small crystals which will include less mother 

 liquor than coarse crystals. In the case of very 

 soluble substances, it is advisable to cool in a 

 mixture of ice and water to obtain a good yield. 

 To remove adherent liquid, rinse the crystals 

 with small portions of cold water. The re- 

 crystallized salt, if sufficiently purified by one 

 recrystallization is then dried at. or above, room 

 temperature, depending upon the material. 

 The solid thus dried will contain included water 

 unless it has been heated to a high tempera- 

 ture. This water may be removed bj^ reducing 

 the crystals to a very fine powder, by grinding 

 in an agate or porcelain mortar, and again 

 heating. The complete expulsion of water is 

 naturally necessary only when the recrystallized 

 solid is to be used as a standard substance. 

 Included water can be removed from hydrated 

 salts by placing them in a desiccator. The 

 crystals tlien fall to a very fine powder, both 

 foreign and hydrate water being lost. The pure 

 dry salt thus obtained should be kept in glass- 

 stoppered bottles to protect it from atmospheric 

 contamination. 



Bottles with mouths ground on the outside 

 instead of inside are suitable for holding certain 

 standard substances. By lightly covering the 

 ground glass surface with petrolatum or stop- 

 cock lubricant, the bottle may be hermetically 

 sealed . This is especially desirable for hydrates 

 which may lose water in air of low humidity. 



13-18 Preparing Glass Tubing for Lab- 

 oratory Use. — Glass tubing is usually supplied 

 in bundles of 4-foot lengths. Tubing must be 

 cut to desired length, bent to desired shape, 

 and the ends fire polished for use. To cut glass 

 tubing, place it on a solid surface and, using a 

 triangular file, scratch the glass in one even 

 stroke at the desired place. Grasp the tubing 

 in both hands with the thumbs back of the 

 scratch. The scratch must face away from the 

 thumbs. Gently apply pressure. The tubing 

 should break cleanly without chipping. The 

 edges are fire polished by holding the tip in the 

 flame of a torch and rotating the tube with a 

 rolling motion between the fingers. 



To bend glass tubing, grasp the tubing a few 

 inches back on either side of the region to be 

 bent and, holding this region in the flame of 

 the torch, roll the tubing back and forth just 

 above the blue portion of the flame. When 

 the glass is soft and pliable, withdraw the tubing 

 from the flame and quickly bend to the desired 

 angle. This bending should be done with a 

 single motion. Doing it this way will prevent 

 the tubing from flattening at the bend and coii- 

 stricting the bore. 



To thread a piece of glass tubing through the 

 hole in a rubber stopper, first wet the fire- 

 polished end, then grasp the tubing a little 

 above the stopper hole and gently work it 

 through the hole. Never cup your hand over 

 the end of a piece of tubing to force it through 

 a stopper. Even if the end is fire polished the 

 tubing may break and a serious cut will result. 

 Never try to put a broken end of tubing or one 

 that is not fire-polished into a stopper or rubber 

 tubing. Alwaj's wet glass tubing before in- 

 serting it into rubber tubing. Never use a 

 piece of glass tubing or glass rod that does not 

 have fire-polished ends. 



13 19 DETERMINATION OF CHLORIN- 

 ITY AND SALINITY OF SEA WATER.— 

 Determining the chlorinity of sea water is of 

 fundamental importance to oceanography. As 

 mentioned in chapter 1, the knowledge of the 

 salt content, or salinity, is of great importance 

 in determining subsurface currents, underwater 

 sound transmission velocities, and water mass 

 transport and identification. The ratios of 

 concentrations of several major constituents of 

 sea water are virtually constant with the con- 

 centration of chlorinity. Of these, salinity is 

 probably the most important. 



Chlorinity is briefly defined as the number of 

 grams of chlorine, bromine, and iodine contained 

 in 1 kilogram of sea water, assuming that the 

 bromine and iodine had been replaced by 

 chlorine. Salinity is the total weight of dissolved 

 solids in grams, found in 1 kilogram of sea water 

 and may be determined from the concentration 

 of chlorinity. 



13-20 The Knudsen Method.— Professor 

 Otto Petterson introduced the titration of 

 chloride, which is now the basis for nearl}^ all 

 measurements of salinity in oceanograpliic 

 work. Refinements were made by Professor 

 Martin Knudsen and the present method was 

 named after him. This method is used ex- 

 tensively both in this country and abroad, 

 ashore and at sea. Chlorinity is determined by 

 titrating a sample of sea water with a solution 

 of silver nitrate, the strength of which has been 

 determined against a known sample. The 

 known sample, or standard, is standard sea 

 water and is explained in section 13-27. 



13-21 Chemicals Required. — The following 

 chemicals are required for salinity titrations. 

 The amounts in parentheses are those required 

 to analyze 10 stations of 24 bottles each. 



Iodine, crystals, C. P., A. C. S. (small 



bottle). (Sec. 13-12). 



Potassium chromate, crystals, C. P., 



A. C. S. (80 grams per jar — 1 jar). 



120 



H. O. 607 



