Densities and Boiling Points of Sea Water Concentrates CLIFFORD A. HAMPEL Armour Research Foundation of Illinois Institute of Technology, Chicago, Ill.
b
weight, its density becomes ENSITY data for sea Densities of sea water concentrates have been deterwater for the various mined over the temperature range of 20" to 90" C. for Sama/aznds. Hydrometers, caliples of the following per mille chlorinities and salinities, brated in 32nds a t definite concentrations encountered in the oceans of the world respectively: 17.69 and 31.96 (the original Atlantic Ocean temperatures with samples of water used for the study); 28.39 and 51.27; 38.16 and 68.90; sea water of known concentrahave been determined with great accuracy by several 51.75 and 93.43; 62.58 and 112.99; and 76.62 and 138.33. tions, are used t o measure the oceanographic laboratories. The last sample was saturated with respect to gypsum "density" of sea water in The values published by (CaS04.2Hz0), the first salt, with the exception of small maritime circles, but no relaamounts of calcium carbonate, to precipitate as sea water tionship between this system Knudsen (3) of the Hydrographical Laboratories, is concentrated. With these density data, a simple and actual density appears to Copenhagen, Denmark, are density measurement at a known temperature will permit have been determined. one to determine very closely the concentration of a sea The knowledge of the denwidely accepted for their high sity of sea water over a wide water sample. Boiling points of sea water concentrates degree of accuracy, but for the over the same concentration range have been measured. range of concentrations and purposes of those engineers A simple method of determining the densities of salt s o h temperatures up t o the point interested in concentrating tions over a wide temperature range is described. at which sea water becomes sea water, they cover only a narrow concentration rangesaturated with respect to calcium sulfate, the first salt namely, from 1 t o 23 per mille to be precipitated when sea water is concentrated (neglecting chlorinity, and a temperature range of 0' t o 35" C. as given in "International Critical Tables." Sea water of 23 per mille the minute quantities of ferric oxide and calcium carbonate which chlorinity is equivalent t o 41.55 per mille salinity, as compared are precipitated at lower concentrations), will make it possible to relate absolute deneity t o the above 32nds system, or t o any other with 35 per mille salinity of average sea water. The widespread and growing use of sea water evaporation as a system of designation. Chlorinity is related to the 32nds concept by the expression: source of fresh water has required the handling of sea water con= 17.31 per mil chlorinity, a straight line function. centrates, whose chlorinities are much higher than the above Thus, 2/aznds = 2 X 17.31 per mil chlorinity, and so on. For range, a t temperatures above 35" C. A few scattered density determinations a t greater concentrations and more elevated the convenience of those who may desire t o convert the chlorinity temperatures have been published. Clarke ( 1 ) calculated a fev basis of expressing sea water concentration t o the 32nds system, points from experimental data obtained by Usiglio (Y), one of Figure 1 is included. It is a plot of chlorinity, in per mil, versua 32nds. which is in fair agreement with Knudsen's. The others relate to SEA WATER SOURCE solutions from which some salt has precipitated, and, therefore, are not applicable t o this problem. Normandy ( 4 ) published The sea water used for these experiments was procured by the United States Coast Guard Cutter Tamaroa, Commander E. A density values for sea water which check neither Knudsen's nor Coffin, Jr., commanding, in the Atlantic Ocean at a locatioq Clarke's figures; his data approximate the density values for approximately 180 miles east of Atlantic City, latitude 39" 24 sodium chloride solutions, and may have been determined on north, longitude 71 0 46' west. The sea water temperature was sodium chloride solutions. Faraday (2) reported one density 76" F. New paraffin-lined, 50-gallon oak barrels were filled