MEASUREMENT OF SALINITY OF SEA WATER JERRY H. SERVICE Special Publication No. 147 ye ATAL LIBR. ARY “EERE! NGE E COLL “GTO; WIL % H 3OL £ OCEANOGRAPHIC merirer * DEPARTMENT OF COMMERCE U. S. COAST AND GEODETIC SURVEY o¢ MBL/WHOI MI A UII DEPARTMENT OF COMMERCE U. S. COAST AND GEODETIC SURVEY ‘poe E. LESTER JONES, DIRECTOR * MEASUREMENT OF SALINITY OF SEA WATER BY JERRY H. SERVICE Junior Hydrographic and Geodetic Engineer U. S. Coast and Geodetie Survey Special Publication No. 147 0 0301 0093729 & PRICE, 10 CENTS Sold only by the Superintendent of Documents, U. S. Government Printing Office Washington, D. C. UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON 1928 PREFACE Excepting the preliminary calibration of the dipping refractom- eter to measure the salinity of sea water and of boiler water, this publication merely studies methods already available. Such slight modifications have been made in these methods as seemed to make them best suited to the hydrographic work of the Coast and Geo- detic Survey. Concerning the manipulation of the hydrometer at sea, valuable suggestions were received from reports on its use in the echo-sound- ing work of the Coast and Geodetic Survey ship Surveyor. In connection with the titration method, valuable help was given by Roger C. Wells, of the United States Geological Survey, and by J. W. Sale, of the Food, Drugs, and Insecticides Administration. Through the courtesy of Mr. Sale, the samples used in the prelimi- nary refractometer calibration were standardized, both by pycnom- eter and by chlorine titration, by the latter organization. For the excellent photograph of the refractometer the author is indebted to the manufacturers, the Bausch & Lomb Optical Co. For information concerning the electrical conductivity method the author is indebted to Dr. Frank Wenner, of the Bureau of Stand- ards, and to Dr. G. Guében, of the University of Liege. The sinker method described by Mr. Cummings and used at the Scripps Institution of Oceanography was made known to the writer by Dr. George F. McEwen of that institution. The tests made, and the present resultant publication, were made possible largely by the interest and encouragement of W. E. Parker, chief of the division of hydrography and topography of the Coast and Geodetic Survey, and K. T. Adams, commanding officer of the survey ship Lydonia. 'Thanks are also due the many other mem- bers of this bureau who cooperated in the work. II CONTENTS Page BERTc@zilee1 ( Comer ee SS a ee a ee II HATA tT CULL GLO lene ee eee eee oe Be ee ee 1 Ply COTE Ge TeueTe € tl © Ce ee ee ee ee 2 Chemicalehiiranonemeth od S-=—ae Ss ee ee 5 DippINomrehrAChOMeLe rene LWOd a= asa ee ee ee 11 Otheranethodsstor useronyboard ship == ee eee 15 DIECHEKGA| COMOlNVE MI MIAy WiC 15 Methodvot balancings columns of liquids] 222) ee 16 PVCHHOdSeLOr ADO ALOT sUSC= == 2 a= ae oe a ee ee ee 17 TEN ROUNOIMAVE NETS Fae aay |e i ee ee eee 19 Sinikkeraem @ til OC ee Soe See ie Ned a ee 19 Nalini bya byaevaporatlonaton OiyneSsue= = sees ee ee 20 ILLUSTRATIONS slew leley CSO 11d SECT SC Leer eet een eee ee aE ee a ee 2 2. Chart for conversion from specific gravity and temperature to salinity_ 4 SSH enmlcalkabited tl ONmOUt tes = ees LS ee ee 6 Hee MN oe Cha CCOMMeL EI sot ale eae eh a ee Se ee Sea ee alt 5. Preliminary refractometer calibration curves for sea water___________ 13 6. Preliminary refractometer calibration curves for boiler water________ 13 7. Electrolytic cell used for preliminary measurements of resistance of sea-water samples of various salinities______________-____________- 17 8. Apparatus for measuring salinity by the method of balancing columns OEE LUGE IS a ca es Ele ene ne BO cae see REN Geo ES 18 TABLES 1. Table for conversion from specific gravity and temperature to salinity_ 3 2. Independent determinations by several observers of the salinity of a given sample of sea water, by the hydrometer method_____________ 4 -3. Independent determinations by several inexperienced observers of the salinity of a given sample of sea water, by the chemical titration HOYT fs Hr © Cl seats aren ge ls a ea na oe eS Ee a 10 4, Independent determinations by several inexperienced observers of the salinity of a given sample of sea water, by the dipping refractometer TAT © Er (0 ees ete