356
ANALYTICAL CHEMISTRY
and would act M a transparent solvent. For either of these possible monomer solvents present, recourse to a physical separat,ion of the polyester from the solvent would be necessary. ACKNOWLEDGMENT
The authors wish to acknowledge the assistance of W. G. Deichert with the styrene distillations and of J. G. Koren with the spectrophotometric data. LITERATURE CITED
(1) American Cyanamid Co., S e w Product Bulletin, ColZ. VoZ. 111, 118 (1954). (2) Feuer, S.S., Bockstahler, T. E., Brown, C. A, and Rosenthal, I., I n d . Eng. Chem., 46, 1643-5 (1954).
Hirt, R. C., King. I'.T., and Schmitt, It. G., A ~ LCHEM., . 26, 1270 (1954). Hirt, R. C., Stafford, R. W., King, F. T., and Schmitt, R. G., Ibid., 27, 226 (1955). Hobart, E., Ibid., 26, 1291 (1954). McGovern, J. J., Grim, J. M.,and Teach, W. C., Ibid., 20, 312 (1948). Newell, J. E., Ibid., 23,445 (1951). Shay, J. F., Skilling, S.,and Stafford,E. W , Ibid., 26,652 (1954). Shreve, 0. D., and Heether, hl. R., Ibid., 23, 441 (1951). Stafford,R. W., Francel, R . J., and Shay, J. F., Ibid., 21, 1454 (1949). Stafford,R. W., Shay, J. F., and Francel, R. J., Ibid., 26, 656 (1954). Witschonke, C. R.. Ibid., 24,350 (1952).
RECEIVED for review October 28,
1954. Accepted December 6, 1954. Presented before the Sixth Conference on Analytical Chemistry and Applied Spectroscopy Pittsburgh. February 1955
Simultaneous Spectrophotometric Determination of Calcium and Magnesium A L L E N YOUNG, T H O M A S R. SWEET, and BERTSIL B. BAKER' McPherson Chemical Laboratory, The O h i o State University, Columbus 70, O h i o
A reasonably rapid and accurate method for the determination of small quantities of calcium and magnesium in water has been developed. This represents the first simultaneous spectrophotometric determination of calcium and magnesium and depends on obtaining absorption measurements at a wave length of 630 mp and at a pH of both 9.5 and 11.7. On the basis of 43 known aqueous mixtures of calcium and magnesium containing from 0.3 to 6.0 p.p.m. (expressed as parts per million of calcium carbonate) of each of these metal ions and not more than a total of 7.8 p.p.m., it was found that the average absolute error of the calcium was 0.12 p.p.m. and the average absolute error of the magnesium was 0.09 p.p.m.
ANY polyvalent cations are able to change the color of the organic dye Eriochrome Black T in basic solutions. This color change has been used frequently for indicating the end points in complexometric titrations. Schwarzenbach and Biedermann (8) have studied the indicator action of the dye with p H changes in the absence of polyvalent cations. The red color of the dye in basic solutions containing polyvalent cations has been shown to be the result of complexes formed between the dye and the cation. Schwarzenbach and Biedermann (8) have investigated the 1 to 1 complexes and have given stability constants for them. Harvey, Komarmy, and Wyatt (1) pointed out that magnesium forms a complex containing two dye molecules per magnesium atom. Young and Sweet (3) have shown that, in addition to these complexes, a 3 to 1 complex is formed with magnesium and 2 to 1 and 3 to 1 complexes exist for calcium. Harvey, Komarmy, and Wyatt (1) used the red color of the magnesium complex as the basis of a colorimetric determination of magnesium a t a wave length of 520 mp and a pH 10.1, They state that calcium interferes and suggest that it be removed from the solution by precipitation as calcium sulfate. The purpose of the present work was to develop a convenient method whereby small concentrations of both magnesium and 1 Present address, Southern Research Institute, 917 South 20th St., Birmingham. Ala.