directly from the ocean through previously flushed canvas-type figure which agrees fairly well with Knudsen's table. contamination* The tightly bunged barrels hose t o avoid Present data, accordingly, are inadequate, and since density is were shipped by rail to Armour Research Foundation from the one of the more valuable basic properties of a solution, it was Coast Guard ~~~~i~ B ~ St,~G ~ ~,Staten~ Island, ~ N. y. ~ ~ decided to measure the density of various sea water concentrates For laboratory use, the water was withdrawn from a barrel via a dass siphon tube t o a 5-gallon glass bottle. It was removed from over a wide temperature range. The study was conducted in conjunction with a project, sponsored at Armour Research E Foundation jointly by the United States Coast Guard and the Navy Department, Bureau of Ships, to investigate the control of 4 scale in sea water evaporators. It is anticipated that accurate knowledge of the density-concentration relationship will allow 8 the determination of the salt content of any given sample of roncentrated sea water without resorting to chemical analysis. The United States Navy and Coast Guard, and other marine E agencies, as well as their British counterparts, all use a simplified means of expressing sea water density, the 32nds system. Actu1 ally, this system of designation refers, not t o density, but t o concentration. It is based upon the assumption that there is 1 part of total salts in 32 parts of sea water, and expresses the concentra0 10 20 30 40 50 60 70 Bo tion of any sea water sample as a ratio of parts of total salts to 32 CHLORINITY, 0100 parts of sea water. For example, if sea water of l/sznd density iF Figure 1. Relation of Chlorinity and 32nds Sysconcentrated to 50% of its original volume, strictly, of its original tems of Expressing Sea Water Concentration
f=
k*
383
,
IN DUSTR IA L A N D EN GI N E E R I N E CHEM I STR Y
384
the stoppered bottle, as required, by mcans of an affixed glass siphon tube. SEA WATER ANALYSIS
The analysis of the sea water is given in Table I, which also includes the analysis of average sea water, as well as the standard ratios of the principal ions present in sea water t o the chlorinity. Although sea water varies in concentration of total salts from ocean to ocean, the compovition of its soluble constituents in relation t o one another is remarkably constant in the several oceans, and t.he ratios of the principal ions to chlorinity are found to he fixed within narrow limits (6, 6).
TaBLE
I. ANALYSISO F 8E.i WATERGSED
(Compared x i t h sea water average) X-ater Used 7.37 a t 30' C. btnsity, g./cc. t ,0197 a t 30" C. 31900 Salinity,, mg./kg. 17692 Chlorinity, mg./kg. 5870 Sodium, mg./kg. 1175 Magnesium, mg./kg. 389 Calcium, mg./kg. 354 Potassiuin, mg./kg. 2475 Sulfate, mg./kg. Bromine (calcd. from chlorin60 i t y ) , mg./kg. 11.4 Free C O Z (dissolved), mg./kp. 91.6 Combined COz, mg./kg. 103,O Total COz, mrr./kg. Ratios 0.5575 Na:CI Ca:Cl 0.0220 0.0665 blg:Cl 0.0200 K:Cl 0.1358 80a:Cl
AT. ITater
7 6-8.4 1,0243 a t 20° C. 34325 lEi000 10561 1272 400 380 2645
io0 0 . 5556
0,02106
0,06G94
0,0200 0.1395
This particular lot of sea water was somewhat more dilute than the average, and its pH also was somewhat lovier. In other respects, it differed from average sea n.ater only t o a negligible degree. The analytical methods employed for the determination of the pertinent ions were those used by oceanographic laboratories (6, 6). An exception was the measurement of the free and combined carbon dioxide. A 100-ml. sample was titrated t o a phenolphthalein end point with 0.01 N sodium hydroxide, and then backtitrated with 0.01 N sulfuric acid to a methyl purple end point (pH 4.70). The first titration is equivalent t o the free carbon dioxide, the second t o the total carbon dioxide, and the difference to the combined carbon dioxide. Inasmuch as it is recognized that chlorinity is the more accurate measure of the concentration of t,otal salts in sea water no salinity determination was made. The preference for chlorinity i s derived from the constancy of the ratios of the various anions and cations t o chlorinity, and t o the inherent inaccuracies of the salinity determination, as adequately discussed by Sverdrup, Johnson, and Fleming ( 5 ) . Chlorinity is related to salinity by the expression, Salinity = chlorinity X 1.805 0.03 (1)