So cle ta Se cae BERS eI Se eT ee a 14 5. Variation with salinity of the electrical resistance of a column of sea SWELL Tp e aecen Serene Se are AS LSM 2 ak pads NE ee ee 16 MEASUREMENT OF SALINITY OF SEA WATER By Jerry H. Service, United States Coast and Geodetic Survey INTRODUCTION This publication has to do with methods available for use on board ship for measuring the proportion of total dissolved solids in sea water. The adoption of echo sounding for hydrographic surveys made necessary a study of the speed of sound in sea water and its variation with the depth and with the temperature and the salinity of the water. The effect of salinity upon the speed of sound is not great; a change of salinity through the whole range encountered in ocean waters produces scarcely more than 1 per cent change in speed. The accuracy requirements of this bureau demand, however, that the hydrographer shall determine the salinity of the water in which soundings are being made to the nearest gram per kilogram (part per thousand) ; that is, to two significant figures. It becomes necessary, then, to provide one or more standard methods that are simple of execution and that will give on board a moving ship and even in unskilled hands an accuracy such that there will be no doubt of the second significant figure of the result, and preferably a range of uncertainty in the third figure of not more than two or three units. Two inexpensive methods have been found that are satisfactory. When the motion of the ship is not great, hydrometers of good quality that have been calibrated have been found to give satisfactory results when properly used; and for use in any ordinary weather suitable for hydrography a chemical titration method has been worked out that requires little or no previous experience for its successful manipulation. The writer has calibrated the dipping refractometer to give salinity of sea water. Although the apparatus involved is rather expensive, so that probably it will not be supplied generally to the ships of the bureau, it is quite simple and easy to manipulate and will give good | results on board ship even in the roughest weather. It seems probable that an inexpensive electrical conductivity method can be worked out that will be easy of manipulation and give results adequate for the needs of the bureau. At the present time, however, 1 2 U. S. COAST AND GEODETIC SURVEY with the possible exception of the apparatus of Monsieur and Madame Chauchard, available conductivity apparatus is too expensive for the purposes of this bureau, principally because more precise than the requirements demand. The method of obtaining salinity by the method of balancing columns of liquids is given brief mention in the pages that follow. Finally, two standard laboratory methods, the pycnometer method and the sinker method, respectively, are discussed briefly. Fic. 1.—Hydrometer set HYDROMETER METHOD Hydrometer sets are furnished to ships of this bureau, as illustrated in Figure 1. The three hydrometers cover the specific-gravity range from that of fresh water to the greatest specific gravity likely to be encountered in ocean waters. Calibration data are furnished for the three hydrometers. The set includes also a 0° to 50° C. thermometer and a hydrometer jar. The following procedure is suggested for the use of the hydrometer : Pour the sea water to be tested into the hydrometer jar to such depth that, when the hydrometer is put in, the water will not over- MEASUREMENT OF SALINITY OF SEA WATER 5) flow. Stir the water with an unpainted stick. Observe the tempera- ture to the nearest tenth of a degree. Put the hydrometer into the jar and steady it from spinning. Holding the top of the jar between the thumb and first or second finger, allow the jar to hang freely, so that the water surface is seen from below through the glass and the liquid. Slowly lower the jar until the ellipse becomes a straight line, which is the bottom of the water meniscus in the jar. The intersec- tion of this straight line with the hydrometer scale will give the correct reading of the hydrometer. This reading should be made carefully to the nearest half division; that is, to one in the fourth decimal place of specific gravity. TABLE 1.