498
ANALYTICAL CHEMISTRY
side. With the outlet of the gas meter thus open to atmospheric pressure, regardless of the rate of flow, a partial vacuum cannot form inside the gas meter container and cause the sides of the gas meter to buckle. The adsorbed lindane was removed from the adsorbent by washing with successive portions of glacial acetic acid (about 150 ml. total volume). If some h e aluminum oxide is carried through, no harm is done. The filtrate was washed into an Erlenmeyer flask with acetic acid and, as in the first procedure, the acid was concentrated to about 100 ml. The lindane analysis was run on this concentrate. To the Erlenmeyer flask containing the lindane-acetic acid concentrate, zinc dust and malonic acid were added, and the flask was connected directly to the dechlorination and nitration apparatus. The flask was then heated in an oil bath a t 150" C. for about 3 hours. The exact details of the procedure are given in ( 6 ) . IVhen the colorimetric method is used to determine lindane, it is first necessary t o prepare a calibration curve employing a standard glacial acetic acid solution of lindane (6).
and precise method for sampling air containing lindane. Sampling can be made a t the rate of 0.2 cubic foot per minute. In an endosed space, n i t h very little air interchange, lindane concentrations of about 90% of saturation calculated from known vapor pressure measurements were found. ACKNOWLEDGMENT
The authors are grateful to R. A. Fulton of the Bureau of Entomology and Plant Quarantine for valuable suggestions. LITERATURE CITED
(1) Balson, E. W., Trans. Faraday Soc., 43, 54 (1947). (2) Ethyl Corp., Detroit, Mich., private communication. (3) Fulton, R. A . , Nelson, R. H., and Smith, F. F., J . Econ. Entomol., 43, 233 (1950). (4)
COKC LUSIOK s
Hoffman, R. A , , and Lindquist, *4.W., J . Econ., Entomol., 42, 436 (1949).
The concentration of lindane in air under optimum conditions can be determined with a precision of about &2% by the Schechter-Hornstein colorimetric method. Adsorption on an alumina column provides a rapid, simple,
( 5 ) Schechter, M. S., and Hornstein, Irwin, A S ~ LCHEM., . 24, 544 (1952). (6) (7)
Slade, R. E., Chemistry & Industry, 40, 314 (1945). Sullivan, W. N., J.Econ. Entomol., 44, 126 (1951).
RECEIVED for review Bugust 16, 1952. Accepted October 15, 1952
Colorimetric Determination of Magnesium with Eriochrome Black T AUBREY E, JURYEY, JR., J . fir. KOMARMY, AND G. AT. WYATT University of .Irkunsas, Fuyetteville, Ark. colorimetric methods for the determination of S magnesium are available in the literature (5, 6, Some methods depend upon the formation of color lakes and EVERAL
9-11),
others upon an indirect determination of magnesium t)>.a suitable colorimetric method. In all these determinations n o method involves a true solution of a colored magnesium complex in which the color intensity is directly proportional to the magnesium concentration. This paper presents a spectrophotometric method for the determination of magnesium using the reagent Eriochrome Black T, 1 - (1 - hydroxy 2 naphthylaxo) - 2 - hydroxy - 5 - nitro - 4 naphthalenesulfonic acid. The Versenate (disodium salt of ethylenediaminetetraacetic acid) method for the titration of total hardness of water ( 2 ) employs Eriochrome Black T as :in indicator a t the pH of 10.1. In the p H range of 7 to 10 t'his reagent forms a very slightly dissociated, soluble, intensely red coinples with magnesium. The reagent itself in unbuffered methanol solution has a deep reddish black color; a t a pH of 10.1it is blue. Interferences from copper, manganese, iron, aluminum, cobalt, and nickel were reported by Diehl, Goete, and Hach (Z'), who gave procedures for eliminating these int,erferences and limiting concentrations, Calcium was found to complex with the dye almost as readily as magnesium, giving an absorption curve similar to that of the magnesium complex. It is shown in the present investigation that the calcium ion may be removed prior to the determination of magnesium.
