nated phosphine ligands with gold vapor is shown by the considerable broadening of the P(2p) signals. Charging effects are not responsible for these broadenings (as evidenced by studies on NaF, an inert compound). Coordinated phosphine ligands, however, do not react with gold vapor. Thus, their P(2p) signals do not broaden. Compounds containing phosphorus(V) atoms do not react with gold vapor. Gold deposition as a means of calibration for phosphines should be used with extreme care. More consistent results for phosphorus compounds under investigation
here have been obtained by using the C(1s) signal of the sample as a reference.
RECEIVEDfor review June 14,1974. Accepted July 8, 1974. Financial support from the National Science Foundation under Grant No. G P 30703 and from the Center of Materials Research, University of Maryland, under contract No. DAHC-15-68-C-0211, Advanced Research Projects Agency is gratefully acknowledged.
Calcium-Selective Electrode Measurements of Calcium Molybdate Solubilities in Water Naim A.A. Mumallah and W. J. Popiel Chemistry Department, University of Jordan, Amman, Jordan
Precipitation as calcium molybdate has been used for gravimetric determination of calcium (1, 2), and occasionally of molybdenum ( I , 3 ) . It plays an essential role in indirect complexometric determinations of molybdenum ( 4 , 5), and also in quantitative separations of calcium and magnesium (61, tungsten and molybdenum (7), and rhenium and molybdenum (8). There have been several attempts to measure the solubility (9-11) and the solubility product (12-14) of this compound in water at various temperatures, but the results are in considerable disagreement. The most extensive study is that of Zhidikova and coworkers (23, I d ) , who measured the solubilities in water and in dilute sodium chloride solutions a t 25, 50 "C, and at six temperatures in the range 75-300 "C. The activity solubility products were obtained by use of the Debye-Huckel equation and extrapolation to zero ionic strength. However, the analytical procedures adopted were subject to an estimated error of &lo% and led to a considerable scatter of results. Also, the possibility of ion-pair formation was not taken into consideration, although association of bivalent ions is often appreciable, even in fairly dilute solutions (15). The recent commercial availability of ion-selective electrodes provides a simple and rapid method of measuring ionic activities in solution. We have employed the calcium electrode for a new estimation of the activity solubility product of calcium molybdate in water at eight temperatures in the range 20--40 "C. (1) L. Erdey, "Gravimetric Analysis." Part 11, Pergamon Press, Oxford, 1965, pp 528 and 638. (2) H. Brintzinger and E. Jahn, Fresenius'Z. Anal. Cbem., 97, 312, (1934). (3) E. F. Smith and R. H. Bradbury, Ber., 24, 2932 (1891). (4) E. Lassner and H. Schlesinger, Fresenius' Z. Anal. Cbem., 158, 195 (1957). (5) A . de Sousa, Anal. Cbim. Acta, 12, 215 (1955). (6) A . de Sousa, Rev. Fac. Cienc. Univ. Lisboa, 2a, Ser. B., 2, 73 (1952-
EXPERIMENTAL Calcium molybdate was prepared by mixing calculated quantities of approximately 0.1M aqueous sodium molybdate and calcium chloride solutions. The precipitate was washed with a large quantity of distilled water and then with ethanol. It was air dried a t room temperature. This procedure yields the hemihydrate CaMoO4Jh HzO (10). Hard-glass test tubes containing an excess of calcium molybdate in distilled water were placed in a thermostat fitted with a thermometer reading to 0.1 "C, and shaken vigorously several times a day. Saturation was generally attained within 24 hours, this being checked by continuing the analyses over longer periods ranging from 8 days to one month. Calcium activities in the saturated solutions were measured by inserting an Orion Research Inc. Model 92-20 calcium-ion electrode and an appropriate reference electrode, connected to an Orion Model 407 specific-ion meter. Standard calcium chloride solutions were used for calibrating the meter a t each experimental temperature. Steady readings were usually obtained within 5 minutes, except a t the two highest temperatures where rather longer times were necessary to achieve stability. At selected temperatures, the saturated solutions were also analyzed by atomic absorption spectrometry using a Jarrell-Ash (Fisher Scientific Co.) Dial-Atom instrument, and by titration ( 1 6 )with EDTA solutions (10-4M) standardized us. calcium carbonate. Aliquots for analysis were withdrawn through a sintered glass filter stick, after allowing the excess solid to settle, and quantitatively diluted with a little water to prevent crystallization.