calcium could be determined spectrophotonietrically without the necessity of making a separation. Since the absorption curves for dye solutions containing either calcium or magnesium have the same general shape, with absorption maxima in the range 520 to 560 mk, it was not practical to base a simultaneous determination on absorption measurements taken a t the same pH and a t two different wave lengths. These absorption curves are shown in a separate publication (3). However, there is a difference in the stability of the complexes of calcium and magnesium. Although the stability of the calcium complexes is lower, increasing the pH sufficientlycauses the degree of complex formation of both calcium and magnesium to be essentially complete. Measurement of a dye blank against a solution containing calcium and magnesium a t a sufficiently high p H gives a reading which is proportional to the total calcium and magnesium present. As the p H is decreased, the degree of reaction of both calcium and magnesium decreases, but it decreases much more rapidly for calcium than for magnesium. Therefore, it is possible to choose a lower pH value a t tThich the degree of reaction of calcium, as compared to that of magnesium, is quite small. Measurement of a dye blank against a solution containing calcium and magnesium a t this lower pH gives a reading which is essentially a measure of the amount of magnesiuni present. Jleasurement a t t8-o pH values provides the basis for the simultaneous determination presented in the present paper. EXPERIMENTAL
Reagents. WATER. The water was triple distilled and was stored in borosilicate glass. Its specific conductance was less than 1 micromho. BUFFER,pH 11.70. Piperidine (78 ml.) was added to about 300 ml. of water. After mixing, 8.5 ml. of concentrated hydrochloric acid were added. This was diluted to about 500 ml. It was next adjusted with hydrochloric acid or piperidine so that it gave DH11.70 under the same conditions that are used in making the balytical measurements-that is, 5 ml. of buffer must prgduce p H 11.70 in a solution having a total volume of 100 ml., 25 ml. of which is the alcoholic dye solution that is described below. All p H measurements were made using a Beckman Model G pH meter equipped with a micro saturated calomel cell and a Beckman Type E micro glass electrode. The meter was standardized with 0.0534 borax a t pH 9.18. Polyethylene bottles were used for the storage of all buffer solutions.
V O L U M E 27, NO. 3, M A R C H 1 9 5 5
357
BUFFER,pH 0.52. A 8.5-ml. volume of concentrated hydrochloric acid was added to about 300 ml. of water. Concentrated (24 ml.) was added and the total volume was brought up to 500 ml. This buffer also requires adjustment to produce p H 9.52 under the conditions of the determination and is adjusted with ammonium hydroxide or hydrochloric acid in a manner similar to that described for the pH 11.70 buffer. However, 10 ml. of the pH 9.52 buffer are used per 100-ml. total. CALCIUM SOLUTION, 100 P.P.M. Calcium carbonate (0.1000 gram) was dissolved in 0.3 ml. of concentrated hydrochloric acid. This was quantitatively transferred to a 1000-ml. volumetric flask and diluted to the mark. The concentration of this solution is 100 p.p.m., expressed as calcium carbonate, 40 p.p.m. expressed as calcium, or 1 X l O - 3 J f . This solution and all other metal ion solutions were stored in borosilicate glass containers.
t-
0.90
Calculations. Using the described procedure prepare standard concentration curvw, such as those shown in Figure 1, for calcium at p H 11.70, calcium a t pH 9.52, magnesium a t p H 11.70, and magnesium a t pH 9.52. From the four concentration curves shown in Figure 1, Equations l and 2 were obtained. These equations are based on t,he best straight lines drawn through each of the curves.
