CONDUCTIVITY OF SiMMl'M HYDKOXIDK IN AQn-inrs SOLUTION. 295 



PART VII. INFLUENCE OF TKMPERATURE ON THE CONDUCTIVITY OK AQUEOUS 



SOLUTIONS OF SODIUM HYDROXIDE. 



Amongst the earliest investigations of the influence of temperature on conductivity 

 is that of KOHLRAUSCH (' Wied. Ann.,' 1879, voL 6, pp. 1 and 145 ; 1885, vol. 26, 

 p. 161), who measured the linear temperature coefficients of most of the solutions 

 examined, and in several cases determined also the coefficient of I 2 in the equation 

 KI = K O ( 1 + at + fit 2 ). He found that the salts gave positive values for ft, and the 

 acids negative values. A dilute solution of sodium hydroxide gave a very smail 

 positive coefficient, the relationship between temperature and conductivity being very 

 nearly linear over the range of temperature studied. Subsequent investigations have 

 been made by ARRHENIUS, who discovered that the coefficients for phosphoric and 

 hypophosphous acids become negative above 54 C. and 75 C. respectively, so that 

 the conductivity actually reaches a maximum value, whilst SACH, HOLLAND, and 

 TROTSCH ('Ann. Phys. Chem.,' 1890, III, vol. 41, pp. 259-287) showed that the 

 curves for certain sulphates and for the chlorides of copper and cobalt were inflected 

 at the higher temperatures, a supposed abnormality which they attributed to loss of 

 water of crystallisation by the dissolved salts. A detailed bibliography of tempera- 

 ture-conductivity measurements is given by KOHLRAUSCH and HOLBORN (' Leitver- 

 mogen,' pp. 145-158, 195-199). 



The first indication of any general law governing the temperature coefficients of 

 electrolytic conductivity was obtained by KOHLRAUSCH (' Sitz. Preuss. Akad. Wiss.,' 

 1901, voL 42, p. 1026), who found that in the case of the most dilute solutions, and 

 within the somewhat narrow range of temperatures investigated, the two coefficients 



in the formula 



Kt = KU {1 +a(t- lS) + ft(t- 18)*} 



were related to one another by the simple equation ft = 0'0163 (a 0'0174). In this 

 way it was shown that if a were known, the value of ft could be deduced without 

 any further experimental observations, so that between 2 C. and 34 C. the whole 

 influence of temperature on conductivity could be expressed by means of a single 

 arbitrary constant. The relationship between the coefficients may be expressed 

 almost equally well by the equation ft = -fa {a. -5^-}, and when this value for ft is 

 substituted in the equation for K/, it is seen that */ becomes zero when t \ 8 = 57, 

 whatever the value of a, in other words, the whole of the parabolic conductivity- 

 temperature curves must, if produced (without taking account of change of the 

 constants), converge to a common point on the axis of temperature, situated at 

 57 18 = 39 C. below the freezing-point of water. This point has been named the 

 " conductivity-zero " for water, and the most remarkable observation recorded in 

 KOHLRATTSCH'S paper is to the effect that the viscosity of water also extrapolates to a 

 limiting value at this point. It was thus evident that the common factor determining 



