32 Papers from the Department of Marine Biology. 



the C0 2 . Any C0 2 in the water will be estimated as non-volatile 

 buffer and influence the result. The equivalent of about 1 c.c. of 0.1 n 

 NaOH is added to the boiling sea- water; the volume is quickly and 

 accurately restored with boiling distilled water, and the flask tightly 

 closed with a rubber stopper. The NaOH solution must be free from 

 C02 and contain little silicate. The method of its preparation and 

 protection is given on page 45. The exact quantity of NaOH to be 

 added can not be stated, but there should not be sufficient to cause 

 precipitation of earthy hydroxides. It is best to have two series of 

 tubes of known P n , one colored with thymolsulfophthalein and the 

 other with either o-cresolsulfophthalein or a-naphtholsulfophthalein 

 in other words, a total range of P H = 7.5 to 9 or greater, but only the 

 first series of tubes are necessary. Two nonsol test-tubes of exactly 

 the same bore as the standard tubes and with etched marks at the 

 volume to be titrated are provided, each with a rubber stopper with a 

 central perforation closed with a glass rod. The required quantity of 

 indicator is placed in each of the tubes and sea-water from the flask 

 poured in (to the mark) and the stopper quickly inserted. Care must 

 be taken never to breathe toward the test-tube unless it is tightly 

 closed. The tube with thymolsulfophthalein is first examined and 

 compared with the standard tubes and its P H recorded. The glass rod 

 is removed from time to time and small quantities of 0.1 n HC1 are 

 run in from the burette. The tube is shaken and its P H is recorded, 

 together with the burette reading. 



This process is repeated until the limit of the indicator is nearly 

 reached, when enough indicator is added to correct for the increased 

 volume and any error due to dilution of the indicator is recorded. The 

 same process is repeated with the other indicator and the results are 

 compared with figure 4. 



If the excess base is known the total C0 2 may be determined from 

 the P H by using the conversion table in figure 5. The three diag- 

 onal lines are for sea-waters containing 23, 24, and 25 excess base 

 respectively. The smallest possible difference in C0 2 per liter that we 

 have been able to determine by means of the standard tubes is 0.1 c.c., 

 but in order to do this it is necessary to have a finer gradation of tubes 

 than are ordinarily used. Such a degree of accuracy is only relative, 

 since the absolute accuracy of the conversion table probably does not 

 exceed 1 per cent. The diagonals in the conversion table are drawn as 

 straight lines, but this is merely an approximation. If they were 

 extended they would have to be curved or lose in accuracy. The sea is 

 of too complex a composition to admit of any simple mathematical 

 relations. Empirical formulae, such as the one developed by Fox for 

 estimating the total C0 2 , may be discovered, but they are only approxi- 

 mations. Exact data may be obtained only by direct experiment, and 

 the result depends on the technique. 



