781
V O L U M E 23, NO. 5, M A Y 1 9 5 1 volume of t h e original filtrate will give the quantity of alkaloid present in the 500 grams of tissue used for the analysis. The results are given in Table I. Each value is given as the average of two concurrent analyses of the same sample. Duplicate values checked within an average of 3%. Should the chloroform solvent contain much more than 6 niicrogrems of the alkaloid per ml., the analysis should be repeated, using a smaller volume of the aqueous extract. Similarly, should the solvent contain less than 1 microgram of alkaloid, a larger volume of the aqueous extract should be processed. h direct comparison of the color of the final chloroform extract of tissue filtrate with that of a standard alkaloid solution processed in the same manner, may also be used. .4n analysis of the data in Table I leads to the following convlusions: 1. The recoveries of most of the alkaloids and other organic ompounds are good. 2. Some antihistamines-for example, Keohetramine-can also be determined by this method with good results. 3. The low recoveries in the case of cocaine are probably due to decomposition during the steam distillation, because aqueous solutions of cocaine that were not heated gave good recoveries. 4. The paminobenzoic acid derivative pontocaine, and nupercaine, gave good recoveries from aqueous solutions under the itlentical conditions of the method. When added to tissues, howw e r , the recoveries were low. This x a s probably due to decomposition by enzymatic action. pAminobenzoic acid is not determinable by this method and the aliphatic base obtained from the decomposition is extractable only a t high alkalinity and with another sdvent. a. Morphine, not included in Table I, gave poor results under the conditions of this method. Further studies involving the re(
covery of morphine by altering experimental conditions % ,conthe templated. The poor results are probably attributgfffe amphoteric nature of morphine. ‘aw
60
IIor ACKNOWLEDGMENT
)nu
The authors are deeply indebted to Milton Levy of New York University Medical School and Bernard €3. Brodie of the Research Service, Third (New York University) Medical Division, Goldwater Memorial Hospital, who placed their laboratory facilities a t the authors’ disposal. Sidney Udenfriend of New York University Medical School merits thanks for his many helpful suggestions. LITERATURE CITED
(1)
Biodie, B. B., and Udenfriend, S.,J . Bid. Chern., 158, 705-14
(2)
Gettler, A. O., Umberger, C. J., and Goldbaum, L., AX\IZL. CHmi.,
(1945).
22, 600-3 (1950).
Kofler, A., “Mikromethoden Bur Kennzeichnung organischer Stoffe und Stoffegemische,”Vols. I and 11,Philadelphia, hrthur H. Thomas Co., 1947. (4) Lehman, R. A., and ditken, T., J . Lab. Clin. Med., 28, 787-93
(3)
(1943).
( 5 ) llarshall. P. B., and Rogers, E. W., Biochen. J . , 39, 258-60 (1945). (6) Oberst, F. W., J . Pharmucol. Esptl. Therap., 79, 10-5 (1943). (7) Prudhomme, R. O., J. pharm. chim., 9,8-17 (1940). ( 8 ) Reimers, F., Anal. Chim. Acta, 2, 1-16 (1948).
RECEIVEO September 13, 1950. Presented before the Division of AnCHEMICAL alytical Chemistry a t the 119th Meeting of the AMERICAX SOCKETS. Boston, Mass. From a thesis submitted in partial fulfillment of the requirements for the degree of doctor of philosophy a t Xew York Universitv.
Colorimetric Determination of Tungsten Study of Variables Involved in Stannous Chloride-Thiocyanate Method H I R R Y FREUND, Oregon State College, Corvallis, Ore. AND
MARK L. WRIGHT
ROBERT K. BROOKSHIER Northwest Electrodevelopment Laboratory, Bureau of Mines, Albany, Ore. AND
Erratic results in the determination of tungsten led to a study of the variables influencing the stannous chloride thiocyanate method. Free acid and chloride concentrations determine the degree of reduction of the tungsten. With 7.00, 5.95, and 3.63 moles of chloride per liter, the lower limits of free acid to achieve complete reduction are 9.5,11.2, and 13.9 moles per liter, respectively. The thiocyanate concentration should be maintained about 0.2 molar for best results. The reliability of the method is thereby improved by selection of optimum operating conditions.
