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LONDON, EDINBURGH, AND DUBLIN

PHILOSOPHICAL MAGAZINE

AND

JOURNAL OF SCIENCE.

[FOURTH SERIES.]

JUNE 1875.

XLVIII. On the Electric Conducting-power of the Chlorides of

the Alkalies, Alkaline Earths, and Nitric Acid in Aqueous
Solutions. By F. KOHLRAUSCH and O. GROTRIAN*.
THIS
HIS memoir is to form the commencement of a systematic

experimental investigation of the work of the current in the interior of electrolytes. In order to arrive at laws in this obscure department, the first requisite is a detailed inquiry into the facts. For the materials we possess are very imperfect, and for the most part inexact; and the measurements, with few exceptions, have not been referred to a sufficiently defined unit; so that up to the present only a slender basis is afforded for more general points of view. Thus much, however, is already known, that the relations are by no means simple; and accordingly, in order to analyze them, it will be advisable to commence with simple chemical combinations and examine these in groups.

On this account we have operated first upon the chlorides of the alkalies and alkaline earths. The observations refer to thirtyfive different solutions of them, and show the dependence of the conducting-power on the amount of salt contained, and on the temperature from 0° to 40° C. Chloride of lithium is the only one that was examined merely in very dilute solutions.

Of the acids, we have previously experimented on sulphuric and hydrochlorict. We now add nitric acid, about the electricity-conducting power of which almost nothing was known. Observations have been made on it in seven proportions of solution; and thereby a sufficient knowledge for all purposes has been gained.

* Translated from an abstract, communicated by the Authors, in the Nachrichten von der königl. Gesellschaft der Wissenschaften zu Göttingen, No. 17, August 5, 1874.

+ Nachrichten, 1868, p. 415; Pogg. Ann. vol. cli. Phil. Mag. S. 4. Vol. 49. No. 327. June 1875. 2 G

The method used for measuring the resistances was the one first described in the Nachrichten (1869, Nov. 14)—that of quickly alternating currents, which, with some subsequent improvements (namely, the production of a convenient inductioncoil for the alternating currents, the application of Wheatstone's bridge to the dynamometer, and the introduction of platinized electrodes *), in simplicity and accuracy leaves nothing to be desired. Indeed the determination of the temperature of the liquid under experiment now presents greater difficulties than the measurement of the resistance, if the same degree of accuracy is demanded for both.

We have been most careful to make sure of the sensibly complete exclusion of polarization of the electrodes, which, when constant currents were used, hindered the exact measurement of the work of the current in decomposable conductors. To mention one of the tests applied in this direction, we experimented upon a solution of sulphate of zinc, first between electrodes of awalgamated zinc (which generally give no polarization), and then between the platinized platinum electrodes of 2000 square millims. surface which were employed for all the subsequent measurements. The greatest difference between the resistances found corresponded to a temperature-error of about to of a degree. With the zinc electrodes, constant as well as alternating currents were used; and at the same time, by the accordance between the results, it was established that the work of the alternating currents follows the same laws as that of a constant current.

From the commencement onward, the materials for observation were so arranged as to facilitate comparison when put together in Tables. The solutions contained approximately 5, 10, ... (or, in the case of nitric acid, 6.2, 124,...) per cent.; and the temperatures were near 0°, 18°, and 40°; so that reduction to exactly these proportions was attended with no risk of error.

Our thanks are due to Professor Büchner, Dr. Heumann, and Dr. Rössler for the preparation and analysis of the most concentrated solutions of each substance. The other solutions were prepared from these by weight.

The specific gravities (at 18°; water at 4° equal 1) present a second definition, independent of the analysis, of each solution.

The electric conducting-powers k, given below, are all referred to that of mercury at 0° as unity. Siemens's standards no. 1135 and no. 1143, which were made use of for the reduction of th

• Pogg. Ann. Jubelband, p. 290, vol. cli. p. 378.

sec

mercury unit to absolute measuret, served for this reduction. The resistance of a column of liquid of 1 square millim. base and 1 millim. length, is found to be, in absolute measure, 9717000 millim.

millim..milligr. This, in

is at the same k

sec.2 time the work of the unit current which passes this column in a second.

Scarcely any property of bodies depends to so great a degree upon the temperature as the conductivity of electrolytes, which at middle temperatures is influenced as much as ten times as powerfully by heat as the pressure of a gas. On this account observations of the resistance without statement of the temperatures of liquids possess but little value.

But even apart from this, the influence of temperature is here of singular importance, precisely on account of its unusual magnitude; for it follows that the electro-chemical work of the current stands in intimate relation with the thermal condition of the liquid, the tracing-out of which relation may supply an invaluable explanation on the nature of electrolysis. We have comprehended the observations of each solution in the formula

ke=ko(1+at+Bt), in which ke signifies the conductivity at temperature t.