RESULTS The calcium activities or concentrations obtained by the three methods are presented in Table I. The solubility ~ ~ ~calculated z - ) ] by assumproducts [ K , = ( a ~ ~ z + ) ( a ~were ing that the activity of the calcium ion (aca2+)is equal to that of the molybdate (uM,,o~z-). A plot of log K , against 1/T, where T is the absolute temperature, yielded a straight line expressed by the formula:
logl,K, = - ( 2 0 6 6 / T ) - 1.704
53). (7) A. de (8)0. A.
Sousa, Anal. Cbim. Acta, 10, 29 (1954). Suvorova and F. G. Karinskaya, Tr. lnst. Met. Obogasbch. Akad. Nauk. Kaz. S.S.R., I,142 (1959). (9) A . N. Zelikman and T. E. Prosenkova, Russ. J. lnorg. Chem., 6, 105 (10) (11) (12) (13) (14) (15)
(1961). V. I. Spitsyn and I. A. Savich, J. Gen. Chem. (USSR), 22, 1323 (1952). I. Ya. Bashilov and P. S. Kindyakov, Tsvef. Metal., 1931, 879. D. V. Ramana Rao. J. Sci. lnd. Res., Sect. 6, 13, 309 (1954). A. P. Zhidikova and I. L. Khodakovskii, Geokhimia, 1971, 427. A. P. Zhidikova and S.D. Malinin, Geokhimia, 1972, 28. L. Meites, J. S. F. Pode, and H. C. Thomas, J. Cbem. Educ., 43, 667 (1966).
This gives the following values of K , X lo9: 5.0 (at 40 "C), 3.9 (35 "C), 3.0 (30 "C), 2.3 (25 "C), and 1.8 (20 "C), the estimated error being less than f0.15. From the slope of the above graph, the molar heat of solution of CaMo04JhHr0 is found to be 9.5 kcal mol-l. Vogel. "A Textbook of Quantitative Inorganic Analysis," Longmans, London, 1962, p 437.
(16) A. I.
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Table I. Calcium Activities or Concentrations (in mol 1.-1) in S a t u r a t e d Solutions of Calcium Molybdate a t Various T e m p e r a t u r e s Ca-selective electrode [activity]
Atomic absorption [concentration] ( l t O . 1 X 10-5M)a ( l t 0 . 3 X 10-5M)a
3C
EDTA titration [concentration]
All values given below should be multiplied by 10-5
20
4.2
21 25 28
4.4
...
4.8 5.1
7 .O
30 32
5.4
...
11.1
...
... 13.0
35
5 .cs 6.3
40
6.9
...
8.4 9.2
... 9.3 ...
14.6
Estimated errors are given in parentheses.
DISCUSSION The results of atomic absorption spectrometry and EDTA titration are notably higher than those obtained with the calcium-selective electrode, and their reliability is in doubt. Possibly the saturated solution samples may have contained a small quantity of colloidally dispersed molybdate. This is suggested by the fact that the Eriochrome Black T indicator used in the titrations reverted to its preend-point color when the titrated solutions were allowed to stand for a short period. Such behavior was not observed with the calcium carbonate standard solutions of similar concentrations. Roth in titration and in atomic-absorption work, the total dissolved calcium would be measured, including that present as ion pairs. Any colloidal molybdate would further increase the results. The analytical procedures of previous workers are also prone to such errors.