—Apparent specific gravity of sea water [As indicated by a glass hydrometer with no errors of graduation] Temperature in degrees centigrade Salinity in parts |_ per 1,000 0 5 10 15 20 25 30 OI See eee ae ae 1, 02212 1. 02191 1. 02141 1. 02062 1. 01961 1. 01837 1. 01690 Oh ee 1. 02292 1. 02270 1. 02218 1. 02138 1. 02036 1. 01911 1. 01764 a) peas ase 1. 02372 1. 02348 1, 02295 1. 02214 1, 02112 1. 01986 1. 01839 Sle eee eee 1. 02453 1, 02428 1. 02374 1. 02292 1. 02188 1. 02062 1. 01914 Some eae ee 1. 02533 1. 02506 1. 02451 1. 02368 1. 02263 1. 02136 1. 01988 SB a5 eee eee 1, 02614 1. 02586 1. 02529 1. 02445 1. 02340 1, 02212 1. 02063 Sh a 1. 02694 1. 02664 1. 02607 1. 02522 1. 02415 1. 02287 1. 02138 SO een wee oe 1. 02774 1. 02744 1. 02685 1. 02599 1. 02492 1. 02363 1. 02213 GG) eee eect pene 1. 02855 1. 02824 1. 02763 1. 02676 1. 02569 1. 02439 1. 02288 Siete woes S! 1. 02935 1. 02902 1. 02841 1, 02753 1. 02644 1. 02514 1. 02363 Two or three readings should be taken, the hydrometer being dis- turbed between readings, to insure against a reading made with the bulb sticking to the side of the jar. It will be advisable to stir the water and make a second temperature observation after the hydrom- eter has been removed from the jar. The mean of the temperatures taken before and after the hydrometer readings should be used. At the conclusion of the observations the hydrometer, thermometer, stirring rod, and hydrometer jar should be well rinsed with fresh water. From the specific-gravity and temperature observations the salinity of the sample can be obtained conveniently by the use of the chart, Figure 2, or by Table 1, below If the temperature is measured cor- rectly to the nearest tenth of a degree centigrade and the specific gravity to one in the fourth decimal place, the resulting value of salinity will be correct to within one or two units in the third sig- nificant figure. In using Figure 2, which will be more convenient than Table 1, the point is found that corresponds to the observed temperature and specific gravity, both corrected for instrumental errors. The posi- tion of this point will show at once the first two significant figures of salinity. The third figure is obtained by estimating the frac- 4 U. S. COAST AND GEODETIC SURVEY tional part of the distance between the curves, measured parallel to the axis of specific gravities, that the point lies to the right of the curve of lower salinity. Thus, if the point lies three- ‘tenths of the way from the 34 curve to the 35 curve, the salinity of the sample is 34.3, and so on. If it noid be desired to use Table 1 instead of Figure 2, it will be found convenient to reduce the observed specific gravity (cor- rected for graduation error) to the temperature in the table that is nearest to the observed temperature. For example, suppose that the observed specific gravity (corrected for graduation error) is 1.0224 and the temperature, 22.°3 C. It will be seen in the table that the change per degree in the specific gravity is 2.5 in the fourth decimal place (in the part of the table corresponding to the observed specific gravity and temperature). Hence the change for 2.3 degrees will be 6 in the fourth decimal place. Thus the observed specific gravity and temperature are equivalent to 1.0230 at 20° C. By interpola- tion between 1.02263 and 1.02340 we find the salinity of the given sample to be 32.5. It will be seen from the table that a change of one unit in the second figure of salinity corresponds to a change of 7 or 8 in the fourth decimal place of specific gravity. Therefore, an error of one in the fourth decimal place of specific gravity will produce an error in the salinity scarcely greater than one in the third significant figure. Experiments were conducted recently on board the Lydonia to determine how consistently a hydrometer could be read on board ship when the ship was under way and rolling and pitching gently to a ight chop and swell. Several officers made independent specific- gravity and corresponding temperature measurements upon the same sample, with the results shown in Table 2. It will be seen from the table that the greatest discrepancy from the mean was one unit in the fourth decimal place of specific gravity. TABLE 2.