V O L U M E 25, NO. 2, F E B R U A R Y 1 9 5 3
27 1
WAVELENGTH, MICRONS
their spectra. Two of them can be readily identified by the following characteristics: 9-Dehydrohecogenin can be identified by is0 configuration from A, monohydroxy from B, a,@-unsaturated ketonic band a t 1680 cni.-’ 9-Dehydromanogenin can he identified by is0 configuration from A, dihydrosy froin B: a,& unsaturated ketonic band a t 1681 cm.-‘ similar systematic identification procedure can be set up for the unncetylated sapogenins in chloroform solution. The spectral tests for F ring, normal us. is0 configuration, carbonyl, and A s unsaturation are the s:tme as for the acetat,es. The distinction hetween mono- and dihydrosy sapogenins is not a3 clear cut as with the acetates, hut can usually lie made in the 1000 to I100 cm. -1 region. I n general, monohydroxy sapogenins have a single hand near 1050 cni. -1, Tvhercas dihydroxy sapogenins have a band that is stronger, broader, or more complex. If these preliminary tests point to a known genin whose spectrum turns out to be substantially identical to that of the unknown, and if the spectrum of the unknown is rlearly different from any of the other known spectra, the identification is considered satisfactory. On the other hand, if the sample is not pure enough to allo~rn. decision among the known spectra, it is best to acetylate and compare with the acetate curves. ACKKOWLEDGRIENT
The authors gratefully acknowledge the assistance of MaryAnne Morris and A4udrryE. Jones in various phases of this in-
vestigation. The sapogenins vere isolated by Arthur Finchler, H. TT’. Jones, If. E. Kenney, R. F. Mininger, and Samuel Serota. LITERATURE CITED
Bergmann, E. D., and Pinchas, S., Rec. trav. chim.,71, 161 (1952). Jones, R. X’., Humphries, P., and Dobriner, K., J . Am. Chem. Soc., 71, 241 (1949). Jones, R. N., Humphries, P., and Dobriner, K., Ibid., 72, 956 (1950). Jones, R. N., Humphries, P., Herling, F., and Dobriner, K., Ibid., 73, 3215 (1951). Jones, R. N., Humphries, P., Packard, E., and Dobriner, K., Ibid.,72,86 (1950). Jones, R. N., Williams, V. Z., Vihalen, 11.J., and Dobriner, K., Ibid., 70,2024 (1948). Krider, hI. M., and Wall, 11.E., I b i d . , 74, 3201 (1952). Rothman, E. S., Wall, h4. E., and Eddy, C. R., I b i d . , 74, 4013 (1952). Wall, M. E., Eddy, C. R., hIcClennan, M. L., and Klumpp, h1. E., h A L . CHE?,f., 24, 1337 (1952). Wall, hI. E., Krider, hI. SI.,Rothman, E. S., and Eddy, C. R., J . Biol. Chem., in press. RECEIVED for review September
4 , 1952. Accepted October 2 2 , 1Q52 Sixth in a series on steroidal sapogenins; for uaper V see (7). Work done as part of a cooperative arrangement between t h e Bureau of P l a n t Industry, Soils, a n d Agricultural Engineering and Bureau of Agricultural a n d Industrial Chemistry (U. S. Department of Agriculture) a n d the ?;ational Institutes of Health (Federal Security Administration).
Induced Reaction Method for Determination of Fluoride Ion JACK L. LARIBERT Kansas State College, Manhattan, K a n .
T
HIS study was undertaken to investigate a fundamentally new and sensitive method for quantitatively determining fluoride ion in solution. The increasing use of fluoride ion in drinking water has heightened interest in methods capable of producing good precision a t concentrations of 1 p.p.m. Methods reported in the literature, if sufficiently sensitive, are usually colorimetric procedures which involve the reaction of fluoride ion with complexes of zirconium, iron, or titanium. The standard method of the American Water Works iissociation ( 1 ) involves the reaction of fluoride ion to displace alizarin sodium monosulfonate, sodium 1,2-dihydroxyanthraquinone-3-sulfonate, from its pink zirconyl complex in arid solution to produce grada-
tions of color through yellow in proportion to the fluoride ion concentration. Xumerous variations of this method utilize other polyhydroxyanthraquinone derivatives. Other proposed methods (3) involve the decolorization of ferric ion complexes, such as the thiocyanate, or pertitanates by reaction with fluoride ion. The method described in this study differs from others in being made quantitative by timing the appearance of color produced by a reaction whose rate is proportional to the fluoride ion concentration in the sample. Although visual color comparisons were used, little practice is needed to obtain precision and the time measurements can be made with any watch having
ANALYTICAL CHEMISTRY
272 Fluoridation of drinking water supplies has increased the interest in quantitative analytical methods for fluoride ion in concentrations of the order of 1 p.p.m. A fundamentally new- method which meets these requirements is w-ortliy of study, as comparatively few methods are sufficiently sensitive for determinations in this range. -4quantitative method is based on the speeding up of the reaction between iodide ion and t h e hydrolysis products of ceric sulfate, which is measured by timing the ap-
a second hand. Few interferences were found. but a number of ions exert influence on the reaction. REAGENTS
Cerate Reagent. Ten milliliters of saturated solution of ceric sulfate, Ce(HSO4)4, which was obtained from the G. F. Smith Chemical Co., C'olumhus, Ohio, and '7.0 ml. of C.P. concentrated sulfuric acid in 500 ml. of solution. (It was convenient to use a solution saturated a t room temperature, approximately 25' C., as ceric sulfate is difficult to dry to constant, weight for weighing. By evaporating and drying a t 105" C. it was found that 10 ml. of the solution, weighing 12.02 grams, contain 2.64 grams of ceric sulfate. Cadmium Iodide-Linear Starch Reagent ( 2 ) . Eleven grams of C.P. cadmium iodide and 2.50 grams of twice-recrystalliaed linear ".&-fraction" potato starch in 1000 ml. of solution. Bromothymol Blue Comparison Standard. Sufficient bromothymol blue indicator in approximately 0.1 F disodium hydrogen phosphate, SazHPOl.'iH~O,to produce a very faint hut perceptible hlue color.
