ANALYTICAL CHEMISTRY
904 Table 111. Precision Acetol Weighed, Mg./L. 0.82 1.64 2.40 3.28 4.09 4.91 5.73 6.56
Found, Mg./L. 0.8,O.g 1.6, 1.6 2.5,2.5 3.2,3.3 4.2,4.2 4.8,4.9 5.7,5.8 6.6,6.5
Formaldehyde, in large excess, forms colored solutions which fluoresce beyond the range measurable by the fluorometer. .4t lower concentrations of formaldehyde, in which case little coloration develops, low values are obtained, possibly due to reaction of formaldehyde with acetol. The presence of furfural results in high values. Acetaldehyde interferes only when present in very large excess, whereas the interference by pyruvic acid is small. The following substances do not interfere when present in a weight ratio of 1000 to 1 : methanol, ethanol, acetone, lactic acid, and levulinic acid. DISCUSSION
This quantitative application of Baudisch’s qualitative test for acetol gives good results in the absence of the interfering sub-
stances mentioned above. As shown in Table 111, the precision of the method appears to be nearly within the limits of the fluorometer. The fluorescence is dependent on temperature. However, the fluorescent compound is stable, exhibiting no decay, in the absence of strong illumination, during 2 hours. A single determination requires approximately one hour, and several can be run simultaneously. As yet, no means has heed found for reducing the effect of any of the interfering substances. This is unfortunate, for the interferences of diacetyl and furfural limit the usefulness of the method in application to certain problems in carbohydrate chemistry. LITERATURE CITED (1) (2)
Baudisch, O., Biochem. Z.,89, 279 (1918). Baudisch, O., and Deuel, H. J., J . Am. Chem. Soc., 44,
1585
(1922).
(3) McIlvaine, T. C., J . Biol. Chem., 49, 183 (1921).
RECEIVED December 27, 1949. This paper reports research undertaken in cooperation with the Quartermaster Food a n d Container Institute for the Armed Forces, and has been assigned number 283 in t h e series of papers approved for publication. The views or conclusions contained in this report ar0 those of the authors. They a l e not t o be construed as necessarily retlecting t h e views or endorsement of the Department of t h e h r m y
Spectrochemical Analysis of Brines R. G . RUSSELL, Gulf Research & Development Company, Pittsburgh, Pa. A spectrographic procedure for the analysis of most of the cations in brine solutions utilizes an alternating current spark form of excitation on 0.5-inch briquets combined with a series of synthetic standards. A relative standard method is used to obtain the concentrations of the major constituents. The shortcomings of the method as well as its advantages are discussed. An error of *lo% is claimed for the procedure.
T
HE complete quantitative chemical analysis of brines has always been a difficult and tedious task. In the determination of the cations this has been occasioned both by the difficult chemical separations as, for instance, small amounts of potassium from large amounts of sodium, or small quantities of barium and strontium from larger amounts of calcium, and the de termination of very small amounts of some constituents. In addition, there is difficulty in determining the anions in the form in which they occur in the original sample. Contact of the brine solutions with the air continually changes the form of the anion-i.e., bicarbonate changes to carbonate. As the result of these chemical difficulties, shortened and approximate methods of brine analysis have been developed in many laboratories, All alkalies are reported as sodium. Barium and strontium are not determined, but are partially precipitated with and determined as calcium. Aluminum and iron are precipitated as the hydroxides, along with any other materials that would be precipitated a t this point, and reported as RzO~. Many of the elements such as boron, manganese, and lithium, present only in small amounts, are not detected in a sample of the size usually taken for chemical analysis. The investigation reported here has been carried out to develop a more rapid method for the quantitative analysis of brines. In addition, more information can be provided by the spectroscopic method than by the usual chemical analysis. QUALITATIVE ANALYSIS
Before any attempt is made to analyze a sample quantitatively, a so-called qualitative analysis is made. The sample is evapo-
rated to dryness over an open flame in the presence of a little hydrochloric acid. Three 10-mg. portions of the dried residue are weighed and arced, using 10-ampere direct current and a large Littrow quartz prism spectrograph. The purpose of using an exact weight of sample is to enable the operator to estimate more accurately the quantities of elementa present. These qualitative results become of value to the quantitative analysis only as the operator learns to place them in the proper percentage groupings. On the basis of many other samples that have been analyzed chemically, the operator is able to place the elementa in their correct groupings by remembering the densities of linea
Table I.
