Flame Photometric Determination of Chloride in Sea Water MINORU HONMA
U. S. N a v a l
Radiological Defense Laboratory, San Francisco
This investigation involves expansion of the flame photometric technique to the quantitative determination of chloride in sea water. By modifying the “Beilstein chloride test” a determination of chlorides in the range of 0.02 to 0.50M has been accomplished. The source of copper is copper nitrate which is introduced directly into the chloride solution; when aspirated into the hydrogen flame the CuCl band systems appear. By measurement of the CuCl hand peak of the D system, which occurs at 435.4 mp, the intensity of the emission is found to be proportional to the concentration of the chloride present. Although the sensitivity is not very good, the method provides a useful extension to existing methods for the determination of chlorinity in more concentrated solutions.
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LAME photometric determinations have been quantitatively applied in the analysis of many cations in various substances during the past 10 years. Other aspects of these determinations have included viscosity effects and particle size affecting the determinations of sodium, potassium and calcium as studied by Caton and Bremner (4). However, very little work has been done on the quantitative estimation of anions, except for studies in which the anionic interferences in relation to metallic cation emissions were investigated (1-3, 5 , 11, I d ) . Recently Dippel, Bricker, and Furman (6) applied the flame photometric technique in the quantitative determination of phosphates by measuring the depression of the calcium emission. Band spectra have been used to a certain extent in flame photometry such as, for example, in the determination of calcium by using the calcium oxide band head a t 554 mp. The CuCl band system which occurs in the blue region has been used extensively in the qualitative identification of chlorides in the form of the “Beilstein chloride test.” This flame test is dependent on the appearance of the CuCl bands of which the 435.4-mp band of the D system is the most prominent (8, 15). The possibility of a quantitative application of this test was first noted in a publication by Honma and Smith (IO),who used the spectrographic technique and photographed the CuCl spectra. By modifying the Beilsteiu test, a flame photometric method for the determination of chlorine in Bea water has been developed. APPARATUS AND REAGENTS
Photometer. The photometer used was the Beckman DU spectrophotometer equipped - .. with a Model 9290 oxyhydrogen - burner attachment. Hood. The flame Dhotometer was daced in an efficient hood and, as a further precaution, a filter system was devised to collect the oxidation products of the flame, particularly noxious copper combustion products. A 2-foot stack equipped with copper heat radiators and a glass wool filter paper collector a t the top was placed about 4 inches over the burner. A positive displacement air pump operating at 14 cubic feet per minute sucked the combustion gases and oxidation products through this filter system. Manometer. A water manometer was placed between the fuel pressure gage and the burner to get more accurate readings of the hydrogen pressure, since the investigation was made a t very low hydrogen pressures. Reagents. All reagents used were C.P.grade chemicals. Synthetic Sea Water. The standard solutions of chlorides were prepared from a stock solution of synthetic sea water prepared by referring to the table of ionic concentrations of natural sea water given by Sverdrup (16). Lfodification involved the replacement
24, Calif.
of all chloride values with equivalent amounts of nitratw. The synthetic spa water stock solution consisted of: Compound NazSOd NaHCOa NaBr .\Ig(NOdz.6H20 Ca(NOn)z.4HzO KNOI Sr(N0s) 2 NaNOs
Grams per Liter 3.917 0.192 0.0823 13.411 2.360 0.981 0.0317 34.0635
Copper Solution. The 1.26M copper solution was prepared by dissolving 304.45 grams of cupric nitrate trihydrate in distilled water and making up to 1 liter in a volumetric flask. This was then filtered through a fine sintered-glass funnel to remove the insoluble materials. PROCEDURE
Preparation of Standard Chloride Solutions. Chloride solutions ranging from 0.025 to 0.5051 were prepared by adding to each 10-ml. volumetric flask 5 ml. of 1.26M copper nitrate solution, 2.5 nil. of synthetic sea water stock, and 2.5 ml. of the a p propriate standard sodium chloride solution. Each standard solution prepared corresponded to a 1 to 4 dilution of the natural Rea water except for the chloride value. A blank solution in which distilled water was used instead of the chloride solution was also prepared a t the same time. These solutions constituted Rtandards for the determination of chlorinity in sea water. About 0.5 hour after preparation in some of the higher chloride samples, a faint cloudy precipitation started which did not settle appreciably even after 24 hours. When shaken thoroughly, this cloudiness did not impair the flame photometric readings. The precipitate was later identified as amorphous basic cupric chloride. Preparation of Sea Water. The sea water unknown was prepared in a manner similar to that used for the standard Five milliliters of 1.2631 