T
HERE are only a few chemical methods for determining
tra’ce amounts of tungsten. Current interest is directed primarily toward the colorimetric method, based on the reduction of tungstate and subsequent formation of the yellow tungsten thiocyanate complex. A bibliography relating to the development of the method is given by Geld and Carroll ( 2 ) . The changes in procedure and reagents suggested in the literature stress the need for studying the reactions involved. This paper discusses some of the variables involved in the stannous chloride thiocyanate method, and makes possible selection of optimum operating conditions, thereby improving the reliability of the method. Experiments on the nature and rate of formation of the color when the reduction and color development
are carried out simultaneously indicate the limitations of this method. A modificd procedure, separating the reduction and complexation steps, is considered in detail. The influence of free acid, chloride, and stannous ion concentrations on the reduction and the choice of proper conditions for color development comprise the major parts of the investigation. The addition of stannous chloride and then potassium thiocyanate t o an acid tungstate solution results in a greenish color, whereas the same additions to an initially alkaline solution yield a yellow color. Two tungstate samples treated according to the general procedure of Sandell ( 4 ) were identical in all respects, except that one was initially acid and the other initially alkaline. The two absorption curves, plotted in Figure 1, show a maximum
ANALYTICAL CHEMISTRY
182
of 4?$f$$,
characteristic of the tungsten thiocyanate complex. The suggestion of Gentry and Sherrington ( 8 )that the color of the green solution is due to the additive effect of blue quinquevalent compounds of tungsten is supported by the lower absorption of the thiocyanate complex and the somewhat greater absorption in the red region. The slow development of the color a t room temperature is another disadvantage. Two to 5 hours are required to achieve complete reduction of the tungsten and stabilization of the color in solutions containing 0.2 to 6 p.p.m. of tungsten. Raising the temperature leads to extensive decomposition of the thiocyanate into hydrogen sulfide.
inltia1Iy a l k a l i n e ( y r l l a w ) l n l l i o l l y acid
____------------___ -
O.'t I
(grrrn)
t
I
400
I
I
I
O
500 WAVE
,
(
I
600 LENGTH mp
I
I
'
l
l
>
4
l
700
Figure 1. Absorption Curves of Tungsten Thiocyanate Complexes
The use of other reducing agents such as amalgams ( 3 ) and titanous chloride ( 1 )has been suggested for more rapid and complete reduction. However, stannous chloride is desirable as it is colorless, readily available, and capable of yielding reliable results. Geld and Carroll ( 2 ) , by first reducing with stannous chloride a t the boiling point of the solution, cooling, and then adding the thiocyanate, achieve complete reduction in a matter of minutes. Under these drastic conditions the yellow complex is formed, even though the reduction is made in an initially acid solution. They applied the method to the determination of tungsten in high-temperature alloys. chloride and free acid concentrations influence the electrode potentials of the redox system and may determine the extent and rate of reduction. The next series of experiments relate the formation of the complex tungsten thiocyanate to these solution conditions, when the reduction and complexation steps are performed separately.
