JAMES H. MOSER' and MAX B. WILLIAMS Oregon State College, Corvallis, O r e .
Samples of murexide used as the complexing reagent in the colorimetric determination of calcium were found to vary widely in purity. Because no satisfactory and reliable method for analysis of the absolute purity of murexide was available, this gravimetric method was developed. Murexide is precipitated and weighed as the monohydrate of the calcium salt, CaCdVs06HrH20. Several samples analyzed by this procedure showed purities ranging from 70 to 100%. The gravimetric assay provides a method for the standardization of a colorimetric analysis of murexide.
C R E X I D E (ammonium purpurate) was used by the present authors as a reagent for the colorimetric determination of calcium ( 4 ) . In that work and in some studies recently completed, samples of murexide obtained from different eources apparently varied quite widely in purity and stability as determined by absorbance measurements. HoIvever, it was not possible to determine the absolute purity of the samples by this method for no reference standard was available. Kuhn and Lyman ( 3 ) and Davidson ( 1 ) titrated murexide with sodium hyposulfite in the absence of oxygen, but the best attainable accuracy found by the present authors with this method was about =!=8%. The present paper presents a direct method for the determination of murexide based upon the limited solubility of the calcium salt under optimum conditions. In this method, the bulk of the murexide is precipitated and weighed as calcium purpurate monohydrate, and if the highest accuracy is desired, the very small amount of the dye remaining in Eolution is estimated Epectrophotometrically. REAGEYTS
Calcium nitrate, c.P., mas weighed out on a trip balance to make a 1-M solution. A carbonate-free solution of 0.200N C.P. sodium hydroxide was prepared. -4concentrated buffer solution of about pH 9 was prepared by dissolving 49.4 grams of C.P. boric acid in 100 ml. of distilled water, adding to this 852 ml. 0.200N of sodium hydroxide, and diluting this solution t o 2000 ml. Four samples of murexide were obtained in the following manner; Samples A and B were prepared according t o Davidson (1) and Hartley (I),reepectively. Samples C and D were purchased from two commercial sources.
0.2N of sodium hydroxide, with the solution being well stirred during the addition of the base. The material adhering t o t h e electrodes was washed back into the beaker with a minimum amount of distilled water and the precipitate allowed to settle and coagulate by setting the beaker aside for 5 minutes a t room temperature. Next, the precipitate was quantitatively filtered by means of suction with a sintered glass crucible, previously dried to constant weight a t 110' C., with the filtrate and washings being caught in a clean suction flask. The precipitate was washed three times with small portions of distilled water and dried to constant weight a t 110' C. in an air oven. Estimation of Murexide Remaining in Filtrate. The amount of murexide not precipitated was usually found to be less than 2y0 of the total amount, and in the case of large samples (about 100 mg.) of the purest dye, this amounted to about 0.5%. Therefore, if the greatest accuracy is desired, the solubility correction should be determined by the following colorimetric method. The filtrate remaining in the suction flask mas quantitatively transferred into a 250-ml. volumetric flask, 50 0 ml. of the borate buffer solution added, then distilled water to the mark. The absorbance of this solution was determined a t 485 mfi against distilled Rater, and the total amount of the murexide contained in t h e filtrate was calculated by means of Beer's law, using a previously determined constant. To obtain the Beer's law constant, 125 mg. of murexide sample, A (n hich the gravimetric method showed to be essentially pure) was dissolved in 250 ml. of distilled water. Various exactly measured amounts of this solution were then added to 250-ml. volumetric flasks containing 50.0 ml. of borate buffer and 0.5 ml. of 0.200X sodium hydroxide (approximately the volume required t o adjust the pH to 8.5). The various solutions were diluted to the mark with distilled water and the absorbance of each solution determined as in the preceding paragraph. If no dye samples of high purity are available (as shown by the gravimetric analysis), then the value of the Beer's law constant for pure murexide may be obtained with an impure sample by utilizing the gravimetric results and a series of successive approximations-i e., the first approximate constant R-ould be obtained by calculating the approximate murexide concentration by the per cent purity of the sample obtained from the gravimetric portion of the method; then, a solubility correction calculated from this constant would be added to the gravimetric reiults. This process is repeated until sufficient convergence is obtained. Calculations. To determine the original weight of murexide taken, the weight of calcium purpurate monohydrate found was multiplied by Molecular weight of murexide = 0.8792 Molecular weight of calcium purpurate monohydrate and the amount of murexide remaining in the filtrate, calculated by Beer's law, was then added t o this value.