popper nitrate were pipetted into a 10-ml. volumetric flask, 2.5 ml. of sea water were added, and the solution was made up to volume with distilled water. Both standards and unknown solutions were run off immediately on the flame photometer. Flame Photometric Technique. Data for the calibration curve were obtained by running the standard chloride solutions prepared with the synthetic sea water matrix and copper nitrate solution. All measurements rvere made at the wave length 437 mp The oxygen pressure was maintained a t 13 pounds per square inch and hydrogen pressure a t 1 inch of water as indicated by the manometer. Because of the low sensitivity a slit width of 0.34 mm. was selected. At this low hydrogen pressure there was considerable gas pressure fluctuation which was barely observable on the manometer and not detectable with the Beckman gage. However, these small changes in the hydrogen pressure were evident on the flame photometric readings. Even a change of inch in the manometer reading caused considerable error. First, the approximate concentration of the unknown chloride was obtained and the standards necessary for bracketing it were run off. I n the final determination the unknown solution was run off betneen two standard solutions a t most 0.04N apart. The solutions were run off consecutively as fast as an accurate reading could be obtained. Usually three repetitions rTere sufficient. The procedure was repeated for the next sample DISCUSSION
Results. The reproducibility of the method was checked b v using the burning procedure adopted for the determination of chlorinity in synthetic sea water. Table J shows the reproducibility data obtained. Accuracy of the determinations was obtained by comparing the flame photometric results with the Mohr titration of chlorides (12). The results of a determination of chlorinity in sea water are given in Table 11.
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V O L U M E 27, N O . 10, O C T O B E R 1955 Table I.
value of 435.4 mfi (15). To produce the desirable spectra of optimum signal to noise ratio, considerable adjusting of the hydrogen was necessary, since the energy of the flame and the quantity of hydrogen introduced presented difficulties. The competing reactions of interest in this flame are:
Reproducibility of llethod
0.22O.M NaCl 0 222 0 220 0 218 0.224 0 220 0 218 0 222 0 221
Table 11.
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,M Found 0.140M XaC1 0 142 0 142 0 140 0 130 0 141 0 140 0 138 0 142
Cu Cu Cu
Determination of Chloride in Sea WateF Mohr's Method, M c10.1368 0.1376 0.1378 0.1379
Flame Method, M C10.137 0.134 0.136 0.133 0.135 9 v . 0.1376 0.138 Chloride value of sea water is for 1 to 4 dilution and includes bromide and iodide. Standard deviation is 0.0018 or 1.3% for flame method. Sample Sea water No. 2
Production of CuCl Spectra. Preliminary investigations on the satisfactory production of CuCl spectra were first attempted with the original Beilstein's method using a copper wire ring with a copper screen. All results were unsatisfactory. It was necessary to find another means of introducing the copper ions into the flame and this was successfully accomplished by dissolving copper nitrate in the chloride solution. Figure 1 shows the CuCl spectra obtained a t the most intense portion of the blue region after a scan of that region. The flame spectra of copper nitrate and copper sulfate are also shown. The peak emission occurs a t 437 mfi shifted to the right from the literature
+ CuO* emits in the same region as CuCl + HP = CuH* emits in the same region as CuCl + C1 = CuC1* desirable a t maximum intensity 0 2 =
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Spectrum of 0 . 4 M copper chloride
Calibration curve for sodium chloride at 437 mfi
Of these reactions the first two constitute the background. The dissociation energies listed are CuO, 4.5 e.v.; CuH, 2.89 e.v.; and CuCI, 3.0 e.v. (9). From the energy consideration a relatively cool flame would reduce the background of the CuO system ( 7 , 1 3 ) . However, the oxygen pressure cannot be reduced since the atomization of the sample as well as the copper ion concentration in the flame are dependent on this pressure. But by lowering the hydrogen pressure, the flame energy is reduced and the formation of the CuH background is also minimized a t the same time, since the concentration of hydrogen is decreased in the flame. Therefore, by maintaining a normal oxygen pressure and a very low hydrogen pressure just sufficient for a flame 0.5 inch high, the flame energy was enough to excite the desired spectra, although there still was considerable background emission. Effect of Some Variables. Of all the variables involved, the hydrogen pressure xas the most critical. A minute change in the pressure gave considerable differences in the CuCl emission. Figure 2 s h o w the calibration curve for the chloride in the form of sodium chloride. A slight difference in hydrogen pressure resulted in the two curves, one run at 0 7 and the other a t 0.75 pound per square inch gage of hydrogen. To find the effect of copper concentration on the CuCl spectra, a series of experiments was performed in which increments of copper nitrate were added to known concentrations of chloride. The chloride concentration was the same for each solution in the series. A parallel family of curves including the curve for the background emission of copper nitrate was formed in this study and when the background was subtracted from the appropriate points in each curve a horizontal family of curves past the 1 t o 1 ratio of copper t o chlorine was obtained. As seen in Figure 3,