—Independent determinations of the salinity of a given sample of sea water by the hydrometer method : Observer No, | Obsegved specif) Temperature, | Specie sanity j ree Maye dt odie 1. 02395 | 21.8 1. 02441 Der ea oe Oe 1. 02395 21.8 1. 02441 3 ec nies) oe ae 1. 02405 Pe yi 1. 02449 Aots 5 eee 1. 0240 PAW Ti 1. 0244 ein) Someevaan 1. 0239 21.9 1. 0244 (i ee ee 1. 02385 21.8 1. 02441 7 en ee ee): 1. 0240 21.7 1. 0244 SURES ae oe 1. 0239 PAS; 1. 0244- Miearil. is es Sa pe tk ae ea 1. 02442 Notre.—The above observations were made on board the Lydonia while she was under way and rolling and pitching to a light chop and swell. APPARENT SPECIFIC GRAVITY 1.0160 180 200 220 240 260 280 300 N i) i (=) ~ <-) = a = ey = i TEMPERATURE IN DEGREES CENTIGRADE = o. CONVERSION CHART H aE FOR SEA WATER FEN 4 SPECIFIC GRAVITY TEMPERATURE SALINITY an FAHRENHEIT to CENTICRADE | B30 RY Oy = Td EH Hie 32 0.0 60 15.6 33, 0.6 61 16.1 Beal secsstict SA) ded 62 16.7 HH = Bis. 7 63 17.2 HH tN 4 360 2B 64 17.8 estes eeeee 372.8 65 18.3 A 2 -38 3.3 66 18.9 F ; 393.9 67 19.4 pees eee Feet ASH £40 4.4 68 20.0 cH fe 415.0 69 20.6 t HE 42 5.6 70 21.15 HH f £45 6.1 71 21.7 Eee i ; Hit 44 66. 72 22.2 BH See tae! geen the Sf 45 702 73 22.8 S ES Reeeen eccere rE 46768 74 23.3 peeee eee : oes HE 4? 8.3 75 23.9 A Ha cH tH tt To 48 8.9 76 24.4 Eee ne EEE 499.4 77 25.0 EH H suagan gaaaes) esdacas Gesssea seseces teeseee. Sees 50 10.0 78 25.6 rH eeseete fe 51 10.6 79 26.1 a oh peas: &S H H H Ht 52 11.1 80 26.7 PEE Ht EEE H ACE 55 1107 81 27.2 14 HEH sudeeatecevaaazce EH H EH HH 54 12.2 82 27.8 HH ipeoesseeeseseaee! pak caine : 55 12.8 83 28.3 Ht HEH SALINITY IN PARTS PER THOUSAND : Beane : 56 (15.3 84 28.9 EEE EEE EEE EEEEEEE EEE EEE EEE ge: SeEeeua NE : HH 57 13.9 85 29.4 H HE } t rH 5814.4 86 30.0 : nEpeee : 5 H 59 15.0 iH Beat 7 He H aittecssiiit seen : es mI t 30 t HH i Ho imo 1 a fesse Boess egaabes) Gaezsen Geaeeas Wits } EEE EEE EH f ; { eeaee : HHH t : f aaa. puee : ry t Peat rH jebesedeees dees +H 13a f ; H HHH t t | eae ! ic HH t i | ff fe : sue : i EE + i fan oa 9 + { : 5 t - cL: ert t at +4 ital . aa FECEEET HEH HEEEEEEEEEEEHEEE EEE 4 isabd cussed becesesss BEEEEERHEES H 1.0160 180 200 220 240 260 280 300 APPARENT SPECIFIC GRAVITY As indicated by a glass hydrometer with no error of graduation Fic. 2 103509—28. (Face p. 4.) rs TEMPERATURE IN DEGREES CENTIGRADE =a) o a - ciesonenecsrss avers eveene e oes Gere ees ae USP rel eaeen OSs ; Tat MEASUREMENT OF SALINITY OF SEA WATER 5) CHEMICAL TITRATION METHOD This method consists in determining how much silver nitrate solu- tion of known strength must be added to a measured sample of sea water to precipitate the chlorine in the sample as silver chloride. The proportion of chlorine in the total dissolved solids is nearly con- stant in ocean waters, so that the amount of nitrate solution re- quired is a measure of the salinity of the sample. The observer is enabled to detect the completion of the precipitation by the addition of a small quantity of potassium chromate solution to the sea-water sample before any silver nitrate is added. As soon as the last trace of chlorine has been precipitated the next drop of nitrate solution added begins the precipitation of silver chromate, which is of a deep red color and brings about a sharp color change. It is customary to let the silver nitrate solution run down into the measured sea-water sample from a burette. The level of solu- tion in the burette is set at zero at the beginning, and the reading of the burette at the change of color will be the number of cubic centi- meters of nitrate solution required to precipitate all the chlorine from the sample. If 10 c. c. of sample is used and the nitrate solu- tion has been made up by dissolving 27.09 grams of chemically pure, dry silver nitrate crystals per liter of solution, then the read- ing of the burette when the color change occurs will be numerically equal to the salinity of the sample. The assumption is made here that the silver nitrate crystals are 100 per cent silver nitrate and that 55.25 per cent of the total dis- solved solids is chlorine. This proportion is in close agreement with Dittmar’s figures on the composition of average sea water and with the data in Clarke’s The Data of Geochemistry, and in fair agree- ment with Knudsen’s formula: Salinity =0.030+ 1.8050 X chlorine content where salinity and