APPARATUS
The p H values of various solutions were measured with a Beckman Model H-2 pH meter calibrated against a p H 7.00 (25" C.) buffer solution obtained from the same company. Absorbance determinations were made in 1.00-cm. matched Corex cells in a Beckman Model B spectrophotometer. The precipitate was collected in small sintered glass crucibles of fine porosity. ANALYTICAL METHOD
Precipitation. A sample of the dye containing between 40 and 120 mg. of murexide was weighed into a 50-ml. beaker, several drops of water were added, and this mixture was made into a smooth paste with a flat-ended stirring rod (this technique was found to decrease greatly the time subsequently required for solution). Then 3.50 ml. of 1M calcium nitrate solution and 16.5 ml. of distilled water were placed in the beaker and the contents stirred until completely dissolved. In the least pure samples, a light colored residue remained, so the solution was quantitatively filtered thiough paper and the residue discarded. The p H of the solution was adjusted to 8.5 by means of the p H meter and the 1
Present address, Shell Oil Co., Martinez, Calif.
Table I.
Analysis of Calcium Purpurate Precipitate
Component % Found Calcium 1 2 . 4 5 =tO0.4Oa Nitrogen 2 1 . 4 f0.36 Water 6.8 2 ~ 0 . 5 ~ 0 Standard deviation. b Average deviation.
Theoretical Percentage C~C~NSO~HVHZOCaCdsOxj 12.40 13.13 21 67 22.93 5 6 ...
DISCUSSION
The calcium content of the precipitates obtained by the precipitation procedure to four murexide samples was determined by ignition of the salt to the oxide. The total nitrogen content of the calcium purpurate was determined by the Dumas method. Water of crystallization was determined by drying the precipitate a t 137' t o 140' C. in vacuo over phosphorus pentoxide to constant weight. The results of these analyses are presented in Table I. 1167
1168
ANALYTICAL CHEMISTRY
The results found for the four samples of murexide, with two runs per sample, are presented in Table 11. Statistical tests showed that the results are not dependent on the weight of sample taken within the limits shown. The essentially pure murexide samples A and C did not differ significantly, whereas the impure samples B and D did differ significantly between themselves and from A and C. The estimated standard deviation of a single run was 0.45% murexide. The anhydrous calcium purpurate was found to be very hygroscopic and could not be conveniently used in this method. The slightly high water content found for the monohydrate (Table I ) could perhaps be ascribed to a slight amount of thermal decomposition of the calcium purpurate taking place during the water determination. Calcium purpurate monohydrate is slightly hygroscopic, but not sufficiently so to make the weighings difficult. For this reason, the monohydrate was selected as the more suitable form for the gravimetric method. The assay results presented in Table I1 were further verified in a relative manner by absorbance determinations on each sample. Sample A was arbitrarily taken as the standard, with an assigned purity of 100%. Using the Beer’s law constant obtained with this sample, the per cent purities of the remaining samples were determined spectrophotometrically.
Colorimetric Procedure. To each of nine 100-ml. volumetric flasks, 14.82 ml. of 0.200N sodium hydroxide and 25.00 ml. of 0.200N potassium phos hate, monobasic, were added (this buffer mixture results in a p 8 of 7.0 when diluted to the mark). Between 40 and 50 mg. of murexide from each of the four sources were weighed to the nearest 0.1 mg., transferred to 250-ml. volumetric flasks, dissolved, and diluted to the mark with distilled water. A 10.00-ml. aliquot of a murexide solution was added to each of two 100-ml. flasks, the solution diluted to volume, and the absorbances determined a t 525 mp (the absorption maximum of murexide under these conditions) in 1-cm. cells versus a blank prepared by diluting the solution in the ninth 100-ml. flask to the mark with distilled water.