pearance of the blue linear starch-triiodide (IS-) ion complex. The method is capable of good precision in the range from 0.3 to 0.9 p.p.m. of fluoride ion, but is subject to influence in varying degrees by several substances commonly found in potable waters. 4s described, this method is probabl? subject to enough influences to make i t inconvenient for routine analyses of puhlic water supplies. Under favorable conditions, the method is shown to be highly sensitive and capable of good precision.
was nie:tsured into il 250-ml. glass-stoppered volumetric flask, 1 .O nil. of cerate reagent was added. and the solution was mixed thoroughly. A 20.0-ml. portion was transferred to a 20 X 200 nim. test tube, which was then placed in a constant-temperature bath. The bath used was a 190 X 100 mm. glass crystallizing dish. which was easily kept a t the desired temperature by the addition of warm or cool water. -4n-hite cloth beneath the hath a i d d in accurate color comparisons.
PROCEDURE
Distilled water, to which ivere added the various sullstances shdied, was used in all the det,erminations. h 250-ml. sample
0 1.0 M L
CERATE REAGENT
0 80% OF ABOVE Ce(IV) CONCN.
A 120% OF
ABOVE C e W ) CONCN
CONCN. OF H&O, TEMPERATURE
CONSTANT
= 25'
C.
P.P.M. F -
Figure 2.
Effect of Temperature
thc simple had been i i i the liuth 10 minutes to adjust to the temperature of the hath, 1.0 nil. of cadmium iotiicle-linear starch reagent was added and thoroughly mixed. Time n-as recorded by a stop watch from the moment of addition of the second reagent until the hlur of the linear starch-triiodide ( k ) ion developed in the sample exactly nintched the color of the hronioth\-niol lilue comparison solution. The division of graph paper in tenths made it convcnielit t o record the time t o the neareet 6 seconds. The colors should tie compared either through the greatest depth of solution. or at an angle of approsinlately 30" from th(, vrrtical. It was ohserved that the comparisons could hest lie made under inrsridercent (tungsten filament) lighting. This niethod might l)e adapted to sDectrophotoiiietric procedures, but it would be difficult t o halt a continuing reaction and maintain temperature control. DISCUSSION OF RESULTS P.P.M. F-
Figure 1. Effect of Varying Amounts of Cerium(1V) i n Cerate Reagent
;is preliminary work indicated that temperature had an influence on the rate of color development. all determinations were carried out a t 25' f 0.1" C. except t,hose shown in Figure 2, in which the influence of temperature v a s studied. The approxi-
V O L U M E 2 5 , NO. 2, F E B R U A R Y 1 9 5 3
273 appreciably (Figure 4 ) . Cations of small ionic radius, such as sodium and magnesium have less effect on the rate of color development-vertical displacement of the line-than do those of larger ionic radius such as potassium and calcium. Chloride ion affects the rate of color development by speeding it up, as indicated by a study of Figures 3 and 4. Several factors may be responsible for the effects noted, either singly or in combination. The presence of electrolytes may alter activity coefficients so that the solubility of the hydrolyzed cerate compound is changed, or perhaps compounds are formed between cations and the cerate compounds. Chloride ion may be oxidized very slowly by the cerate compounds to chlorine, Tvhich would in turn oxidize iodide ion to iodine.
