The Hydrogen-ion Concentration, etc., of Sea-Water. 45 



No special precautions are necessary in making the solutions. 

 Reagent borax and boric acid needs no recrystallization, but com- 

 mercial products must be recrystallized by dissolving in hot distilled 

 water and filtering and cooling, then removing the mother Hquor as 

 much as possible and rinsing the crystals in ice-cold distilled water. 

 The boric acid may be dried in a desiccator, but must not be heated 

 above 70° to hasten the drying. Borax crystals effloresce in air on 

 the surface, and powdery crystals must be discarded. It is usually 

 sufficient to discard the surface layer of crystals from the bottle. The 

 Na content of the borax may be titrated with 0.1 n HCl, using methyl 

 red or dibrom-o-cresolsulfophthalein as indicator. It was found, how- 

 ever, that the eye was more capable of detecting efflorescence of the 

 crystal before it was dissolved than by the use of methyl red afterwards. 

 The other indicator is more sensitive and a flask containing the equiva- 

 lent quantity of boric acid and NaCl may be used to get the end-point. 

 The titration does not, however, distinguish between impurity and loss 

 of water. 



The borax solution may be prepared from boric acid and CO2- 

 free NaOH, but it takes more time to do this unless it is done at the 

 same time that the solutions for determining the non- volatile buffer are 

 prepared. Reagent NaOH contains more carbonate than is apparent 

 on the surface of the sticks and must be purified. For this purpose, a 

 saturated solution in distilled water is prepared (100 grams NaOH-j- 

 100 c.c. H2O), and after thorough mixing it is sealed in a number of 

 nonsol test-tubes by fusing the glass. These are placed upright in a 

 thermos bottle (Dewar flask) or wrapped in cotton wool, in order to 

 retard convection currents that stir up the precipitate. In 2 or 3 days 

 the solution in the middle of each tube will be clear, the insoluble 

 carbonate having settled, except for a little on the sides and the surface 

 film. The only way to prevent a trace on the surface film is to com- 

 pletely fill the tubes, which can only be approximated. Owing to the 

 high viscosity of the solution it can not be measured with delivery 

 pipettes, and it is very convenient to have pipettes whose capacity is 

 known (which can be determined by weighing when empty and when 

 full of water and multiplying the difference by the density of water at 

 that temperature). A tube is opened by cutting off the sealed end, 

 the pipette tip is plunged to the center, and the sample quickly drawn. 

 The meniscus is adjusted, the outside is wiped off, and the NaOH 

 washed out of the pipette into a beaker with distilled water and 

 titrated. If the opened test-tube is to be used again it must be quickly 

 sealed with a lump of soft paraffin and inclosed in an air-tight container 

 or C02-free compartment, preferably in a thermos bottle, for the 

 disturbed carbonate to settle again; but it is advisable to use a tube 

 only once. The solution of boric acid is prepared in a volumetric 

 flask with distilled water freshly boiled in a metal container, but 