- -
reagent dye solution in absolute methanol was prepared. These solutions were prepared by dissolving 0.5000 or 0.1000 gram of dye in methanol and diluting to 100.0 ml. Standard Magnesium Solution. This solution was prepared by dissolving 8.3636 grams of reagent grade magnesium chloride hexahydrate, RlgClz.6H20, in distilled water and diluting to 1 liter. Magnesium was determined gravimetrically by weighing as Mg2P20, and its concentration in the solution was found to be 1.000 mg. per ml. More dilute solutions were prepared from this solution by careful dilutions with distilled water. Buffer Solutions. The buffer solution of p H 10.1 was prepared according to the directions of Diehl, Goetz, and Hach (I) by mixing 6.75 grams of reagent grade ammonium chloride with 57 ml. of C.P. concentrated ammonium hydroxide and diluting to 1 liter with distilled water. 0 5
EXPERIMENTAL
Instruments. A Beckman quartz spectrophotometer, Model DU, with 10-mm. Corex or silica absorption cells was used for absorbancy measurements made a t the maximum sensitivity of the instrument with distilled water in the reference cell. Silica cell spacers were used to obtain a 1-mm. optical path when s eci fied. Absorbancy is defined as the negative logarithm o r t h e ratio of the transmittancy of the solution to that of the solvent, A , = --log T m i / T a o l v . A Beckman Model G pH meter was used to check the pH of all buffer solutions. SSS, Reagent Solution. Eriochrome Black T, C ~ O H ~ ~ O ~was X furnished by the Hach Chemical Co. The dye solution was prepared fresh prior to each determination. For higher concentra- Figure 1. Magnesium Complex with Eriochrome Black T tions of magnesium a 0.5% reagent dye solution in absolute x d.0f2.27 x 10-3 Mdye a d d d to (1 - x)mi. ofequimolar magnesium and diluted to 25 ml. 540 mp, pH 10.1, Lam. cells methanol was used and in the lower concentration range a 0.1%
V O L U M E 25, NO. 3, M A R C H 1 9 5 3
499
pH decreases, but only the calcium complex absorption curve becomes identical with the reagent absorption curve a t pH 7.75. These curves suggested the possibility of determining magnesium \\ ithout preliminary separation from calcium. However, the difference betmeen the dye and the magnesium complex absorption curves a t pH 7.75 was not sufficient for an accurate method for the determination of magnesium. It was therefore decided to remove calcium from the solution and then determine niagncsium at a higher pII value.
I .6 Campier
Reoqent
I
II
IV
Ill V Vlll
VI1
pH
10.1
8.4
6.0 7.75
PROCEDURE FOR REMOVAL OF CALCIUM A h D DETER?IINATIOY O F MAGYESIUM
1Iany Irocedures for the removal of calcium were investigated. The most satisfactory method was the precipitationaof calcium as the' wlfnte from a 90% methanol solution ( 1 ) . 16
0.41
I
1 00
Complex
9
Reoqent IV
1
v vi
500
600
pH 10 I 8.4 7.75
70
Wavelength ,n rnp
Figure 2. Effect of pH on Absorption of Calcium Complex and Reagent
Buffers of the pH range from 7.75 t o 10.1 were prepared in the same way by varying the concentrations of ammonium chloride and ammonium hydroxide. EMPIRICAL FORMULA