Besides these constants ko, a, and B, the following Table con. tains the conducting-power at 18° multiplied by 108, and, finally,

1 dk under

the increment for 1° in the vicinity of 18°, exk dt pressed in fractions of the conducting-power at 18°.

The percentages denote parts by weight of anhydrous salt, or of nitric-acid hydrate, in parts by weight of the solution. The specific gravities are for 18°.

The solutions marked with an asterisk (*) have not been analyzed; but their content was taken, according to the specific gravity, from R. Hoffmann's Tabellen für Chemiker. The conducting-powers &c. set down for the bracketed percentages were interpolated from a graphic representation of the results, and are here and there uncertain to a few units in the last place. The most concentrated solution of NH, Cl precipitated some crystals at 0°, when a leap in the conducting-power was not observed. Two solutions of MgCl, were examined only at 18° and 30°; and two of SrCla, at 18° only. The strongest nitric contained a little nitrous acid.

+ Nachrichten, 1870, p. 513. It is not unimportant to remark that the present comparison of the two standards gave, to within gobo, the same ratio as that made four years previously.

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NaCl. 5 1.0346 10 1.0710 15 1•1089

1.1482 24 1.1802

KCI. 5

1.0309 10 1.0639 15 1.0978 21 1.1410

LiCl. 5 1.0274

1.0562 NH, Cl. 5 1.0142 10 1.0289 15 1.0430

1.0570 25 1.0724

Caci, 5 1:0409 10 1.0853 (15) 1•1312 20 1.1795 25 1.2306

1.2843 35* 1.3420

MgCl, 5 1.0416 10 1.0861 (15) 1.1295 20* 1•1765 (25) 1.2257 30* 1.2780 34 13212

Baci. 5 1.0446 10 1.0940 15 1.1475 (20)

1.2051 24 1.2564

SrCi, 5 1.0443 10 1:0931 15 1:1456 (20)

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452 829 1151 1398 1480

0.0218

204

-0.000037 -0.000025

1•2023 22 1•2259

HNO3. 6.2

1.0346 12:4 1.0717 (18.6) | 1:1105

24.8 1.1525 31.0 1:1946 37.2

1.2372 (43.4) 1.2786

49.6 13190 (55.8) 1.3560 62.0 1.3871

2118 3731 4830 5402 5462 5206 4790 4274 3770 3296

184
190
198

-0.000003
-0.000008
-0-000001

2924
5072
6460
7185
7319
7062
6550
5935
5290
4646

0.0148

143 138 138 140 146 152 158 16) 158

212 +0.000020

232-0.000027

The dependence of the conducting-power of the chlorides on the temperature shows, according to the above, great simplicity in many respects. The universally small amount of the coefficient B proves that, with all solutions, the conducting-power increases in nearly equal proportion with the temperature; the positive sigu of B, that each of the slight deviations consists of an acceleration.

With so strongly pronounced a dependence as we have here (with which 30° rise of temperature about doubles the conducting-power), this nearly equal proportional augmentation could not à priori be expected. It has, however, been observed also in sulphate-of-zinc and sulphuric-acid solutions*, and appears to be a universal property of liquid conductors. Viscous substances only, such as concentrated solutions of chloride of calcium, chloride of magnesium, and sulphuric acid, exhibit greater inequality.

A further, very remarkable fact is the near approximation to equality of the temperature-coefficients for the different chlorides in dilute solution. Those at 18°, for example, for all 5-per-cent. solutions, lie between it (for LiCl) and a (for NH, CI); the graphic representation permits the conjecture that with further dilution they would come still nearer together; nay, it is probable that they tend to the same limit (about 13). And certainly this limit cannot signify the temperature-coefficient of pure water, since the conducting-power of this is generally a vanishing

quantity in comparison with the numbers in the above Table. The temperature-coefficient of sulphate-of-zinc solution, too, observed by Beetz, appears as the dilution is increased to approach towards about the same limit.

With increasing amount of salt contained, all the temperaturecoefficients at first diminish. Afterwards the substances divide theniselves into two groups: KCI, NH, CI, and BaCl, show a diminution of the coefficients up to the greatest concentration, the coefficient sinking in the case of NHACI to the lowest value, 4. NaCl, CaCl,, and MgCl2, on the contrary, have a minimum between 10 and 20 per cent.; and thence onward the coefficient rises, that of MgCl, even to 3's. This group-difference appears to be connected with a maximum of conducting-power with the salt-content, exhibited by the latter substances, but not by the former. (Compare what is stated below.)

Nitric acid connects itself with the latter group. In the sign of B changing from – to + it agrees with sulphuric acid; yet the inequality of the augmentation between 0° and 40° is generally slight. The absolute amount of the influence of temperature is less than with the chlorides, and not very different from that observed with hydrochloric and sulphuric acids.

* Beetz, Pogg. Ann. vol. cxvii. p. 21 ; Grotrian, ibid. vol. cli. p. 394,

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