/
/
0.80
070
4
1L
served as a vent, was placed on a reciprocal motor-driven shaker for 0.5 hour. The stock dye solution was stored in the dark until it was used. Procedure. .4dd 50 ml. of the solution to be analyzed to a 100nil. volumetric flask. Add 25 ml. of the dye solution, followed by 5 ml. of the pH 11.70 buffer. Dilute to the mark with water. +ifter mixing, store in the dark for 60 minutes. Measure the absorbance a t 630 mp with a Beckman Model DU spectrophotometer equipped with I-em. Corex cells. Adjust the slit width so that the sensitivity knob is kept a t or near the counterclockwise limit. .is the blank absorbs more light a t this wave length than the sample, balance the instrument with the sample in the light path; then place the blank in the light path and determine its ahsorbanre. The blank must be prepared in exactly the same way and a t the same time as the sample. Repeat the procedure using 10 ml. of the pH 9.52 buffer in place of the, pH 11.70 buffer.
c
0.60
z
+ 0 . 1 2 7 ~- 0.026 0.1202 + 0 . 0 1 3 3 ~- 0.024
An,, = 0.1522
(1)
-49.5
(2)
0.50
a
m
a
0.40 m U
t
1
0'30[ 0.20
MINUTES AFTER MIXING (0
w
u a
7 I
m
a o'4000
-
0.00
u
I
2
3
4
5
6
CON CE NT RATION, RR M. Figure 1. Concentration curves
c
20
40 60 80 100 MINUTES AFTER MIXING ( b)
120
Concentration, 0 t o 6 p.p.m., expressed as calciuin carbonate 1. Calcium a t p H 9.5 and 630 m p 2 . lfagnesium a t p H 9 5 a n d 630 nip 3. Calcium a t pH 1l.i and 630 mp 4 . Magnesium at p l i 11.7 and 630 m p
CaLcIrlr SOLVITOS,6 l'.P..\I. Sixty niilliliters of the 100p,p.m, calcium solutio11 Tvrre diluted to 1000 ml. with water. MAGXESIUM SOLUTION, 100 P.P.M. The solution was prepared by treating 0.0913 gram of 3 MgCOa.Mg(OH)2~3H~O in the same way as the calcium carbonate in the preparation of the 100-p.p.m. calcium solution. The resulting solution has a concentration of 100 p.p.m. expressed as calcium carbonate, 24.32 p.p.m. expressed as magnesium, or 1 X lO-3-lf. hIAGmsIuar SOIXTIOS,6 P.P.M. Sixty milliliters of the 100p.p.m. magnesium solution were diluted to 1000 ml. with m-ater. K N O W X SAMPLES O F C.4LCIUM AND MAGNESIUM.These were prepared by appropriately diluting the calcium and magnesium solutions. DYESoLu.rIos. One hundred milligrams of Eriochrome Black T (W. H. and L. D. Betz) Tvere transferred to a 250-ml. volumetric flask, using small portions of water, all totaling about 15 ml. One milliliter of p H 11.70 buffer was included in thelasbportion. The solution was brought u p to the 250-ml. mark with 95% grain alcohol. After brief manual shaking, the solution was transferred to a 500-ml. Florence flask. This flask, which was stoppered with a rubber stopper containing a constricted glass tube that
MINUTES AFTER MIXING (C)
Figure 2.
Time study
Concentration, 3.0 p.p.m.; 630 mp a. Calcium a t p H 11.7 b. Magnesium a t p H 11.7 c. Calcium a n d magnesium a t pH 11.7 All,, is the nieasurcd absorbance a t p H 11.70 and a t 630 nip; Ag.6 is the measured absorbance a t pH 9.5 and a t 630 mp; z is the concentration of magnesium in the 50-ml. sample in p.p.m. (expressed as calcium carbonate); and y is the concentration of calcium in the 50-ml. sample in p.p.m. (expressed as calcium carbonate). The experimental values of All., and Ag.6 were substituted in these two simultaneous equations and solved for z and y. This has been done for 43 known mixture8 of calcium
358
ANALYTICAL CHEMISTRY
and magnesium and the results are shown in columns 4 and 6 of Table I. Equations 3 and 4 were developed in order to make allowance for the slight deviations from linearity in several of the s t a n d a r d c u r v e s n e a r the upper or lower ends of the concentration range.
+
0.1522 0.1267~0.026 0.0216(1.2 - 2 ) - O.OlO(y - 4) ( 3 )
Ail.?
