DETERMINATION OF SALINITY 



The salinities were measured by the conductivity 

 method using a Wenner salinity bridge (Wenner, Smith, 

 and Soule, 1930). This instrument was of the type de- 

 signed by Dr. Frank Wenner, of the Bureau of Standards, 

 originally for the International Ice Patrol Service. It 

 consists essentially of an alternating current Wheat- 

 stone's bridge, two adjacent arms of the bridge being 

 formed by two similar electrolytic cells, the other two 

 arms being made up of two fixed coils of manganin wire 

 between which is a slide wire. The electrolytic cells 

 are immersed in a stirred water bath which is thermo- 

 statically controlled at constant temperature. A sub- 

 stitution method is employed so that cell constants and 

 absolute conductivities need not be known. Sea water, 

 the salinity of which is unimportant within limits, is 

 placed in one of the c§lls and sea water of known salinity 

 is placed in the other cell. A small resistance in series 

 with the first cell is then adjusted until the bridge is 

 balanced when the slide-wire reading corresponds to the 

 salinity of the known sample. This samp'e is then with- 

 drawn and replaced by the unknown sample which is to 

 be measured. The bridge is balanced this time by the 

 adjustment of the slide wire, thus giving the conductivity 

 of the unknown in terms of the known. The conductivi- 

 ties may be converted into salinities, but it is custom- 

 ary to calibrate the instrument by the measurement of a 

 number of samples of known salinity so that the slide- 

 wire reading may be converted directly into salinity 

 without a knowledge of the relation between salinity and 

 conductivity. 



It can be assumed that the relation between salinity 

 and conductivity is linear, but not proportional, over the 

 range encountered in sea water. On this assumption the 

 relation between slide -wire readings and salinity in an 

 instrument of this sort can be expressed by an equation 

 of the type 



S=S' [1 + A(s-s') +B(s-s')2 +C(s-s')3....] (1) 



in which s is the slide-wire reading corresponding to 

 any salinity S, and s' is the slide-wire reading corre- 

 sponding to the salinity S', and A, B, C are numeri- 

 cal constants depending on s', S', the relation between 

 salinity and conductivity, and the constants of the bridge 

 circuit. The numerical limits of the salinity range of 

 such an instrument are fixed by the ratio of the resist- 

 ance of one division of the slide wire to the resistance 

 of the two bridge arms which include the slide wire, and 

 by the arbitrary selection of the slide-wire reading s' 

 which will correspond to the salinity S'. In the Carnegie 

 instrument the slide wire had 1000 divisions, each of 

 which had a resistance of 1/15,000 of the sum of the re- 

 sistances of the two adjacent bridge arms which includ- 

 ed the slide wire. Under these conditions, terms in 

 equation (1) involving (s - s') to exponents greater than 

 3 are negligibly small and the third-degree term need 

 only be considered when (s - s') is numerically large. 

 When s' is selected near the middle of the slide wire, 

 the second-degree equation can be used with negligible 

 error. 



In the case of this instrument a slide-wire reading 

 s' of 699.5 was selected as corresponding to a salinity 

 S' of 35.00 per mille, so that a second-degree equation 

 can be used to express the calibration curve. If any 



irregularities existed in the slide wire the calibration 

 curve would have had departures from the curve of such 

 an equation since the development of the equation as- 

 sumes direct proportionality between slide-wire reading 

 and slide-wire resistance. 



Time did not permit of a test being made for uni- 

 formity of the slide wire before the departure of the 

 Carnegie in May 1928. The preliminary calibration of 

 the bridge was therefore made in the following manner. 

 Standard water from the International Bureau at Copen- 

 hagen having a salinity of 34.99 per mille was placed in 

 the test cells and the bridge was balanced with the slide 

 wire set at a reading of 098.5. The slide-wire readings 

 at balance were then determined for five other samples 

 of known salinity furnished by the Scripps Institution of 

 Oceanography, and titrated against Copenhagen standard 

 water by H. R. Seiwell. A curve was then drawn through 

 these six well-distributed points". From time to time, 

 as the cruise progressed, some of the samples which 

 were measured in the bridge were also titrated against 

 standard water in a Knudsen burette by the silver nitrate 

 method. Each of these samples furnished an additional 

 point on the calibration curve. All such points ultimate- 

 ly obtained are shown in figure 1. The origin of these 

 samples and the comparison values are given in table 1. 



Because of the considerable range of room tempera- 

 ture encountered in a cruise such as that of the Carnegie , 

 two regulating temperatures were provided for. In colder 

 weather the water bath was regulated at a temperature of 

 about 30° C and in the tropics a temperature of about 40° 

 C was used. In the hope that the slope and curvature of 

 the calibration curve at 40° C would be practically the 

 same as for 30° C, the same arbitrary point, namely, 

 salinity 34.99 per mille at slide-wire reading 698.5, was 

 selected for each temperature. The points determined 

 at a regulating temperature of 30° C are shown in figure 

 1 by circles and those determined at 40° C by crosses. 

 The arbitrarily selected point is shown as a solid circle 

 and cross. As there were no systematic differences be- 

 tween the points determined at the two temperatures, all 

 points could be used in determining the calibration curve. 

 This meant further that the exact temperature of regula- 

 tion was unimportant as long as it did not change materi- 

 ally during a series of measurements. 



Figure 1 includes all com.parisons made on the Car - 

 negie between bridge and titration methods. None was 

 discarded. It includes all differences arising from both 

 instrumental and observational error in both bridge and 

 titration measurements, as well as any differences aris- 

 ing from variation in salt ratios in samples from differ- 

 ent localities. As an individual bridge measurement is 

 accurate to about 0.01 to 0.02 per mille salinity, and as 

 an individual titration is subject to a similar error, it 

 was expected that the points would scatter over from 

 0.02 to 0.04 per ftiille on each side of a smooth curve. 



The second-degree equation whose curve fits the 

 points shown in figure 1 is 



S = 35 [1 + 295.7 X lO'^ (s - 699.5) 



+ 46. X 10-9 (s . 699.5)2] (2) 



The slide-wire readings of all the points shown in figure 

 1 were converted into salinities by this equation and 

 their differences from the titration values plotted against 



67 