used to obtain 100% traiismittaiice. The aliquota are analyzed for stannous ion, free acid, arid total chloride. Methods of Analysis. T h 5.00-nil. aliquot is diluted to 100 ml. and suitable fractions are removed for the various analyses. TINDETERhiIX.4TION. The stannous ion concentration is determined in the conventional nia1inc1' by titrating with standard iodine solution, which has been standardized previously against. arsenious oxide. FREEACID DETERMIXAIIOX. The method for the total iweacid is based on the procedure recommended for the determination of free acid in the presence of a hydrolyzable salt. Best results are obtained by a preliminary oxidation of tin with hydrogen peroxide followed hy complexation with tartrate. Five granis of disodium t,artrate are dissolved in 100 nil. of water, 2 ml. of 3Oy0 hydrogen peroxide arc d d e d , and the pH of the solut.ioii is adjusted to a phenolphthalein end point with standard sodium hydroxide. .hi aliquot of the sample to be analyzed is added and the solution is again brought, to the pheiiolphthalein end point. with standard sodium hydroxide. TOTAL('HLORIDE D E T m . v I s . i n o s . Following oxidation o f the stannous ion with hydrogen peroside and neutralizatiori of most of the free acid with sodium h>droxide, the total chloride is determined by titrating with standard silver nitrate, using dichlorofluorescein indicator. Two milliliters of 30% hydrogen peroxide and about 0.1 gram of dextrin are added to 100 ml. of water. A suitable aliquot of the sample is added together with sufficient chloride-free sodium hydroxide to neutralize 99% of the free acid. Standard 0.1 N silver nitrate, prepared by the direct solution of the salt in water, is used to titrate the chloride. Despite recipitation of the tin, the results on synthetic samples and furtfm checks with the Volhard method have shown the adsorption indicator method to he accurate.
Effect of Chloride Concentration on Reduction of Tungstate with Stannous Chloride. Those factors influencing the strength of the stannous chloride m a reducing agent should exert the most profound influence on the reduction step. The significance of the ionic activities is evident from the equation for the reduction potential of the stannous-stannic system.
117' E = EQ - - In (us,,+-/us,,----) nF The standard electrode potential, Eo, is equal to f0.13 volt, and a represents the activities of the ions. R, T , n, and F have their conventional meanings. The more negative the value of E, the stronger is the reducing agent. Hence, increasing the ratio of stannous to stannic ion activities will result in a more negative reduction potential.
'+/
I 8 3 equ8w. H O * / l 0.183 mol. Sn I
0.01 m g . W / m / .
GENERAL EXPERIMENTAL PROCEDt'RE
Solutions. Sodium tungstate (0.276 gram, 72.591, tungsten) is dissolved in water containing a few tenths of a gram of sodium hydroxide and then diluted to 2 liters. A tungsten concentration of 0.10 mg. by weight per ml. is obtained. Stannous chloride, 2 M , is prepared by dissolving 112.9 grams of stannous chloride dihydrate in concentrated hydrochloric acid and making up to 250 ml. Rith concentrated hydrochloric acid. Potassium thiocyanate, 20%, is made by dissolving 20 grams of potassium thiocyanate in 80 ml. of water. Magnesium chloride, 8 M , is made by dissolving 250 grams of magnesium chloride hexahydrate in 100 ml. of water. The solution is assayed by means of a chloride determination. The system to be studied is placed in a 50-ml. borosilicate glass volumetric flask and adjusted to the mark, and a 5.00-ml. aliquot for analysis is transferred to a 100-nil. volumetric flask. The reaction flask is immersed in a boiling water bath for 5 minutes and cooled in running water a t 13" to 15' C. for 2 minutes. The reduction mixture is poured into 10.00 ml. of potassium thiocyanate and 25.00 ml. of n-ater in a 100-nil. vnlumetric flask. Water IS added approximately to the mark and the samples are thermostated a t 25.0" C. Extinction readings a t a wave length of 400 mp are taken after 1 hour, using a Beckman D U spectrophotometer. A blank sample containing no tungsten or magnesium chloride, hut otherwise identical, is carried through the procedure and
i
2
3
4
CONCENTRATION OF CHLORIDE
Figure 2.
5
6
7
8
MOLS/LiTER
Effect of Chloride Concentration on Reduction of Tungsten
Bivalent and tetravalent tin in hydrochloric acid solutions are known to exist primarily as complex ions, SnClc-- and SnCI6--, and the activities of stannous and stannic ions will depend upon the dissociation of these complexes. If the activity of the SnC14-- ion is assumed to be large and relatively constant (as it would be in these experiments), and the dissociation of SnClO-- is assumed to be small, an increase in the chloride activity will cause a relatively greater decrease in the stannic activity than in the stannous activity. Thus the strength of stannous chloride as a reducing agent may be increased by increasing the chloride activity.