chlorine content are both expressed in parts per thousand by weight. Figure 3 shows the outfit needed for the chemical titration method. The locker can be constructed by the ship’s carpenter from Figure 3. The dimensions given in the figure correspond to those of the locker made up in the office for tryout on board the Lydonia,; the height can be reduced by 4 inches, the width by 2 inches, and the depth by 1 inch, if necessary; all measurements are inside measurements. The following chemical apparatus will be needed and can be purchased from any laboratory supply house: One half-gallon aspirator bottle, plain. One one-hole rubber stopper for aspirator bottle. NotEe.—Some aspirator bottles require 2 rubber stoppers, 1 at the mouth and 1 at the outlet. One rubber bulb, with hard-rubber valves, for pressure. 103509—28 2 6 U. S. COAST AND GEODETIC SURVEY One length soft glass tubing, about 14 inch inside diameter. Two feet rubber tubing, black, pure gum, heavy wall, 14 inch inside diameter. One burette, 50 ¢. c., plain (for use with pinch cock). One plain rubber stopper to fit top of burette. One burette attachment, for refilling from reservoir, consisting of glass tip, T-tube, rubber connections, and 2 pinch cocks. One glass tube, elbow shape, about 4 inch inside diameter, 114 inches straight tubing on each Jeg. This can be made on board by the use of a blow torch, if desired. Fic. 3.—Chemical titration outfit Two beakers, pyrex glass, 150 ¢. c. One evaporating dish, 250 c. c. One washing bottle, pyrex, complete, 500 c. c. One glass stirring rod. One transfer pipette, 10 ¢. ec. One transfer pipette, 1 c. c. One bottle, tincture, 4 ounce, with glass stopper. One volumetric flask, 2 liters. MEASUREMENT OF SALINITY OF SEA WATER i, The arrangement of the apparatus should be clear from the figure. Glass tubing is cut by marking it with a three-cornered file, placing the thumb-nails opposite the file mark and pulling the ends apart with a slight inclination away from the scratch. Raw ends of glass tubing should be “ fire polished ” by heating to redness in a blowtorch flame. : It will be found convenient to set the evaporating dish in a wooden block mounted in a set of boat-compass gimbals. This has not been done in the outfit shown in Figure 3. The parts of the apparatus, all of which appear in the figure, are designated there as follows: A.—Aspirator bottle (for silver nitrate solution). B.—Rubber pressure bulb (for developing a slight air pressure in the aspirator bottle). C.—Glass elbow tube. D.—Burette (should be fitted with plain rubber stopper at top). #.—Parts of burette attachment. F.—Hvaporating dish (the wooden block shown may be mounted in boat- compass gimbals; if the locker is made of the height shown in the figure, there will be plenty of room for this change). G.—Beaker (for supply of sea-water sample). H.—Stirring rod (not shown in the figure; should be where J is). J.—Transfer pipette, 10 c. c., for measuring out sample of sea water (out of place in the figure; should be in position 7). K.—Tincture bottle (for supply of potassium chromate indicator). L.—tTransfer pipette, 1 cc. (for adding potassium chromate, indicator to measured portion of sea-water sample in the evaporating dish). M.—Volumetrie flask, holding exactly 2 liters of solution to etched mark in neck (for use in preparing silver nitrate solution). N.—Extra beaker (for use in preparing silver nitrate solution). H P.—Wash bottle (to be kept filled with distilled water). Before the apparatus is assembled all tubing, stoppers, and con- tainers should be carefully washed and then well rinsed with distilled water. The silver nitrate solution must be precisely of the strength already mentioned, 27.09 grams of silver nitrate per liter of solution—54.18 grams for one filling of the volumetric flask. Enough silver nitrate solution for 50 to 60 titrations can be prepared quite easily as follows: Making sure that the wash bottle and volumetric flask are clean, fill the wash bottle and partly fill the volumetric flask with distilled water so that the volumetric flask can still receive at least a beakerful of solution below the mark in the neck. See that the beaker reserved for silver nitrate is clean, rinse it with distilled water, and carefully empty the weighed portion of silver-nitrate crystals into the bottom of it. Using the wash bottle, carefully rinse the inside of the bottle that held the crystals and pour all the rinse water into the beaker. Carefully wash down the inside of the sides of the beaker by means of the wash bottle and get the crystals into solution with as little agitation and as little water as possible. Pour the solution carefully 8 U. S.