltameter

Volhard titration analychem Determination of the halogen content of a solution by titration with a standard thiocyanate solution. fol,hart tT tra-shan voltameter See coulometer. val tam-ad-ar ... 

Pure water is a very poor conductor of electricity because there are so few ions in it. However, it can be made to decompose if an electric current is passed through it in a Hofmann voltameter, as in Figure 5.11. 

Figure 5.11 A Hofmann voltameter used to electrolyse water.

Account for the following observations which were made when concentrated sodium chloride solution, to which a little universal indicator had been added, was electrolysed in the laboratory in a Hofmann voltameter. 

In this chapter, E is used to represent any potential measurement that can be made versus a reference electrode. Eapp and E are measured relative to some reference electrode scale. The potential difference AE can also be measured at any two points in the electrolyte between separated anodes and cathodes. Two reference electrodes connected between a high impedance voltameter can make this measurement. Alternatively, the potential between the reference electrode and anode or cathode may be determined at any two points. AE is determined by taking the difference, which in this case eliminates the reference potential scale. VIR is an absolute ohmic voltage (not referenced to any scale). 

Coulometer — (previously coulombmeter, or also voltameter) A coulometer is an instrument to measure charge, i.e., to perform -> coulometry. Richards and Heimrod [i] suggested in 1902 the name coulometer to replace the previously used term voltameter . Modern coulometers perform electronic integration of - current over time. However, the first coulometers utilized - Faraday s law, e.g., by weighing the amount of silver that has been deposited on a silver electrode in a silver electrolyte solution the charge could be calculated (silver... 

Rosa, E. B., Vinal, G. W. (1917) The silver voltameter as an international standard for the measurement of electric current. Proc. Natl Acad. Sci. USA. 3, 59-64... 

The amount, or quantity, of electricity passing through a circuit is measured in coulombs, which are, in turn, the product of the amperes and time in seconds.2 At an international electrical conference in London (1908) the ampere was adopted as one of the fundamental units. The ampere is defined as the steady current which, when passed through a solution of silver nitrate in water, under definite conditions to be described later, deposits silver at the rate of 0.00111800 gram per second. This value of the ampere is one-tenth of the c.g.s. (electromagnetic) unit within a few parts in one hundred thousand. The instrument used for measuring current in terms of deposited silver is called a silver coulometer or a silver voltameter.8... 

The term voltameter has heen longer in use, but the expression coulometer, suggested by T, W. Richards, is more descriptive of the use of the instrument and will be employed in this booh. 

Reference may also be made here to the new measurements of specific heat by means of the vacuum calorimeter, to which we have already alluded on page 233. The authors mentioned made use of an apparatus which was essentially the same as that described by Schwers and myself the temperature was measured by means of a platinum wire, through which passed an extremely constant current, so that the resistance could be measured by the potential drop, using a potentiometer. During the heating a silver voltameter was included in the circuit as a control on the measurement of time convenient though this is, I think that as a rule the determination of the duration of the heating current can be made with sufficient accuracy to allow the somewhat troublesome silver estimation to be avoided. 

For example, the electrolysis of water, using the Hoffinan voltameter apparatus, could be followed by presenting diagrams of what the mass spectra of the water and the gaseous products would look like - what chemists would find if they tested the three different substances (the water and the gases collected at each electrode) in their elemental analyser (Figure 1.11). 

Figure 8.3 depicts a special electrolytic cell, called a Hoffmann voltameter, which demonstrates the famous electrolysis of water experiment. Note that pure water (a covalent compound - see Chapter 4) does not conduct electricity, so cannot be readily electrolysed. In the demonstration an electrolyte, such as sulfuric acid or a sodium hydroxide aqueous solution, is added to the water to allow electrolysis to occur at an observable rate. The oxygen and hydrogen evolved can be tested in the usual ways. The observations from this experiment provide data that support many chemical models, such as the composition of water and its H2O formula. 

Faraday used the name voltameter . He speaks of water being decomposed with facility when rendered a better conductor by the addition of acids or salts ( 706). Three forms of voltameter are shown in Fig. 10. He did not use a galvanometer. 

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