+
and is itself defined as "the number of grams of halogens, calculated as chlorine, capable of precipitation by silver nitrate from a kilogram of sea water." The sea water used in t'hese experiments did not change appreciably during the period over which the tests were made, in fact, a t the end of 1 year, its analysis was almost identical with the original analysis. PREPARATION O F SEA WATER CONCENTRATES
The concentrated samples of sea m-ater were prepared by evaporating the sea water in a lowside chemically resistant glass beaker under a n infrared lamp a t about 75" C. (169 F.). During the evaporation, a stream of carbon dioxide-free air was passed over the surface of the solution. Evaporation is effected a t the surface by this method, and heat does not enter through the O
Vol. 42, No. 2
container bottom or xvall, 'Thus, a temperature gradient through a heat transfer surface is avoided, and no precipitation of salts on the container bottom or wall is experienced. Another advantage of this evaporation technique is that no portion of the solution reaches a temperature much higher than that of the main body of the liquid. Natural circulation results from t.he increase in density a t the surface as bhe concentration of the surface layer rises; this density difference causes the upper layer to sink toward the bottom of the beaker. The method essentially is identical with solar evaporation. Each solution was analyzed for chlorinity by titrating a weighed sample with 0.1 N silver nitrate by the Mohr method, using sodium chromate as the indicator. DENSlTY O F SEA WATER CONCENTRATES
Determination of the density of a salt' solution a t elevated temperatures is difficult and subject to many unavoidable errors. A somewhat unorthodox method was chosen which reduces both the errors and the number of readings. This method also is applicable over a wide range of temperatures. A yeighed quantity, a t least 250 ml. in volume, of a, solution o f knomm chlorinity was placed in a 250-ml. Cassia flask, a volumetric type, thin-walled, glass flask, whose long neck contains 25 ml. The neck is calibrated in 0.1-ml. divisions, the zero point corresponding to 230 ml. I n other words, the flask is a 250-ml. container with a 25-ml. buret as a neck. This flask r a s heated in a glass battery jar, 18 inches tall and 6 inches diam-ter, containing water. The bath was heated slowly, a t t'he rate of 0.5 t o 1 C. per minute, t o various temperatures, as measured by a thermometer hung in the bath and outside of, but touching, the flask. Temperature and volume readings were taken simultaneously. Heating was done in a step-wise fashion SQ that a t intervals t,he volume and t,emperature remained static. From these "hesitation points," a smooth curve of temperature versus volume was constructed. By dividing the original weight of water in the flask by the volume reading a t any one t,emperature, the density could be calculated, and a density-temperature curve n-as drawn for the oarticular concentration of s o l u ~ion used.
250
252
254
256
298
2@3
262
VOLUME OF LIPUtD,CC.
Figure 2. Calibration of Cassia Flask for Volume Occupied by 250.6 Grams of Water at 20" to 90" C.
The apparatus was first calibrated with a weighed quantity of distilled water (250.6 grams) in the Cassia flask, and corrections over the temperature range of 20' t o 90" C. (68" to 194" F.) were determined. T h e maximum error of volume observed by this calibration was 0.4 cc. in a total volume of a t least 250 cc., an error of 0.16%. Tests were made of the temperature lag between the flask contents and the bath by comparing readings of standardized thermometers in the flask and in the bath. At the slow heating rate used, practically no difference was detected. The calibration curve shown in Figure 2 is based upon the density of water, in grams per cc., a t various temperatures. The flask and its contents were weighed on a triple-beam pan
February 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
385
0.
Figure 3.