- COAST AND GEODETIC SURVEY into the volumetric flask, rinse out the beaker carefully with the wash bottle, and pour the rinsing water into the volumetric flask. Rinse down the inside of the neck of the flask with the wash bottle. Care- fully fill the flask with distilled water up to the mark in the neck so that the mark is just tangent to the bottom of the meniscus. If these instructions have been carried out with reasonable care, the solution will be quite accurately of the required strength. The rubber stopper and bulb may now be removed from the top of the aspirator bottle and the silver-nitrate solution poured in, making sure beforehand that both burette pinch cocks are closed. Replace the rubber stopper and bulb into the aspirator bottle, pump up a lit- tle air pressure, loosen the rubber stopper in the top of the burette, and by means of the upper pinch cock fill the burette partly full of solution. Place the beaker that is reserved for silver-nitrate solution under the burette and allow solution to run into it until all parts of the burette attachment, especially the glass tip, are full of solution, with no air bubbles. Again remove the rubber stopper and bulb from the top of the aspirator bottle, pour into the bottle the solution that has run down into the beaker, and replace securely the rubber stopper and bulb in the top of the bottle and the rubber stopper in the top of the burette. The purpose of the rubber stopper in the top of the burette is to prevent evaporation. It must be loosened when the apparatus is in use, but must be kept securely in place when the apparatus is not in use. Silver-nitrate crystals undergo chemical change when exposed to unfiltered sunlight, and it is for this reason that they are usually stored in amber-colored glass bottles. This colored glass seems to protect them quite effectively from the short, chemically active light waves. Care should be taken that the crystals are not exposed to the light unnecessarily. Silver-nitrate solution apparently does not deteriorate seriously under the action of light. J. W. Sale, of the Food, Drugs, and Insecticides Administration, says that they keep their solution in a clear-glass bottle exposed to light and that the solution appears to keep its strength indefinitely. He calls attention, however, to the recommendation in the United States Pharmacopeia that sil- ver nitrate volumetric solution be kept in an amber-colored, glass-stoppered bottle. An article by A. H. Clark, in the Journal of the American Pharmaceuti- eal Association, volume 1, 1912, page 228, states that a tenth-normal solution of silver nitrate was standardized in December, 1909, and when restandardized two years and eight months later was found to have the same concentration to the fifth significant figure. The solution apparently had been kept in an amber-colored, glass-stoppered bottle during the interval. It is considered quite safe, however, to keep the solution in a clear-glass aspirator bottle, especially since it will be exposed to light only when the apparatus is in use and the iocker is open. It must be admitted that the use of rubber connections in the apparatus is not desirable. Several chemists have stated, however, that trouble from this source is not likely if the glass tubes are placed close together at the rubber ~ MEASUREMENT OF SALINITY OF SEA WATER 8) connections, because the solution need not be kept for long periods. It might be well to keep a large portion of a given sample of sea water and determine its salinity at intervals with the apparatus. If the nitrate solution deteriorates, there will be an apparent increase in the salinity of the sample. The chemical titration outfit here proposed may undergo many improvements. For example, it may be found advantageous to use a Squibbs’s automatic burette, which could be connected to the aspirator bottle in such a way that