6332 HYDROGRAPHIC MANUAL PaGE 586 



Salinity may also be determined by chemical titration. This method is based on 

 the fact that the proportion of chlorine in the total dissolved solids is nearly constant 

 in all sea water (see 633). A silver nitrate solution of known strength is used to precipi- 

 tate the chlorine as silver chloride and a small quantity of potassium chromate solution, 

 free from chlorides, is used as an indicator. The instruments and method of chemical 

 titration, as well as other methods of determining salinity, are described in Special 

 Publication No. 147, Measurement of Salinity of Sea Water. 



Water samples are ordinarily not retained unless they are desired for scientific 

 purposes*. Field parties sometimes survey in areas where few scientific analyses of the 

 water have been made and water samples from such areas are frequently desired by 

 oceanographic institutions. Special instructions will be issued by the Washington 

 Office in such cases and the oceanographic institution will generally furnish specimen 

 bottles and issue directions for preserving the samples and instructions for shipment, 

 which is usually made at the expense of the institution. The specific gravity of such 

 water samples is immediately measured by hydrometer in the usual manner, so that the 

 salinity may be determined for immediate use, and the sample is then bottled and 

 marked for identification. The salinities of water samples sent to oceanographic 

 institutions are generally furnished later to the field party. They should be recorded 

 in the proper spaces on Form 717 and used in the velocity computations, if received in 

 time. 



A graph of the salinities with reference to depth should be plotted for every serial 

 temperature observation, in the space provided on Form B-1 528-5 on which the serial 

 temperatures are plotted. 



6332. Frequency of Determinations 



The range of salinity is usually small, and compared to temperature it has com- 

 paratively little effect on the velocity of sound. It usually increases uniformly with 

 depth. In general a water sample for use in determining the salinity should be 

 obtained at the surface and bottom as a part of each serial temperature; and a sample 

 at one intermediate depth should be obtained occasionally. The salinities for other 

 depths may usually be interpolated with sufficient accuracy from the salinity curve. 

 Determinations of salinity should be well distributed as to area and throughout the 

 season and should yield sufficient information to ensure the accuracy specified in 6313. 

 Where the temperature varies erratically, the salinity will usually vary in a similar 

 manner and additional salinity determinations will be required. 



634. Records and Computations 

 6347. Records of Observed Data 



The observed temperatures and the observations to determine the salinity in a 

 serial temperature, and all scattered observations of temperatures and salinities, must 

 be recorded on Form 717, Record of Temperatures, Salinities, and Theoretical Veloci- 

 ties. This form contains a number of columns in which the data should be entered; 

 one horizontal line should be used for each observation at each different depth. 



The form is designed for use in recording both serial temperature observations and 

 scattered bottom or surface observations. A separate copy of the form should be used 

 for each serial temperature, the date of the observation being entered only on the date 

 line at the top of the form. Wlien the form is used to record scattered bottom or 