+
The fourth term is used when x < 1.2 and the fifth term is used when y > 4.0.
+
As., (0.0968 0 . 0 0 4 7 3 ~ ) ~ 0 . 0 1 3 3 ~- 0 . 0 2 5 ( ~ 4)’
+
-
(4)
The third term in Equation > 4.0. After solving for x and y by Equations l and 2, the indicated terms of Equations 3 and 4 may be used in order to improve the results. This has been done for all of the mixtures of Table I and the results are shown in columns 5 and 7 of Table I. 4 is used when x
Table I. Analytical R e s u l t s
Run
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43
1.2
1.2
DISCUSSIOR
Alcohol is used in the preparation of the dye solutions, because it yields a solution that is more homogeneous and is easier to handle. Pure water solutions of the dye foam during the process of shaking and during pipetting operations, whereas solutions containing sufficient alcohol do not. I n addition, the use of alcohol results in final solutions that change less rapidly with time. The rather high absorption coefficient of the dye limits the total concentration of the dye and hence of both calcium and magnesium. The total final concentration of dye that was used was 0.01 gram per 100 ml. or 21.65 X 10-5-zI. If the formation of 3 to 1 complexes were complete] this would limit the total metal ion concentration to 7.22 X 10-5JI, w>hichcorresponds to a 50-ml. sample which is 14.44 p.p.m. (eupressed as parts per million of calcium carbonate). For a simultaneous determination, y parts per million of calcium in a mixture of y parts per million of calcium and x parts per million of magnesium muet produce the same change in absorbance as y parts per million of calcium does in the absence of magnesium. This requires an eycess of dye above 3 to 1 and hence reduces the maximum total metal concentration to much less than 14.44 p.p.m. I n addition] the high absorption of the dye necessitates the use of a dye blank. There were two possible choices with regard to the wave length of the measurements. The first was to measure the absorbance in the region of 520 to 560 mp, w here the complexes have their mayima. This region was used in some of the preliminary experiments. However, the results were somewhat erratic. This can be explained in the following way. Although 520 to 560 mp is the region of the maxima for the complexes, it is also a region in which the absorption curve of the dye is steep, thus making the wave-length setting very critical. The second region that was considered R-as between 630 and 650 mp, the region of the dye’s maximum. Here the absorption of the complexes is low
Experimentally Determined Results Calcium Magnesium Linear Nonlinear Linear Nonlinear eqs., eqs., eqs., eqs., p.p.m. p.p.m. p.p.m. P.P.~. 0.6 0.7 0.6 0.6 0.3 1.0 0.3 0.9 0.4 0.4 1.o 0.9 0.5 0.5 1.4 1.3 1.3 0.6 0.6 1.3 0.5 0.5 2.0 2.0 0.4 0.3 2.3 2.4 0.8 0.8 3.0 3.0 3.0 0.6 0.6 3.0 3.1 0.4 3.1 0.5 2.8 0.8 0.8 2.8 4.8 0.6 0.5 4.8 5.7 0.8 1.0 5.8 5.3 1 .o 1.2 5.5 0.8 5,s 5.5 1.3 1.2 0.7 1.2 0.6 1.2 1.2 1.3 1.2 1.4 1.4 2.3 2.3 1.4 2.4 1.4 2.4 1.2 3.0 1.2 3.0 1.0 1.5 5.6 6.1 5.4 1.6 5.6 1.8 5.0 1.6 1.1 6.0 1.9 1.9 0.6 0.5 1.8 1.8 0.6 2.3 1.8 5.9 2.5 2.6 0.6 0 .5 2.4 2.4 0.7 0 6 2.4 2.4 2.4 2.4 3.1 3.1 0.4 0.3 2.9 2 9 0.8 0.6 3.0 3.1 0.7 0.6 3.1 3.2 1.0 0.9 3.1 1.2 3.0 1.2 3.2 2.2 3.2 2.2 3.1 2.2 3.1 2.2 3.1 2.3 3.1 2.3 3.0 3.0 3.0 3 0 2 9 2.9 3.1 3.1 4.9 4.9 0.6 0.5 6.0 0.7 5.9 0.6 1.7 5.8 5.9 1.7 1.9 5.8 5.6 1.9
;:;
Errors Calcium Magnesium Linear Nonlinear Linear Nonlinear eqs., eqs.. eqs., eqs., P.P.~.