the solution would not come into contact with anything except glass. In the meantime, while rubber connections are used, they should be renewed whenever the rubber ceases to be pliable. The potassium chromate solution must be made up with chemically pure chromate and good distilled water. Its concentration need not be precise, however, and it may be made up by a pharmacist or other competent person, as follows: Dissolve 5 grams of chemically pure potassium chromate crystals in less than 100 c. c. of distilled water. Add silver nitrate solution slowly until a blood-red precipitate is formed that does not disappear upon stirring. Filter and add enough distilled water to the filtrate to make 100 c. c. of solution. This solution should be kept in tincture bottle X. The outfit should now be ready for use. The procedure for deter- mining the salinity of a given sample of sea water is as follows: 1. Place some of the sample in the beaker that is toward the front of the case. Rinse the evaporating dish, 10 ¢. c. pipette, and stirring rod with some of the sample and then shake them clear of rinsing water. 2. Pump up a little pressure in the aspirator bottle and loosen the rubber stopper at the top of the burette. Fill the burette with silver nitrate solution, controlling the flow by means of the left-hand pinchcock until the zero mark on the burette is precisely tangent to the bottom of the meniscus of the nitrate solution. 3. Using the 10 c. c. pipette, measure out 10 c. c. of sea-water sample into the evaporating dish. Using the 1 c. c. pipette, add about 14 ec. c. of potassium chromate solution. 4. Allow silver nitrate solution to run down into the evaporating dish from the burette, stirring constantly, until a red tint appears that does not disappear upon stirring. The reading of the burette at the bottom of the meniscus will then be the salinity of the sample in parts per thousand (by weight) of total dissolved solids. The solution should be stirred vigorously near the “end point ” to coagulate the white precipitate of silver chloride and soak out the sodium chloride. The observer can usually estimate a minimum value for the salinity and let the nitrate solution run out of the burette rapidly down to the mark on the burette corresponding to the estimated minimum salinity. After that he should let the nitrate drip down drop by drop. Possibly one or more drops will come down after the color 10 U. S. COAST AND GEODETIC SURVEY change occurs. The observer should subtract 0.05 c. c. from the burette reading for each of these drops (including the one left hang- ing from the tip of the burette). 5. Replace the rubber stopper snugly in the top of the burette. Rinse the beaker, evaporating dish, stirring rod, and 10-c. c. pipette with fresh water or distilled water (from the wash bottle), and the 1-c. c. pipette with distilled water. The apparatus will then be ready for the titration of the next sample. Experiments were conducted on board the Zydonia to determine how consistent would be the values obtained for the salinity of a given sample by the above method by several officers that were wholly inexperienced in titrations. Each one made his determina- tion independently, without knowing the results obtained by the others. The ship was rolling 10° to 15°. The results are shown in Table 3. It is reasonable to expect that the results would be more concordant after each of the officers had made a few titrations. TABLE 3.—Independent observations of the salinity of a given sample of sea water by the chemical titration method | Oboaver No, | pustozeiting | saline j (ep Maier ec 34. 4 34. 4 pala ete Ne Sie a Be 34. 3 34. 3 oo on ha abe Se 34. 65 34. 65 pe eR Ly et 34. 4 34. 4 15 ae I gee SS 34. 5 34. 5 Note.