ANALYTICAL CHEMISTRY
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Figure 3. Effect of copper nitrate on copper chloride emission at 437 m p (corrected for background)
the c'orrected curves were parallel to the abscissa indicating that excess copper beyond the 1 to 1 ratio had no effect on the CuCl emission over the range investigated. Since copper was added in the form of the nitrate to develop the CuCl flame bands, the nitrate concentration became important in its effect on these spectra. A study was conducted b>adding increments of nitric acid t o the cupric chloride solution. The cupric chloride concentration was kept the same for each solution in the series and only the nitrate concentrations were changed. The changes in pH were neglected. The maximum emissions occurred when the nitrate to chloride ratio reached 1.0, from which point the emissions remained constant as seen it1 Figure 4. By referring to the first point on each curve (shown a t the ordinate) m-hich represents the cupric chloride solution without the nitrate, an evaluation of the nitrate enhancement may be obtained. In the 0.2.11 chloride solution there was an 8.6% increase, for the O.&M chloride a 10.6% increase, for the 0.6.14 chloride a 11.4% increase, and for the 0.831 chloride a 20% increase in the CuCl emission over that for the standard cupric (ahloride solutions. The addition of cupric nitrate then served two purposes: It added the required caopper ions to the solution and it enhanced the CuCl emission, Jvhich was desirable since the sensitivity was low. Some Interferences on CuCl Spectral Emission. Carbon compounds interfered in the radiative range of the CuC1. Thus, gases such as acetylene could not he used as the fuel because of the strong emissions of the C H and C2 band systems a t 438 nip. Because of the presence of small amounts of earhonaceous materials in the natural sea water, bicarbonate was added t o the synthetic sea water which \vas used as the matrix for the standards. The presence of copper in the solution had an inhihitive effect on the background emissions of the other metallic cations in the synthetic sea water. Particularly in the case of sodium there was considerable reduction in its background interference. The sulfate had an increasing inhihitive effect on the CuCl emission, with increasing concentrations of sulfate. A 0.4M chloride with 0.2-44 sulfuric acid reduced the emission 17.9% and for a 0.4M sulfuric acid, the repression was 44.2y0. Calibration Curves of Other Chlorides. Hydrochloric acid which does not have any metallic cation presented the idealized case for the production of CuCl spectra. Neglecting the p H effect, a flame of much higher potential (4 pounds per square inch gage of hydrogen) was used in this determination without undue
interferences. Aluminum chloride behaved very similarly. Satisfactory calibration curves were also obtained for magnesium and calcium chlorides but a t 0.75 pound per square inch gage of hydrogen. Only very low hydrogen pressures gave satisfactory curves for potassium and ferric chlorides similar to that used for sodium chloride Generally, with the euception of iron, the excitation potential of the cation gave an excellent indication as to the hydrogen pressures necessary for the determination of the chloride (the higher the escitation potential the more hydrogen could he used). Figure 5 s h o w the ferric chloride and the hydrochloric acid calibration curves. Limit of Determination. The limit of determination with any accuracy for the chloride in its sodium salt 1% 0 02-14 chloride and,
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Calibration curves for ferric chloride and hydrochloric acid at 437 mp
V O L U M E 27, NO. 10, O C T O B E R 1 9 5 5 in the idealized cases for such substances as hydrochloric acid or aluminum chloride, the limit is 0.01M chloride. In the case of sea water the chlorinity is high and by dilution to 1 to 4 the optimum working range of 0.05 to 0.4M chloride was used for this determination. The reading required for a chloride determination by the burning procedure can be obtainrd in about 1 minute if all solutions are previously prepared. The method fails to distinguish between the chloride, bromide, or the iodide. The B and C band systems of CuBr and the D and E band systems of CUI emit in the same region as the chloride. However, the concentrations of the bromide and iodide in sea water are belox the detection limit of the method and so for practical purposes only the chloride value is obtained. The sensitivity leaves much to be desired and may be improved by using a more sensitive detection system, but even with this improvement it is insensitive compared with the determination of metallic elements. However, for the more concentrated chloride solutions this method should provide a useful extension to the existing methods of chloride analysis. LITERATURE CITED
(1) Raker, G. L., and Johnson, IT. C., hi