Table 11. Murexide Found for Various Samples Sample Taken, Mg. 100-116 59-73 a
Murexide Found, % Sample B C D
A 100.3a 101.0
74.8 74.1
99.9 100.4
69.8 70.3
100.7 74.5 Mean Each number represents one run.
100.2
70.1
-
Mean 86.20 86.45
Table 111. Relative Purity of Murexide Samples by Absorbance Measurements Sample Purity,
a
Mean Arbitrary standard.
Aa
B
C
D
99.5 100.5
74.2 75.0
99.7 98.6
69.1 69.7
100.0
74.6
99.2
69.5
The results of the colorimetric procedure are shown in Table 111. These figures compare favorably with those reported in Table 11, within the experimental error. Thus the proposed gravimetric procedure for the assay of murexide provides an absolute method for the standardization of a colorimetric analytical curve for this compound. LITERATURE CITED
(1) Davidson, David, J . A m . Cheni. Soc., 58, 1821 (1936). (2) Hartley, W. K.,J . ChenL. Soc. (London),87, 1791 (1905). (3) Kuhn, R., a n d L y m a n , J. C., B e r . , 69, 1547 (1936). (4) Williams, RI. B., and Jloser, J. H., ANAL. CHEM.,25, 1414 (1953). RECEIVEDfor review February 8, 1954. Accepted April 16, 1954. P a r t of a thesis of James H. Moser presented to the Graduate School of Oregon State College in partial fulfillment of requirements for degree of doctor of philosophy. Approved for publication b y the Oregon State College Monograph Committee, Research Paper No. 243, Department of Chemistry,
School of Science.
Determination of Traces of Antimony in Soils and Rocks F. N. WARD and H. W. LAKIN Geochemical Prospecting Research Laboratory, Denver Federal Center, Building 25, Denver, Colo.
A relatively simple, rapid, and moderately accurate method for the determination of traces of antimony in soils and roclrs is based on the reaction of pentavalent antimony with rhodamine B in isopropyl ether after extraction of the antimony from 1 to 2M hydrochloric acid. The suggested procedure is applicable to samples containing from 0.5 to 50 p.p.m. of antimony, and with modifications it can be used on samples containing larger amounts. Four determinations on two rocks containing less than 2 p.p.m. of antimony agree within 0.4 p.p.m., and four determinations on seven soils containing 2 to 10 p.p.m. of antimony agree within 1 p.p.m. of the mean. The conditions for oxidation of the antimony and the subsequent extraction of the pentavalent form with isopropyl ether have been established. Experiments show that the antimony-rhodamine B compound is stable in isopropyl ether for more than 3 hours. The suggested procedure permits the determination of 2 y of antimony in the presence of 30,000 y of iron, 250 y of arsenic, and 300 y of gold and/or thallium. Data are given to show the applicability of the method to routine laboratory and field use. Under field conditions the method has been used to determine traces of antimony in as many as 20 soil samples in an 8-hour day.
T
HE U. S. Geological Survey is currently engaged in a program designed to investigate the usefulness of geochemical prospecting in locating new ore bodies and extending older ones ( 1 , 8, 10). The chemical analysis of soils or rocks for various elements such as copper, lead, zinc, molybdenum, silver, or arsenic is a primary step in geochemical prospecting. Although antimony is not abundant, it is common in sulfide deposits; and the chemical analysis of soils or rocks for antimony is worth while, because the soil or rock sample containing an abnormal concentration of antimony may indicate a sulfide deposit. A search of the literature revealed that existing methods were inadequate for determining the traces of antimony in such soils or rocks. Therefore, it was necessary to develop a new procedure, which was sensitive enough to distinguish anomalous values from background values. In this paper the background value of antimony is defined as the amount found in representative soils from nonmineralized areas. Recent estimates of the abundance of antimony in the earth’s crust ( 1 4 ) are all in the range of 0.3 to 1 p.p.m Because very few determinations of antimony in soils and rocks have been made, three of the estimates were made only on igneous rocks. As igneous rocks constitute about 95% of the earth’s crust ( B ) , the antimony content of igneous rocks can serve as a background value in the establishment of the range over which a method of