0 0 0.3 0.2 0.1 0.0 0.1 0.1 0.2 0.0 0.1 0.2 0.1 0.4 0.6 0.7 0.0 0.0 0.2 0.2 0.0 0.5 0.6 0.5 0.1 0.0 0.5 0.1 0.0 0.0 0.1 0.1 0.0 0.1 0.0 0.2 0.1 0.1 0.0 0.1 0.1 0.1 0.2 0.4
P.P.~.
0 0 0 0
0 3 2 1
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1 0 2 0 2 2 0 2 4 2 0 0 2 2 0 2
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1 1 0 0 2 0
0 0
0 4
0 1
1 1 2 1 2 1 1 0 1 1 0 1 2
P.P.~.
0.1 0.4 0.4 0.2 0.1 0.2 0.1 0.0 0.0 0.1 0.2 0 0 0.3 0.7 0.5 0.1 0.1 0.1 0.0 0.0 0.4 0.6 0.5 0.0 0.1 0.5 0.0 0.1 0.0 0.1 0.2 0.1 0.1 0.0 0.2 0.2 0.1 0.0 0.1 0.0 0.1 0.1 0.1
D.D.~.
0.0 0.3 0.3 0.1 0.1 0.2 0.0 0.0 0.0 0.1 0.2 0 0 0.2 0.5 0.2 0.0 0.0 0.1 0.0 0.0 0.1 0.4 0.0 0.1 0.0 0.1 0.1 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.2 0.2 0.1 0.0 0.1 0.1 0.0 0.1 0. I
and fairly level. Thus the wave-length setting is not so critiral. In this region, measurement of a dye blank against the sample solution gives a reading which is primarily due to the disappearance of the dye as a result of complex formation. The exact wave length used was 630 mp. A u-ave length of 640 mp might have been a somewhat better choice. One of the pH values used in the analysis should be high, so that both the calcium and the magnesium will react as completely as possible with the dye. However, several difficulties arise which place an upper limit on the pH that can be used. Solutions having high pH values are hard on glassware, especially absorption cells. Second, a satisfactory buffer system must be used. Sodium hydroxide and potassium hydroxide are not pure enough. Xeither these substances nor any amine hydrochloride were found satisfactory. In so far as could be determined, piperidine reacts with neither calcium nor magnesium and could be obtained free from heavy metal impurities. The pKb for piperidine is about 11.2. I n order to remain safely within the limit of this buffer system’s usefulness, a pH of 11.70 wae selected. This exact value is arbitrary, although once selected, it is important to maintain it consistently. Because it was felt desirable to use the same buffer system a t the lower p H measurement, a p H of 10.25 was first selected. At this pH the concentration curve for magnesium was lowered only slightly as compared with pH 11.70. The calcium curve, on the other hand, was considerably lower. Honwer, when the absorbances of mixtures were determined a t p H 10.25, they were found to be less than the sums predicted from the individual calcium and magnesium concentration curves. The final choice was pH 9.52. Here the calcium concentration curve is much lower and the magnesium curve is still not less than 75% of its value a t pH 11.70. 4 t this pH the absorbance of a mixture of calcium and magnesium is equal to the sum of the individual components. The exact value of 9.52 is not critical, providing that once decided upon it is maintained constant.