V O L U M E 2 3 , N O . 5, M A Y 1 9 5 1
783
Experimental confirmation is obtained by selecting concentrations of free acid and st,annous chloride known to yield only a partial reduction of tungsten. Then the effect of increasing the chloride ion concentration is studied by adding varying amounts of Ptrong magnepiuni chloride solution to a series of samples.
1
,0A0
2
4
CONCENTriATIOV
Figitre 3 .
6
8
IO
I2
OF F R E E ACID -EQUIVALENTS
acid. The experimental technique and methods of analysis are the same as described before. The results are plotted in Figure 3 and show the desirable effect of operating a t high acidities. Combined Effect of Chloride and Free Acid on Reduction of Tungstate with Stannous Chloride. The experiments thus far were not conducted on practical systems, owing to the presence of large amounts of magnesium chloride and the independent variation of the chloride and free acid. I n actual practice, varying amounts of sulfuric acid may he present, while t,he addition of hydrochloric acid will siniultaneouslg increaye both the free acid and chloride concentrations. With this in mind, a family of curves is prepared using a series of fixed total chloride concentrations as parameters and varying the free acid by means of sulfuric acid additions. Only stannous chloride and hydrochloric and sulfuric acids :ire used to control the concentrations of chloride and free acid.
I4
H30+/ L I T E R
Effect of Free Acid on Reduction of Tungsten
The procedure coneist,s of placing 5.00 nil. of concentrated Iiythrhloric acid, t,he desired amount of niagnesiuni chloride solution, and 5.00 ml. o f sodium tungstate solution in it X-ml. volunletric flask. Five milliliters o f stannous chloride are added, the sample ip dilutwl t.o the mark with water, and a 5.00-ml. aliquot is removed for analysis. The reduction, color develop'merit, and analyses &re carried out, by the methods already desc~itietl.
Thc~results of thew experiments, plotted in Figure 2, show the pronounced effect of the chloride concentration in increasing the reduring power of the stannous-stannic system. At the high conrentlation levels u e d , little is known about the activity coefficients except that they are considerably greater than 1. Thus the effective concentration is evcn greater than the analytically determined values. Unfortunately, the solubility of magnesium chloride does not permit chloride concentrations above those uwd, but it appears that essentially complete reduction is attained ahove 7 moles per l i k r of chloride ion. Attrmpts to u w stannous sulfatr as the reducing agent gave results t,hat were qualitatively similar t o the above runs. Because of the low solubility of stannous sulfate, a weak reducing syskm resulted, even with ttheaddition of chloride; consequently, only a sinall percentage incrmse in the rstent of tungsten reduction was noted. Effect of Free Acid on Reduction of Tungstate with Stannous Chloride. The dependence of the rate of reduction on acidity was suggested by SQndell (6),although as hydrochloric acid was used the combined c.ffect,s of both acid and chloride were ob,u:rved. Increased acidity will deciwsc the dissociation of b0t.h H2YnC1a and IT2SnC16, but should not markedly influence the ratio of stannous t o stannic activities. The exact nature of the tungsten thiocyanate complex is not kiiown. Fl'ork in progirss at Oregon &ate College indicates that tungsten ha2 a valrnw state of 5. A possible half-c~llreaction might lx : IVO4-
t 81130+
+ l(e)
=
W+
+ 12H2O
The corwspondirig clectrode potential equation indicates the role of the free acid. .4s the acidity is increased, the electrode potential is made more positive, thus increasing the oxidizing power of the tungetcn system. Consequently, its reduction with stannous chloride is facilitated. Experimental verification is obtained in a manner analogous to the chloride experiment. Samples are prepared in which the stannous chloride and magnesium chloride concentrations are known to produce only slight reduction of the tungsten. The total free wid is then varied by means of additions of sulfuric
2 4 6 8 IO 12 14 16 CONCENTRATION OF F R E E ACID E Q W V A L E N T S H30t/ L I T E R
Figure 4.
