SEA-FISHERIES LABORATORY. 253 



curves by Walpole, 16 which may be utilised instead of the 

 tables. Minute directions are given by Sorensen for the 

 preparation of the stock solutions from which the mixtures are 

 made up at the time of each experiment. In making up the 

 mixtures matters are so arranged that the volume of the 

 comparison mixture is always 10 c.c. For example, calling 

 the solutions A and B, a series of mixtures is made containing, 

 say — (1) Nine of A and one of B, (2) Eight of A and two of B, 

 (3) Seven of A and three of B, and so on. In an additional 

 tube 10 c.c. of the sea- water is taken of which the hydrogen-ion 

 concentration is to be determined. All the tubes are placed 

 in a special test tube rack so that they slope at an angle of 45° 

 to the horizontal and rest on a white surface such as milk- 

 glass. An equal and definite volume of the coloured indicator 

 to be used is now added to each tube and the tube containing 

 the sea-water to be analysed for its hydrogen-ion concentration 

 is carefully compared as to tint of colour with each tube, and 

 that one selected with which it most closely matches. After 

 a rough approximation a series of tubes lying closer in their 

 steps of variation can be chosen. That one which matches 

 closest is taken as having the same hydrogen-ion concentration 

 as the sample to be tested. In the case of sea-water, after 

 identification and establishment of the hydrogen-ion potential 

 by table or chart, the salt error deduction must be made. 



Naturally, different coloured indicators, and different 

 comparison mixtures, are selected according to the level of 

 hydrogen-ion concentration of the solutions to be tested. In 

 our experiments on sea-water about Port Erin we have always 

 loin id phenol-phthalein appropriate as a coloured indicator, 

 and the most useful comparative solution-mixture that known 

 as " borate mixture " and hydrochloric acid. The " borate 

 mixture " contains 12404 grams of boric acid and 100 c.c. of 

 normal caustic soda in one litre, and was carefully prepared 

 according to Sorensen's directions. The hydrochloric acid 