Densities of Sea Water Concentrates us. Temperature
balance, accurate to within 0.1 gram. This error, when carried over t o the density calculated from the weight of solution used, results in an error of 0.0004 gram per cc., and is the major error in this experimental method. One possible source of error in the volume-temperature experiment is the loss by evaporation of water from the flask. Such loss would result in a smaller volume, and a higher density by the calculation based upon the original weight of water in the flask. This error proved t o be negligible. A loosely fitting glass stopper was kept in the month of the flask during the test, and the only loss of water from the flask was equal t o the volume present as vapor in the air displaced from the neck of the flask as the temperature was raised, a n amount less than 0.01 gram. When checked by weighings before and after a test, no difference could be detected. Owing t o the fact that the water bath was maintained a t a high level on the flask neck, very little condensation was obtained on the upper wall of the neck of the flask. Such condensation would tend t o decrease the observed volume. That it was small and of minor effect was shown by taking volume readings at the same room temperature before and after a test; no difference was noted in these readings. Density data were determined for sea water whose chlorinity was 17.69 per mil, and for five samples of concentrated sea water whose chlorinities were 28.39, 38.16,51.75, 62.58, and 76.62 per mil, respectively. The last chlorinity represents a concentrate saturated with respect to gypsum (CaSOr.2HzO). No published data are available for comparison with any concentrated sea water, but the values for the original sea water match the Knudsen values ( 3 ) closely over the range of 20' to 35" C. In this range, the values found were 0.3 t o 0.6 part per thousand higher than the reported values, as shown in Table 11. The Knudsen values were calculated by a straight arithmetical extrapolation of figures from the tables given in "International Critical Tables" ( 3 ) . The experimental results are listed in Table 111. The volumes given are corrected volumes based upon the calibration of the Cassia flask. The family of curves obtained from these experimental data is presented in Figure 3, a plot of density, in grams per cc., versus temperature in degrees centigrade. Included therein are a curve for distilled water and a curve of the density values of Knudsen for the original sea water over the range of 20' t o 35" C. A family of isotherm curves for the density of sea water concentrates versus chlorinity in per mille is given in Figure 4. The
curves are smoothed averages of points taken from the data plotted in Figure 3; the temperature in Figure 4 is expressed in degrees Fahrenheit, the temperature scale most commonly used in this country in evaporator operation.
TABLE11. DENSITYVALUESFOUND FOR SEA WATEROF 17.69 O / O O CHLORINITY
Temp., C. 20 25 30 35
(Compared with Knudsen values). Density, Density, Found Knudsen, G./Cc.' G./Cc. 1.0231 1.0225 1.0216 1.0211 1.0198 1.0195 1.0181 1.0177
7c
Difference +0.06 +0.04
+0.03 +0.04
TABLE111. TEMPERATURE-VOLUME-DENSITY EXPERIMENTAL DATA OF SEAWATERSOLUTIOKS Temp., C.
Volume, Co.
Density, G./Cc.
SALINITY(31 g60/00) CHLORINITY (17.6g0/oa), WT: O F S d L N . (256.9 G . )
Temp., C.
Volume, Cc.
Density, G./Cc.
SALINITY(51 27O/oo) CHLORINITY (28.3g0/oo), w>.O F &LN. (261.9 G.) 25.0 1.0354 33.0 1.0325 1.0282 43.8 1.0236 53.3 1.0169 66.8 1.0163 67.7 1.0110 76.8 1.0046 87.0
SALINITY(68.gOO/oo) CHLORINITY SALINITY(93 43O/oo) CHLORINITY (38.16°/aa), WT. O F S 6 L N . (263.9 G.) (51.75O/oo), W;. O F S d L X . (272.0 G.) 22.7 251.36 1.0499 25.0 254.58 1,0684 1.0481 39.3 255.99 1.0625 28.0 251.79 41.0 253.03 1,0430 49.3 257.15 1.0578 52.7 254.44 1,0372 61.5 258.68 1.0515 63.6 255.85 1.0315 75.1 260.59 1.0438 86.8 262.29 1.0370 82.0 258.58 1,0206 88.3 259.63 1.0165 SALINITY (1 12.99°/oa), CHLORINITY OF SOLN.(275.2 G.) (62.580/00), WT. 253.75 1,0845 22.8 253.94 1.0837 25.4 255.20 1 0784 38.1 256.51 1 0729 49.5 257.72 1.0678 59.3 259.96 1 0586 76.0 261.49 1.0524 86.4
SALINITY(138 33O/oo) CELORINITY (76. 62O/oo), WT. OF S O ~ N(280.7 . G.) 254.54 1,1028 24.6 38.6 256.00 1.0965 257.25 1.0912 49.8 258.45 1.0861 59.4 260.71 1.0767 76.3 86.0 262.14 1.0708
386
Vol. 42, No. 2
INDUSTRIAL AND ENGINEERING CHEMISTRY
Densities of Sea Water Concentrates us. Chlorinity at Various Temperatures
Figure 4.