Zhidikova and Khodakovskii (13) measured calcium by flame photometry and molybdenum colorimetrically using ammonium thiocyanate. At 25 "C, they obtained an activity solubility product approximately in the range 2-5 X 10-9, and they report that the solubility a t that temperamol L-'. Our calcium-selective electrode ture is 0.9 X and atomic absorption measurements also give Ks values within this range. Our solubility result obtained by titration is close to that reported by Zhidikova and Khodakovskii, but the agreement is probably fortuitous. Ramana Rao (12) proposed a value of 1.24 X 10-5 for K,; however, neither the temperature nor the method of determination were stated. Using a calcium molybdate prepared by a method similar to ours, Spitsyn and Savich (10) found the solubility to be mol 1.-l a t 22 "C, while Bashilov and Kin2.3-2.6 X dyakov (11) reported it to be about 18 X mol I.-' a t 18 "C. Zelikman and Prosenkova (9) obtained a value of 6.25 x at 20 "C for a molybdate prepared by heating a mixture of calcium oxide and molybdenum trioxide to 600 "C for several hours. In a critical review of past work, Zhidikova and Khodakovskii ascribe discrepancies between results of various workers to differences in particle size and method of preparation of the molybdate. Their own material was subjected to a calcination above 1200 "C before use (17).However, no clear relation between solubility and preparation method emerges from the results quoted above. It would seem more likely that the disagreement is attributable to differences in analytical procedure.
RECEIVEDfor review March 15, 1974. Accepted June 26, 1974. (17) E. S. Khristoforov L. I. Grosman, and S. N. Mineral. Obshchest. 81, 205 (1952).
Kalashnikova, Zap. Vses.
Quantitative Analysis by Dynamic Single Ion Detection Stephen R. Bareles and Joseph D. Rosen' Department of Food Science, Rutgers University, New Brunswick, N.J. 08903
The major drawback to the more widespread use of mass fragmentography ( I ) is that changes in magnetic field intensity and accelerating voltage alters the focus of specific m/e values (2), making it difficult to obtain reproducible analytical data. This problem was particularly severe on our Du Pont 21-490 mass spectrometer and has led us to build a circuit to overcome the problem. This low-cost circuit incorporates a low amplitude sweep across a specific m/e value instead of static focusing for reasons explained by Klein et al. (3).In the present application, the fluctuating electron multiplier output is demodulated by a gated analog integrator-sample/hold system for chart recorder presentation.
T o whom inquiries should be addressed. (1) A . E. Gordon and A . Frigerio, J. Chromatogr., 73, 401 (1972). (2) J. F. Holland, C . C. Sweely. R. E. Thrush, R. E. Teets, and M. A. Bieber, Anal. Chem., 45, 308 (1973). (3) P. D. Klein, J. R. Haumann. and W. J. Eisler, Anal. Chem., 44, 490 (1972).
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EXPERIMENTAL Instrumentation. A Du Pont Model 21-490 Mass Spectrometer equipped with a digital mass marker and interfaced to a Varian Model 2740 Gas Chromatograph (equipped with a flame ionization detector) via a glass single-stage jet separator was used. In addition we employed a dual pen Hewlett-Packard Model 7128A Strip Chart Recorder and an RCA Model 158 Cathode Ray Oscilloscope. A Tektronix Type 541 Oscilloscope with Model 53/54 K single trace plug-in could also be used. Dynamic Single Ion Detection (DSID) Circuit Description. A block diagram of the DSID system is shown in Figure 1. The Timing and Control Logic, driven by a 500-Hz Clock, performs all sequencing functions. The Accelerating Voltage Sweep/Offset section produces a 10Hz triangle function derived by integration of a *lO-volt zenerclamped square wave generated by an operational amplifier comparator. optically coupled with the Slope Polarity logic pulse train. To this triangle function are added, by a summing amplifier, one or more adjustable offset voltages. Maximum offset obtainable is 15% of the mle set on the mass spectrometer control panel. The summing amplifier is followed by a 741 type buffer operational amplifier and is connected t o the (low impedance) scan sweep inputs of the mass spectrometer through a 1-kohm resistor. The sim-
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