Qualitative Analysis of a Typical Brine Residue (% refen, to cations detected) %
>
10
NS
1-10
CS
Mg
0.1-1
K Sr Si
B
0.01-0.1
Fe
< 0.01
Pb
Li A1
cu
Ba Ti
2
V O L U M E 2 2 , NO. 7, J U L Y 1 9 5 0
905
of samples of known composition and comparing them with the unknowns. However, such results are obtained by visual means. The sum of the elements with the exception of sodium, calcium, and magnesium is estimated for use in the quantitative analysis of the aforementioned elements. Table I illustrates a typical qualitative brine analysis.
0.1
0.2
0.40.6
10 .
2
4
6 8 IO
20
40 60
IC0
QUANTITATIVE ANALYSIS
Once the constituents of a brine are known, it is possible to prepare standards for their quantitative analysis. One of the assumptions made at the beginning of this investigation was that the major constituents were present as the normal hydrated chlorides. In a few cases this is a poor assumption, and then a poor analysis results. However, even in the worst case, where the calcium is present as sulfate rather than chloride, an error of only about 15y0of the amount present would result because the molecular weight of calcium chloride dihydrate is 147 whereas that of calcium sulfate dihydrate is 172. In the case of sodium chloride against sodium carbonate, the error would only be 10%. In most samples the constituents of the brine are present as chloride and little error is introduced. The same approximate composition of both standards and samples is attained by evaporating the brine to dryness with hydrochloric acid. From the qualitative analysis it is possible to bracket the impurities-that is, potassium might be between 0.1 and 1%. Using these estimated ranges of values, standards are made up by adding known quantities of impurities to a base mixture containing sodium chloride, calcium chloride dihydrate, and magnesium chloride hexahydrate. I t is the usual practice to add the highest desired percentage of impurities to a sample of the base mixture and t o make other standards by diluting portions of the high standard with varying amounts of base mixture. In this manner the author has prepared a series of brine standards from approximately an order of magnitude below the ordinary concentration of impurity in a brine to an order of magnitude above this normal impurity level.
A series of standards was prepared for the major constituents, sodium, calcium, and magnesium. The sodium chloride wm varied from 70 to 99% of the total salt mixture, the magnesium chloride hexahydrate from 10 to 0.5%, and the calcium chloride dihydrate from 10 to 0.50j,. In the case of both series of standards, the materials were dried a t 110” C. before weighing, and efforts were made to see that no moisture was picked up during the weighing step of the procedure. The materials were thoroughly ground together by means of a mortar and pestle after weighing. Both the samples and standards were hygroscopic and special care was taken in order to be certain that the ma-
terials were dry and in a reproducible state. These standards were then available for any form of excitation. Because it had been used previously with nonmetallic samples, an alternating current spark form of excitation was used. After some experimentation, aliquots of the standards were wighc 1 up with natural graphite and internal standard in the ratio of 4:l: 1. Natural graphite was chosen over synthetic graphite because of its favorable briquetting properties. Copper oxide was chosen as the internal standard for the minor impurities, and a relative standard method was chosen far the major constitutents. Table I1 gives the conditions used for the analysis of the constituents in brine solutions. Figure 1 gives working curves obtained upon running the series of standards. The curved lines are those obtained without making any correction. Theoretically, such a curve should have been a straight line if there had been no background, no self-absorption of the spectrum lines used, or no contamination or impurities present. The background was low and corrections applied to this background did not appreciably change the curve. The curvature of the lines was in the wrong direction to be explained by self-absorption, and self-absorption does not usually occur a t such low concentration levels. Because both background and self-absorption can be ruled out, it has been assumed that the curvature arose from contamination or impurities within the standards themselves. Qualitative analyses of the base materials substantiate this assumption. Corrections have been applied to the added percentages of impurities with the resulting straight lines shown. These corrections are relatively large, but chemical analyses in this range are usually too poor to prove or disprove the hypothesis. Figure 2 is a series of similarly corrected working curves for some of the minor constituents, --
Table TI.
