made with a known electrolyte. Kohlrausch gives the following list of liquids suitable for the purpose :—

(1) Maximal' sulphuric acid: 30 per cent. H2SO, by weight: density at 18°, 1°223. 206 c.c. strong acid are made up to a litre with water. Commercial "pure" acid

FIG. 22.

is sufficiently good for use.

(2) Maximal MgSO4: 17'4 per cent. : density at 18, 1*190. 424 gm. dry, not effloresced, MgSO, + 7H2O are made up to 1 litre. The commercially pure salt may be recrystallised with advantage.

(3) Saturated NaCl: About 450 grm. salt are made up to a litre. Some of the salt should be very finely powdered, and only added when the rest is mostly dissolved, in order to ensure saturation. The excess of salt is allowed to settle, and the liquid drawn off when required. The dry salt should be heated to drive off any free acid.

(4) Normal KCl solution; 74'59 grm. per litre at 18°; density, 1'04492 at 18°. 74 555 grm. is the apparent weight in air required. If the temperature is ± 1° from 18° the volume is to be taken ± 0.3 c.c. greater. The density should be measured for control.

The salt should be recrystallised, and strongly heated and cooled in a dessicator before weighing. It should colour a bunsen flame only moderately yellow, and show no noticeable reaction with sulphuric acid.

n. n. and on. KCl are prepared by diluting the normal solution.

(5) Saturated CaSO, solution. The substance must be finely powdered and well washed; the solution may be syphoned off into the conductivity vessel while still slightly turbid.

The following table shows the range of conductivity vessels for which the various liquids are suitable :—

1 H2SO4, MgSO, and some other solutions increase in conductivity up to a certain concentration, and then fall off again.

shows a convenient shape. The electrodes are in the form of horizontal plates, and can be raised or lowered through the stoppers which support them; the graduations indicate the ratio conductivity conductance, and are determined by measurements with a standard liquid. With a vessel of this character the slide wire may be dispensed with; the plugs of a resistance-box forming the other three arms of the bridge are set to a definite value, and the electrodes moved up and down till there is silence in the telephone.

Finally, it may be mentioned that standard liquid resistances may be constructed, and sometimes used with advantage. It is, of course, important that they should be as little influenced by temperature as possible. Nernst recommends as having a negligibly small temperature coefficient a solution containing 121 grms. of mannite, 41 grms. boric acid, and o'06 grm. KCl in a litre. Its conductivity is 0'00097 at 18°. By using such a liquid resistance to balance the electrolyte cell the arrangement is made more symmetrical, and it is easier to obtain satisfactory readings with the telephone.

FIG. 23.

Water. In conductivity measurements it is indispensable to use very pure water.

The conductivity of the purest water

ever obtained1 is only o'04 X 10-6; but this is only possible when distilled and preserved out of contact with air. In air the conductivity rises to at least o'7 x 106, chiefly on account of absorption of carbon dioxide; and usually it is much greater than this, on account of ammonia compounds (and in laboratories hydrochloric acid) from the air, alkali dissolved from glass, dust, salts derived from contact with corks, the fingers, etc. Hence, when making up solutions to measure conductivity, the water should always be tested first: it cannot be considered satisfactory if its conductivity exceeds 5 x 10".

To obtain a satisfactory specimen, the water should be distilled with a little caustic alkali; the first fractions rejected; if necessary, distilled again from sulphuric acid. Glass apparatus may be used provided it is of good quality (glasses vary very much as to solubility in water) and has been well steamed out beforehand. The collecting vessel must, of course, be protected from dust, flame-gases, etc.

Water that contains much calcium bicarbonate, on distillation contains an excess of carbon dioxide, and may be improved by drawing a current of air through it.

Porcelain vessels are recommended by Kohlrausch for keeping water in. Corks and india-rubber should be avoided.

20

Sources of Error in Conductivity Measurements.-Conductivity is very largely influenced by temperature, the average coefficient for an aqueous electrolyte being about 2 per cent. per degree. Consequently, in order to obtain an accuracy of only one-tenth per cent., it is necessary to make sure of the temperature to degree. The conductivity vessel should therefore always be placed in a thermostat.2 The thermometer may best be put in the conductivity vessel itself, provided it does not come between the electrodes; or it may be put in the water of the thermostat near by. For high temperatures a vapour jacket is the most convenient heater, and the conductivity vessel should be adapted to suit it (Fig. 22).

1 Kohlrausch, Pogg. Ergzb., 8. 1 (1878); K. and Heydweiller, Wied., 53. 209 (1894); Kohlrausch, Zeitschr. phys. Chem., 42. 193 (1902).

"For construction of thermostats, see Ostwald-Luther, Physiko-chemische Messungen (Leipzig, 1902).

The current used for measuring generates heat in the electrolyte, and so warms it. Care must be taken to avoid introducing error in this way; the current from the induction coil should not be too strong, and should be broken when not wanted. The arrangement of Fig. 15 is good in this respect, as the current then only flows for a few seconds, while the sliding contact is actually in use, and pressed down on the wire.

Electrolytic polarisation, self-induction, and electrostatic capacity in any of the four arms of the bridge affect the balance to a certain extent. Fortunately, their influence shows itself first in want of sharpness in the telephone minimum; the point of balance is only affected secondarily, so that no error in measurement is to be feared so long as the "minimum" is sufficiently definite to observe with precision. The mathematical theory1 shows that the influence of

Self-induction is diminished, as already mentioned, by double winding the coils of the resistance-box. It may similarly be diminished in the leads, etc., by laying out and return wires close alongside one another. Short straight conductors, such as electrolytic cells, have very little self-induction. With ordinarily good arrangements its effect should be negligible.

Polarisation, of course, exists in the liquid cells only. It may always be made vanishingly small by making the electrodes. large enough, and covering them well with platinum or palladium black. If two arms of the bridge are made of liquid conductors it may be made approximately to balance out.

Capacity occurs in high-resistance coils, from which, however, it may be practically eliminated by Chaperon's method of winding; it is also appreciable in electrolytic cells when the electrolyte is of low conductivity, and between the electrolytic cell and a conductor outside the glass, such as the water of the 1 See Kohlrausch and Holborn, Leitrermogen d. Elektrolyte, p. 70.

thermostat. The last difficulty may be got over by placing the cell in a beaker of non-conducting liquid such as paraffin, and that inside the thermostat.

a

K

If, despite these precautions, capacity causes trouble, a small condenser should be inserted in another arm of the bridge, for the effect is always lessened by symmetry. Thus, if there is too much capacity in the arm c (Fig. 24), the condenser K (made of tinfoil or paraffined paper) is placed in the adjacent arm d. The resistance balance is first found as nearly as may be, and the capacity of K then varied until the minimum in the telephone is quite sharp.

FIG. 24.

§ 6. EQUIVALENT AND IONIC CONDUCTIVITIES.

In order to express the results of conductivity measurements in a convenient form it is necessary to introduce a new definition that of the equivalent conductivity. This is the conductivity divided by the concentration. We shall use the symbols κ for conductivity, and A for equivalent conductivity; then

where, as before, is the concentration in gram-equivalents per cubic centimetre, and v is the dilution, or volume, in cubic centimetres per gram-equivalent.

We have seen that the conductivity may be expressed in terms of the properties of ions by the equation-

It follows that

K=

k = 90000 m(U,+U)

A = 96600 у(Ua + Uc)

A

We may form a picture of the meaning of equivalent conductivity in this way. Suppose two metal plates of indefinite extent placed parallel and one centimetre apart; place between them I c.c. of solution (in the form of a prism or cylinder of 1 sq. cm. base). Then the conductance between