The Hydrogen-ion Concentration, etc., of Sea-Water. 47 



to the small part of the time during which we had sunlight, or even 

 sufficient daylight, we tried mazda lights and found that we could 

 compare o-cresolsulfophthalein (with some loss hi accuracy) by means 

 of a 250-watt stereopticon lamp, but this would not do at all for thymol- 

 sulfophthalein. The best artificial light we have tried is a 100-watt 

 "daylite" mazda (blue bulb) with aluminium reflector, but we have great 

 difficulty in making the light symmetrical, probably owing to irregu- 

 larity in the thickness of the blue glass. This adjustment is made 

 easier by a further dispersion of the light by means of a sand-blasted 

 glass placed several inches behind the opal glass, but in every case it is 

 necessary to verify a color-match by interchanging the tubes. The 

 lamp was placed as close as possible to the tubes, but it might be 

 supposed that the use of a larger lamp or thinner opal glass, thus 

 enabling the lamp to be placed at a greater distance, would more 

 nearly approximate the conditions with sunlight. We did not ascer- 

 tain that the opal glass is colorless, although it appears pure white. 

 The sand-blasted glass had a slight greenish hue noticeable on the 

 edge, but we were unable to obtain colorless sand-blasted glass before 

 the close of the experiments. The first attempt at using the color- 

 imeter may lead to the supposition that the tubes may be matched 

 more accurately by simply holding them before a piece of opal glass hi 

 the window, but this is due to failure of symmetrical lighting in the 

 colorimeter. With a little experience it is possible to distinguish tubes 

 in the colorimeter, which appear alike without it. 



The indicators may be used in aqueous solution, since dilution of the 

 sea-water has little effect on the P H . In our experiments, however, it 

 was necessary to use very concentrated solutions of the indicators, 

 because we used the same sample of sea-water for the hydrogen 

 electrode, the colorimeter, and the analysis of total CC>2, and dilution 

 has an enormous effect on the C0 2 content at constant COo tension. 

 We made up 0.1 per cent solutions of the indicators in ethyl alcohol 

 redistilled over sodium. The o-cresolsulfophthalein dissolved on 

 heating, but the thymolsulfophthalein had to be assisted by a little 

 CCVfree NaOH. Enough NaOH to bring the color of the indicator to 

 the middle of its range makes it more sensitive, but unless it is pro- 

 tected from C0 2 it will absorb quantities of this gas and become 

 correspondingly insensitive. If NaOH is used to assist solution, it is 

 safer to add an equivalent of HC1, as the small amount of this salt 

 formed will not be appreciable when mixed with sea- water. Since we 

 used only 0.1 c.c. of 0.1 per cent alcoholic solution of the indicator for 

 10 c.c. of sea- water, we do not believe the alcohol influenced the 

 indicator method appreciably, but we suspect that both alcohol and 

 indicator tended to depolarize the hydrogen electrode and increase the 

 time necessary for equilibrium. 