Combined Effects of Chloride and Free Acid on Reduction of Tungsten
The results of these runs, plotted in Figure 4,supply the essential data required for the selection of suitable reduction conditions. From the three curves the limiting free acid concentrations were found t o be 9.5, 11.2, and 13.9for curves with 7.00, 5.95, and 3.63 moles of chloride per liter, respectively. Free acid concentrations above these values produce no increase in extinction; hence, it may be assumed that complete reduction of the tungsten has taken place. The decrease in absorbancy, below the critical concentrations of chloride and free acid, shows clearly why erratic rwults would be obtained with the stannous chloride method of reduction, if adequate control were not maintained. I
0.3 -
I
/
oP--o
O
D
-
-
O
1
,
o----.o\
'P
I
!
1
0.3 0.4 0.5 06 0.7 OS C O N C E N T R A T I O N OF S T A N N O U S I O N MOL / L I T E R 01
Figure 5 .
01
Effect of Stannous Chloride Concentration on Reduction of Tungsten
Effect of Stannous Chloride on Reduction. Cnder conditions such that stannous chloride is a sufficiently strong reducing agent, and with a preponderance of tin over tungsten, a variation in the tin concentration would not be expected t o result in a significant change in the amount of tungsten reduced. This is borne out in Figure 5, which shows a wide variation in the permissible stannous chloride concentration.
ANALYTICAL CHEMISTRY Table I.
Effect of Time on Color Development
(0.46 mg. of tun sten reduced with 10 ml of concentrated &SO4 20 ml of concentrated H A , 5 ml. of SnCll per 50 ml.;color developed wit; 10 ml: of 2 M KCNS per 100 ml.) Absorbancy after Sample 0.5 1 Null 2 3 4 Cell Cell hour hour hours hours hours 0.002 0.005 0.009 Water Blank 0.014 0.018 Water Tungsten 0.278 0.282 0.287 0.293 0.299 0.277 Blank Tungsten 0.278 0.279 0.279 0.280
Table 11.
Effect of Potassium Thiocyanate Concentration on Color Development (0.45 mg. of tungsten reduced as in Table I) Absorbancy after 0.5 hour 1 hour 0.255 0.247 0.272 0.268 0.272 0.268 0.277 0.277 0,281 0.280
Volume of 2 M KCNS, hI1. 3.00 5.00 8.00 10.00 20.00
necessity for maintaining a constant trmperature. S o significant difference in extinction or stability could be detected over a period of 4 hours. -4 large blank due to a greater rate of decomposition of the thiocyanate a t high temperatures makes it desirable to maintain the temperature around 25' C. or less. RECO.MMENDED PROCEDURE FOR REDUCTIOS . i N D COIIPLEXATION O F T U S G S T E N
The sample containing from 0.1 to 1.5 mg. of tungsten is placed in a 100-ml. borosilicate volumetric flask, water is added to adjust the volume to 15 nil. and 10 nil. of concentrated sulfuric acid are added, mixed, and cooled. This is followed by 20 ml. of concentrated hydrochloric acid and 5 nil. of 2 M stanuous chloride. The solution is then placed in a boiling mater bath for 5 minutes. After cooling for 3 minutes in running cold water (10' to 15' C.), 10 ml. of 2 Jf potassium thiocyanate are added, and t.he volume is adjusted to the mark with distilled water. The extinction is determined after 15 minutes, using a wave leiigth of 400 in@. A reagent blank carried through the Bame procedure may he used in the null position. .i standardization curve prepared according to this procedure shows strict adherence to Beer's law. The absorbancy inder, a,in the equat>ion