Only one complex is formed between magnesium and the reagent dye a t a pH of 10.1. This was determined by measuring the absorbancj- of a series of solutions in which the molar ratio of magnesium to reagent dye was varied between the limits of 1:1 and 1 : 6 and plotting log absorbancy against wave length. The resulting superposable series of curves indicated the presence of the single complex ( 7 ) . The empirical formula of the complex was investigated by the method of continuous variations proposed by Vosburgh and Cooper ( 1 2 ) . Figure 1 sho\\s that when n is calculated it is nearly 2, resulting in the empirical formula of MgRz. This formula was confirmed by applying the method of continuous variations a t 10-mp intervals in the Tyave-length range from 500 to 600 nip. The formula of the complex in solution a t pH 10.1 does not agree n i t h that reported in the literature (8). Data from concentration curves indicate that 2.5 moles of reagent are required to convert 1 mole of magnesium completely to a practically undissociated complex. EFFECT O F pH
The absorption curves for the d j e , the magnesium, and the calcium complexes were determined a t a number of pH values. Absorbancy values were measured a t 10-mp intervals in the wavelength range from 450 to 700 mp, using maximum sensitivity of the instrument. A constant concentration of dye was used for all curves because of the absorption of the dye itself. All solutions were prepared by adding 5 ml. of the proper buffer to 1.00 ml. of 0.1% reagent dye solution and diluting in a volumetric flask to 25 ml. Solutions of the complexes also contained 100 p.p.m. of magnesium and calcium, respectively, which was sufficient excess to convert the dye completely to the complexed form. Figures 2 and 3 present the absorption curves taken from solutions of the reagent, the magnesium complex, and the ~ a l c i u m complex as the pH varies from 10.1 to 7.75. The color intensity of both the magnesium and calcium complexes decreases as the
0.0
I 500
I
I 600
700
Wavelength i n mp
Figure 3. Effect of pH on Absorption of RIagnesium Complex and Reagent
Adjust a water sample or the solution of a solid sample to contain not more than 5 mg. of calcium and 1.2 mg. of magnesium. Evaporate to a volume of 4.5 ml. Add 0.5 ml. of 9 S sulfuric acid follolved by 45 ml. of methanol with stirring. Allow the precipitate to settle for 30 minutes. To separate larger quantities of calciun~from small quantities of magnesium, the above volumes may be doubled and time for alloiving complete precipitation increafed, or calcium may be precipitated from ethanol solution ( 3 , 4). Filter the solution through Khatman No. 40 paper into a 100-ml. volumetric flask and wash the precipitate carefully n i t h 9Oy0 methanol. Add 25 ml. of pH 10.1 buffer t o the filtrate. Dissolve any precipitate which forms at this point by the addition of a little xater. .4dd 4.0 ml. of 0.1% dye solution if the concentration of magnesium is less than 1.4 p.p.m., or 10.0 ml. of 0.5Cjb dye solution for higher concentrations of magnesium. Dilute to the 100-ml. mark with distilled water. Measure the absorbancy a t 520 mp, using minimum slit width. Use a 10-mm. optical path for the lower dye concentration, a 1-mm. path for the higher concentration. Standard concentration curves were obtained for the range from 0.2 to 1.4 p.p.rn. of magnesium and the range from 1 to 14 p.p.m. of magnesium. Both curves followed Beer's law.
Figure 4 shows the absorption curves for the magnesium complex and the reagent dye solution as the time changes from 0 to 15 days. In each case the solution of the complex was prepared immediately before measurement from the stock dye solution,
500
ANALYTICAL CHEMISTRY
which was stored in the dark during the study. Excess magnesium was used to complex completely tthe dye which was present. The vertical dotted line represents the wave length of 520 mM which was used in determining the concentration curves. 16
Complex I
-
I
Reagent V
Table 1. Determination of Magnesium i n Water Sample No. 9983
12.7
Days
0
I1
VI
5
Ill
VI1
10
Colorimetric Bnalysis Average AIg, p.p.m. 12.7 12.7
Rlg, p.p.rn.