Conditions for Determining C o n s t i t u e n t s in Brines B, Si, Fe, hi K , Ba, Sr Large Littrow quartz prism 2413-3100 3687-8000 SA 1 1L 2 2 0,021 0.021 0.36 0.36 75 75 5 5 40 15 18 18
Na, M g . Ca
Spectrograph h u g e , A. 2739-4071 Emulsion used SA1 Power, kv.-amp. Capacitance, sf. 4630i4 Induotance, mh. 0.36 Primary voltage 75 Prespark, 8ec. 5 Exposure time, sec. 15 Distance from slit, 33 inches Slit width, microns 30 30 30 Upper electrode 0.3-inch briquet Lower electrode Hemispherical cone, 0.25-inch graphite rod Internal standard CUO cuo None Development D-19, 4 minutes .4nalysis lines B 2496.8 Cri 2703.2 K 7699.0 Cu 5153.2 Na 3 3 0 2 , J Si 2516.1 Cu 2703.2 Ba 4554 .O Cu 3153.2 M a 2779.8 E’e2599.4 Cu2703.2 S r 4 6 0 7 . 3 C u 5153.2 C a 3 1 8 0 . 5 AI 3082.2 Cu 2703 2
The relative standard method is convenient for use in determining major constituents (1-3). To the extent that the working curves formed using this method have a 45’ slope, the method is theoretically sound. As the working curves depart from the 45’ slope, it is necessary that standards of similar composition be used in order to bracket the concentration ranges covered. This method is based on the fact that the sum of all the constituents in any mixture is equal to lOO’%,. Therefore, by assuming all of the major constituents of the brine to be chlorides, we are justified in writing Equation 1. The hydrates given are those normally present from room temperature to about 110” C. The percentage of “other constituents” can be estimated from the qualitative analysis. The error involved in this estimation, although it may be large for the constituent itself, is relatively minor when compared with the 100% total. Dividing Equation 2 by NaCl and transposing, Equation 3 is obtained. NaCl
+ MgC12.6H20+ CaC12.2H20+ other constituents = 100% (1)
906
ANALYTICAL CHEMISTRY
Hence: NaCl
+ M~CIZ.GHZO4- CaC12.2H20 =
Table 111. Comparison Analyses of Brines
100% - other constituests (2)
SaCl =
1007, - other constituents CaCl 2H20 1 + IclgC12.GHz0 ___.-_+ Sac1 1aC1
+_
Concentration, Mg.[L.yo ~ ~ , . Chemical Spectrographic from Chemical
Element
Sample 3
"
Sa Ca &I 0
(3)
8607 1160 294 134 8 220 5 3 8 21 24 8749 1454
1,030 398
... ... ...
Li Sr Fe A1 Fe A1 Si B Na, K , Li hIg Ca Total dissolved solids
...
+
12 24
...
+ +
8 663 1,428 27,476
0.7 12.6 26
... ... ... ...
33 12
...
0.9 1.9
Sample 8 Na Ca
(I
964 289
hig
..
K
Li
B
a
...
+ Fe
14 24
+ +
Na, K , Li 6,613 Mg 1,253 Ca Total dissolved 21,460 solids Figure used here was total Na, K ,
Table IV.
0.4 16 19.7
78 6 2 8 21 23 6710 1348
42.8 13.3
.1...5 7.6
...
+ Li.
Comparison Analyses of Brines, Sample S-I
Element Na Ca h,I g Total dissolved solids Li
Sr
...
...
concenThe standards for the major constituents are made up of known proportions of sodium chloride, calcium chloride dihydrate, and magnesium chloride hexahydrate. The quantity that is measured on the photographic plate is the transmittance of light through the spectrum lines. These values are converted to intensity ratios by referring them to the calibration curve of the particular emulsion involved. Thus, while the value obtained is the intensity ratio of either calcium and sodium or magnesium and sodium, this value is a measure of the concentration ratios of calcium chloride dihydrate to sodium chloride or magnesium chloride hexahydrate to sodium chloride. The values for the intensity ratios are plotted against the percentage ratios of the compounds involved and a working curve is drawn. The percentage ratios of samples treated in the same fashion are obtained by referring the intensity ratios to this working curve and are substituted in Equation 3. This results in an expression with only one unknown-namely, sodium chloride-which can be solved, Upon getting the percentage of sodium chloride, this value can be substituted into the ratio expression-Le., calcium chloride dihydrate to sodium chloride-and a value obtained for the other membcr. Once percentage values have been obtained for the three compounds, sodium chloride, magnesium chloride hexahydrate, and calcium chloride dihydrate, the parts per million can be calculated in a straightforward manner. Figure 3 gives working curves of the percentage ratio of compounds to intensity ratio of elements. Tables I11 and I V give typical results obtained with the method as well as a limited number of routine chemical analyses. Although the individual errors of the calcium and magnesium are rather large, the combined calcium and magnesium result in a much smaller error. This could be caused by compensating errors. Table V shows the reproducibility of the method on different photographic plates taken on different days. This table illustrates the type of accuracy and precision that could be obtained if accurate chemical values for standards and samples were available. I t is possible that the reproducibility of the method coincides with the accuracy, but in the absence of an independent check, such a conclusion would be open to doubt.
...
4
SI Fe A1 41 Si
6587 1116 232 114
tration .4dded, Mg./L. 17,098 1,038 90.1 47.136