A Color Development with Potassium Thiocyanate. Some difficulty is encountered in obtaining a stable color system. T17hen small amounts of potassium thiocyanate are used the color fades on standing; with increasing amounts the color intensifies. Presumably the fading is due to reoxidation of the tungsten and occurs when insufficient thiocyanate is available to repress thts dissociation of the complex. The intensification of the color IS due to the formation of colored decomposition products of the thiocyanate. The data in Table I confirm the stability of the tungsten complex and show clearly the role of the reagent blmk in increasing the absorbancy of the system. Table I1 emphasizes the need for an adequate concentlation of thiocyanate both to develop the color fully and to prevent fading. In the previous work the samples were thermostated at 25.0 O C. to minimize any error due to temperature variation. This is an undesirable requirement for an analytical method, and several runs a t 12.4', 25.0", and 34.6" C. were compared to ascertain the
=
log (Io/I)
=
a
xcx
I
is 62.5 when the concentration is espressed in milligrams per milliliter and the length of the optical pat,h in centimeters. ACKNOWLEDGMENT
Special acknowledgment is made to E. C. Gilbert for critical review of the manuscript. LITERATURE CITED
(1) -4~111,J. C., and Kinard, F. TY., J . B i d . Chem., 135, 119 (1940). (2) Geld, I., and Carroll, .J., - 4 r . k ~ .CHEY.,21, 1098-101 (1949). (3) Gentry. C. H. R., and Yherrington, L. E., Analyst, 73,57-67 (1948). (4) Sandell, E. E., "Colorimetric Determination of Traces of RIetals," pp. 427-30, New York, Interscience Publishers, 1944. ( 5 ) Sandell. E. B., IND.ESG. CHEY.,; I N . ~ED., L . 18, 163-7 (1946). RECEIVED August 16, 1950. Presented at the Nort,haest Regional 1Ieeting , AmzIcaN CHEJ~ICA SocIhri-, L Richland, Wash., June 1950. Published b y iicrmission of the director, Bureaii of Mines, LT. 8. Department of the Interior.
Instrument for Internal Standard Flame Photometry Application to Determination of Calcium in Rare Earths ROBERT H. HEIDEL AND VELMEK A . FASSEL I n s t i t u t e f o r A t o m i c Research, Iowa S t a t e College, ilnaes, Iowa
I
N THE application of direct photoelectric methods to the measurement of inteniities of flame spectra, a new analytical method known as flame photometry has been developed (2) Flame photometric techniques using a monochromator in place of glass and interference filters for spectral isolation have become thr preferred method because of the reduction of spectral interference and over-all background radiation. The practice with several of the flame photometers of the monochromator type commercially available (1, 9) is to measure the spectral line intensity of an element of interest and to compare this with measurements made on standard samples. This technique of measurement does not take advantage of the increasedprecision and greater freedom from extraneous influences afforded by application of the well-known internal standard principle (8). A ratio flame photometer using this principle and its commercial counterpart have been described ( 3 , 15), but this instrument is restricted to a single element, lithium, as an internal standard. With a few relatively simple modifications of a laboratory monochromator, it is possible to have an instrument offering the
advantages of internal st,andarclizatioii, and in addition, greater fleribilit,y in choosing internal standard lines best suited for various analyses. Furthermore, the need for the more elaborate amplification circuits of the commercial flame photometers is eliminated by the use of multiplier phototubes for measurement of spectral line-intensity ratios. The fact that line-intensity rat,ios in flame excitation under controlled conditions do not undergo the continuous changes characteristic of electrical excitation allom the use of simple electrical circuits with suitable band pass to eliminate small, short-period fluctuations. Although this instrument is constructed around a commonly available monochromator, the design can readily be extended to any spectrometer or spectrograph. INTERNAL STANDARD FLk'clE PHOTOAMMETER
Monochromator. The schematic diagrams in Figures 1 and 2 show the instrument as built around a Gaertner constant-deviation monochromator. The burner WLS positioned to place the