Gravimetric Analysis (Av.), Mg, P.P.M. 12.6
9984
13 7
13 0
14 5
996
16 0 1 6 .J
16.3
15.5
9986
1.5 3
12.4
14.1
3 2
4.9
4.9
2 5
2.9
10009
1.i j
I.i.5
4.5 10010
I
2 3 2 .i
Geological Survey Department, Institute of Science and Technology, Water Analysis Laboratories, University of Arkansas. The results obtained by the new method agreed with those from the gravimetric procedure within the usual error encountered in water analysis and the precision was excellent. The data obtained from both the gravimetric and colorimetric analyses are presented in Table I. It is hoped that the method can be extended to include a calcium determination either by difference or by using the calcium sulfate precipitate directly. It is also proposed that the method be adapted for the determination of magnesium in soils and plants and other materials having small magnesium content,
I I I
I I
LITERATURE CITED
This series of curves shows a considerable change in the dye curve a t longer wave lengths as the aging progresses. The variation in the absorption curves with time indicates a probable structural change of the dye in solution. The variation in the absorption curves of the magnesium complex solution could probably be attributed to the structural variation taking place in the dye solution during the aging process. Further studies have shown that with the dye in excess, the variation of the absorption curve of the magnesium complex prepared from dye aged up to 5 days is negligible a t the xave length of 520 mp. APPLICATION TO WATER SAMPLES
Analyses of water samples by the proposed method were compared to gravimetric determinations as reported by the U. S.
(1) Caley, E. R., and Elving, P. F., IXD.ENQ.CBEM.,ANAL.ED., 10,264 (1938). (2) Diehl, H., Goetz, C. A., and Hach, C. C., J . Am. Waterworks Assoc., 42,40 (1950). (3) Harvey, C. O., Analyst, 61,817 (1936). (4) Hoffman, J. I., Bur. Standards J. Research, 9, 487 (1932). (5) Hoffman, W. S., J. Bid. Chem., 118,37 (1937).
(6) Ludwig, E. E., and Johnson, C. R., IND.ENG.CHEM.,ANAL.
ED.,14,895 (1942). (7) Mellon, M. G . , “Analytical Absorption Spectroscopy,” p. 309, New York, John Wiley & Sons, 1950. (8) Schwarzenbach, G . , and Biederman, W., Helv. Chim. Bcta, 31, 678 (1948). (9) Snell, F. D., “Colorimetric Methods of Analysis,” 2nd ed., p. 470, New York, D. Van Nostrand Co., 1936. (10) Theil, A., and Van Hengel, E., Ber., 71B, 1157 (1938). (11) Thrun, W. E., IND. ENQ.CHEM.,ANAL.ED.,4, 426 (1932). (12) Vosburgh, W. C., and Cooper, G. R., J. Am. Chem. SOC.,63, 437 (1941). RECEIVED for review August 18, 1952. Accepted October 24, 1952.
Volumetric Determination of Tellurium in Organic Compounds F. H. KRUSE’, R. W. SANFTNERZ, AND J. F. SUTTLE Department of Chemistry, University of New Mexico, Albuquerque, N . M . v PREVIOUS volumetric determinations of tellurium, the element ‘.was present in alloys or in inorganic compounds. This paper describes a volumetric method which can be applied to organotellurium compounds where rather vigorous conditions are required to decompose the organic constituent. The sample is digested in perchloric acid and then titrated with potassium dichromate using sodium diphenylamine sulfonate indicator in the back-titration with ferrous ion. Though numerous volumetric methods for the determination of tellurium have been developed, those applicable to the types of address, University of California, Los Angelea, Calif. Present address, University of Illinois, Urbana, Ill.
1 Present 2
organotellurium compounds studied in this laboratory proved too tedious or unsatisfactory. Lenher (8) has written a review of the analytical methods, volumetric and gravimetric, available up to 1926. Volumetric methods using potassium dichromate (9, 1 4 , potassium permanganate ( 7 ) ,and ceric sulfate (IS)for the oxidation of tellurium from an oxidation state of I V to VI have been reported, as well as iodometric (4, 6),instrumental (1, 6), and gravimetric (9, 3, 10) methods employing elemental tellurium. Only the gravimetric methods by Drew (2) and by Tsao (12) have been specifically developed for organotellurium compounds, although solution of the elemental tellurium and titration of the