—The first three observers had had no previous experience with this method. Professor Giral* states that variation in the proportion of chlorine in the total dissolved solids in different regions is such that no more than three significant figures are justified in salinity values computed from the results of chlorine titrations. From data obtained from Clarke’s The Data of Geochemistry it has been estimated that the variation from 55.25 per cent chlorine in total dissolved solids may cause an extreme error of three or four units in the third significant figure but not more than one unit in the third significant figure in the waters along the Atlantic and Pacific coasts of the United States. While the method described above will doubtless meet all the needs of the echo-sounding work of the United States Coast and Geodetic Survey, attention is called to the more precise, although somewhat more difficult, technique involving the use of the salinity outfit sup- plied by the Copenhagen Laboratory of the Conseil permanent inter- 1Conseil permanent International pour l’Exploration de la Mer. Publication de Cir- constance No, 90. MEASUREMENT OF SALINITY OF SEA WATER idutt national de la Mer. This apparatus is used by the United States Geological Survey, the Bureau of Fisheries, and other organizations. The outfit is described briefly by Dole and Chambers in Carnegie Institution Publication No. 213, 1918, page 309, as follows: An essential part of the apparatus is a calibrated burette so graduated that its reading igs approximately grams per kilogram of chloride if 15 c. ce. of sea water is titrated with a solution containing about 37 grams per liter of silver nitrate. A float in the burette assists in estimating tenths of the smallest divisions. 'The strength of the standard solution of silver nitrate is determined by carefully analyzed sealed tubes of standard sea water supplied by the Copenhagen Laboratory as a necessary part of the outfit. The apparatus is designed and constructed with the object of attaining maximum accuracy in titration. Special precautions observed in titration are precise measurement of the sample in an automatic pipette, vigorous stirring of the liquid by means of a glass rod flattened at the end, and observation of exactly Similar tints as end points. DIPPING REFRACTOMETER METHOD Figure 4 shows the Bausch & Lomb dipping refractometer, together with the heating trough and some of the glass cups (test tubes) into which the samples of liquid to be tested are placed. The dipping refractometer affords a convenient means of determining the salinity of sea water with fair precision, and it can be used satisfactorily on board ship in very rough weather. The instrument is designed to measure the refractive indices of liquids at known temperatures but must be used in connection with calibration charts, since it gives indications directly only in terms of an arbitrary scale. Looking into the telescope of the instrument the observer sees part of the field illuminated and part dark, with the dark and illuminated parts quite sharply separated along a chord in the focal plane of the objective. In the focal plane of the objective is mounted also a scale graduated from 0 to 100. By means of a micrometer screw the observer can measure to four significant figures the position on the scale of the chord of separation. The Coast and Geodetic Survey has purchased one of these instru- ments, and a careful preliminary calibration has been made, both for sea water and for boiler water. This preliminary calibration was carried out in July, 1927, as follows: A quantity of sea water of average composition was taken from the Atlantic Ocean off Beaufort, N. C., and divided into four por- tions; the first portion was left as it came and was of salinity 33.90; the second was partially evaporated to salinity 40.22; the third was diluted to salinity 31.25; the fourth was diluted to salinity 5.34. These four samples were standardized by the Food, Drugs, and In- secticides Administration, both by chemical titration for chlorine content and by pycnometer for density. Each of the four samples and a sample of distilled water was run several times with the refractometer from 3° to 26° C. The results 12 U. §. COAST AND GEODETIC SURVEY with the 31.25, the 33.90, and the 40.22 samples were plotted on one sheet, with scale readings as abscissas and temperatures as ordinates; Fic. 4.—Dipping refractometer the results with distilled water and the 5.34 sample were plotted on another such sheet. By interpolation between the 31.25, 33.90, and ae te se eS Pha = apes ive Refract ee Refractometer = f Di 0.5045 for Measurement of Hee pping ity of Sea Water Scale ion 0 In Sal a HH nites? HH N Calibrat apeibijuag saaubeg ui aunjyesedway HHH Scale Reading i=} IG, No.1 28. (Face p. 13.) 509. 103: 5