V O L U M E 2 7 , NO. 3, M A R C H 1 9 5 5 Figure 2 illustrates the effect of time on the absorbance measurements. As indicated in these curves, the best time for taking readings is 60 to 80 minutes after the solutions are mixed. hnother time factor is the age of the stock dye solution. All analytical measurements were made using stock dye solutions 3 to 5 hours old. Use of stock dye solutions on days subsequent to that on which they were prepared led to lower absorbance values. For the 43 known mixtures that were tested by the present method, the average absolute error of calcium was 0.17 p.p.m. using the linear equations and 0.12 p.p.m. using the nonlinear equations. The average absolute error of magnesium was 0.17 p.p.ni. xith the linear equations and 0.09 p.p.m. with the nonlinear equations. Several determinations were made in order to investigate possible interference of iron and copper. ;1 sample known to contain 3.0 p,p.m. of calcium, 3.0 p.p.m. of magnesium, and 1.0 p.p.ni, of copper was found to contain 2.7 p.p.m. of calcium and 3.9 p.p.m. of magnesium. % . sample containing 3.0 p.p.m. of calcium, 3.0 p,p.m. of magnesium, and 1 p.p.m. of iron was found to contain 2.9 p.p.m. of calcium and 3.9 p.p.m. of magnesium. This interference is to be expected, not only with iron and copper, hut also wit,h other polyvalent cations. Possible interference of Bodium and potassium was investigated by analyzing solutions containing varying knoL5-n concentrations of sodium and potassium in addition to 3.0 p.p.m. of calcium and 3.0 p.p.m. of magnesium. The results are summarized in Tnhle 11. If the concentration curves that were used in developing the
359 Table 11.
Effect of Sodium and Potassiuma
Concn., P.P.R.1. 10 ?a: 100 1 K 10 K: 50 K
C a Found, P.P.hI. 2.8 3.1
iy
Q
2.8
3.0 3.3 100 K + 4.1 Sodium and potassium added a s chloride salts
M g . Found,
P.P.M. 3.1 3.2 3.2 3.3 3.3 3.1
equations for the simultaneous determination (Figure 1) are extended, they show a pronounced tendency to level off with increasing metal ion concentration. Although it would not hr advisable to extend the simultaneous determinations much above 6 p.p.m., standard concentration curves may be used, preferabll a t pH 11 70, for the determination of solutions known to contain only calcium or magnesium in the range 0.3 to 15 p,p.m, LITER4TURE CITED
(1) Harvey, A. E., Jr., Komarmy, J. 31 , and K y a t t , G . 31.,ASAL CHEY.,25, 498 (1953). ( 2 ) Schwarzenbach, G., and Biedermann, W., H e h . Chint. Acta, 31, 678 (1948). (3) Young, A., and Sweet, T. R., A N ~ LCHEY., . 27, 418 (1955). RECEIVED for review J u n e 15, 1954. Accepted October 4, l Q 5 4 . Taken in p a r t from a thesis presented t o the Graduate School of T h e Ohio State Unlversity by Allen Young in partial fulfillnient of the requirements for the degree of master of science. J u n e 1954.
Polarographic Behavior of Some Alkyl Phthalate Esters GERALD C. WHITNACK, JOAN REINHART, and E. ST. CLAIR GANTZ
U. S.
N a v a l Ordnance Test Station, Inyokern, China Lake, Calif.
The general behavior of the methyl, ethyl, butyl, and octyl esters of o-phthalic acid were investigated at the dropping mercur>-electrode. In ethanolic solutions, containing quaternary ammonium salts as supporting electrolytes, two well-defined diffusion currents were produced. The reduction process was found to be diffusion-controlled and irreversible for each wave. The effect of pH on the diffusion currents and half-wave potentials of dimethyl phthalate was studied in 75% ethyl alcohol solution buffered over the range of 2 to 12 with Britton and Robinson buffers. Diffusion coefficients were experimentally determined and used to calculate the value of n for the first and second waves. Four electrons are involved in the first reduction and t w o in the second. Phthalide was established as the intermediate reduction product. Optimum analytical results are secured by measurement of the first wave (n = 4). Neutral and alkaline solutions offer the best conditions for measurement of this wave, and the diffusion currents in these media are a linear function of the concentration of phthalate ester.
S"'"