BOILING POINTS O F SEA WATER CONCENTRATES
No data on the boiling points of sea water or sea water conuericrates are given in the commonly used handbooks, in “International Critical Tables,” or in such works on oceanography as (6) and (6). The boiling points of sea water and its concentrates are of interest in the operation of evaporators, and it was decided to determine them experimentally.
Various samples of known chlorinity were subjected t o slow boiling in a small 100-ml. flask, equipped with a reflux condenser. A short, National Bureau of Standards calibrated thermometer, range 42” t o 105? C., was hung in the flask with an immersion of about 1 inch. Boiling was controlled by use of a small, shielded gas flame under the flask. The tests were made a t a barometric pressure of 747 mm. of mercury. The boiling point of water a t this pressure is 99.6” C (21 1.3”F.).
vi L-
9 3O
IO
Figure 5.
TABLE 1V. BOILISGPOINTS OF SEAWATEROF VARIOUSCHLORISITIES [Barometer, 747 m m . (29.41 inches) Hg] Chlorinity Boiling Point
CHLORINITY, PER MILLE
20
30
40
50
60
70
80
90
Boiling Points of Sea Water Concentrates
bonate that precipitates prior t o reaching this point is so minor that its effect upon density changes is nil.
I _
B/oo
18.0 20.4 29.4 48.3 61.2
c. *
0.1 100.3 100.7 1 0.1 100.9 * 0 . 1 101.3 1 0 . 1 101.9 * 0 . 1
F. 212.6 213.3 213.6 214.3 216.4
* 0.2 * 0.2 * 0.2 1 0.2 * 0.2
ACKNOW LEDGM E N 1
The writer wishes to thank Maurice Kayrier for his analysis of the sea water used in the study. Louis G. Smith aqsisted in the preparation of the graphs used in this article. BIBLIOGRAPHY
Table I V contains the boiling points and corresponding chlorinities of the samples of sea water tested. The data are plotted in Figure 5 . CONCLUSION
The density determinations reported herein fill a gap in the knowledge of the basic properties of sea water and sea-water concentrates. As a measure of the concentration of salts in sea water, they should be useful, for one can determine very closely the concentration of a sea water sample by making a simple density measurement at a known temperature. The density-concentration relationship, of course, is valid only up t o the concentration at which salts, with the exception of calcium carbonate, arc precipitated from solution. The amount of calcium car-
(1) Clarke, F. ‘A‘,, U . S. Geol. Survey, Bull. 770, 218-220 (1925) (2) F a r a d a y , M., “Faraday’s Diary,” Val. 1, p. 66, London, George Bell & Sons, L t d . , 1932. (3) Knudsen, M ., “Hydrographische Tabellen,” Copenhagen, 1901; see “International Critical Tables,” Vol. 3, p. 100, New York. McGraw-Hill Publishing Co., 1926. (4) Noimandy, F r a n k , “A Practical M a n u a l of Sea W a t e r Distillat i o n , ” p. 26, L o n d o n , Charles Griffin & Co., L t d . , 1909. ( 6 ) Sverdrup, H. U., Johnson, M . W., a n d Fleming, R. H., “The Oceans,” New York, Prenticc-Hall, Inc., 1942. (6) T h o m p s o n , T. G., a n d Robinson, R. J., Bull. Nail. Research Council, 85, 95-203 (1932). (7) Usiglio, J., Ann. chim. et phya., 3rd series, 27, 92-172 (1849). RECEIVED April 14,1949