4
I
0
Co-50
A
Mp"50ppm,
v
M~"SOppm.Ci~146pprn
ppm.CI-90ppm 50,*200ppm
- - DISTILLED WATER TEMPERATURE
i
I
I
I
01
02
03
(FIG I1 \
25'C I 0 4
05
0 6
07
08
09
1.0
P.P.M. F-
Figure 3. Influence of Calcium, Magnesium, Chloride, and Sulfate Ions
TEMPERATURE = 2 5 . C i
I 01
I
I
I
I
I
I
I
I
02
03
0.4
05
0 6
07
OB
09
I 10
P.P.M. F-
Figure 4.
Influence of Sodium, Potassium, Chloride, and Sulfate Ions
mate concentrations of ceric sulfate and sulfuric acid in the cerate reagent were roughly worked out and the optimum concentration of cerate ion in the reagent was determined by varying its concentration a$ sholvn in Figure 1 Xumerous determinations were made a t 25" C. to obtain an idea of the accuracy and precision of the method. The size of the points on the graphs indicates variability of z t 6 seconds. The region of uncertainty in Figure 1 is the concentration range between 0.0 and 0.3 p.p.m., nhere the precision is not good. Results were best between 0.3 and 0.9 p.p.m., but above 0.9 p.p.m. the difficulty in timing short intervals and obtaining thorough mixing quickly probably caused the departure from linearity. Figure 2 indicates that close temperature control is necessary. Divalent cations have a marked effect on the slope of the lines (Figure 3), while monovalent cations do not change the slope
I
I
\
I
Ions which would ordinarily occur in much smaller concent,rations were investigated a t concentrations that might be expected in potable waters. Figure 5 shows that 0.2 p.p.m. of nianganous ion has no effect, while ferric ion (0.3 p.p.m.) and phosphate ion (0.5 p,p.m.) have a slight effect on the slope of the line obtained. Aluminuni has a serious effect a t 0.2 p.p.m., but much less a t 0.05 p.p.m., which is probably due to the adsorption of fluoride ion by colloidal aluminum hydroxide. The alkalinity of the sample has a very marked influence on the rate of color development (Figure 6), but its effect can be compensated by increasing the acidity of the cerate reagent. If the alkalinity of the water varied, a series of empirical curves might be determined to include the estremes normally expected. Residual chlorine or hypochlorite, if not removed, would make any determination by this method impossible, as it n-ould immediately oxidize iodide ion to iodine to give a blue color. Water containing approximately the inaximuni concentration of residual chlorine allowable in public water supplies (0.4 p.p.ni.) was passed through a Jones reductor consisting of a 150-mm. column of 20-mesh zinc metal contained in a 50-ml. buret of 10-mm. inside diameter. The chlorine was completely reduced without affecting the fluoride ion concentration, although the line was displaced upward, as shown in Figure 6. This slowing up of the rate of color development can be attributed to the alkalinity of the hypochlorite solution (Zonite) used. Enough Zonite was added to oxidize the easily oxidizable matter in the distilled water
274
ANALYTICAL CHEMISTRY relative reactivity of the hydrolyzed ceric sulfate. Ten minutes was selected as a convenient time to equilibrate the sample thermally and was constant in all determinations. It was observed, however, that the period for color development increased from 24 to 102 seconds after a sample containing 0.95 p.p.m. of fluoride ion and cerate reagent stood for 1.5 hours a t room temperature. Qualitative observations indicated that a-hydroxy acids such as tartaric completely halted the color development, while oxalic acid had a marked accelerating effect. CONCLUSION
P.P.M. F-
Figure 6. Corrections for Alkalinity by Added Sulfuric Acid, and Results Obtained after Removal of Residual Chlorine and, after several hours, more was added to give the equivalent of approximately 0.4 p.p.m. of residual chlorine. The effect of the zinc reductor on other ions possibly present, such as ferric ion, was not studied. No determinations were made on samples of fluoridated city water supplies because the variability of concentration of substances which influence the results, particularly alkalinity, would make them have little meaning. It is possible that a buffered solution would eliminate the difficulty due t o alkalinity, but that would necessitate further study, as the conditions of the method would be altered. A factor not investigated further was the effect of time on the
This brief study has indicated that although the analytical procedure described is exceedingly sensitive and capable of good precision under controlled conditions, it apparently is subject to enough influences in its present state of development to be inconvenient for routine analyses of public water supplies. I t s sensitivity, however, would allow dilution of the sample by as much as 100% a t I p.p.m., which would halve the concentration of interfering substances. The method is fundamentally new and is of interest because it shows t h a t an induced reaction type of analysis can be accurate and precise a t extremely small concentrations. Uses other than the analyses of fluoridated water supplies may be found for which this method would be more satisfactory. The need for temperature control is offset by the fact that the only measuring instrument required is a stop mratch or ordinary watch having a second hand. LITERATURE CITED
(1) Am. Public Health Assoc., Yew York, and Am. Water Works Assoc., “Standard Methods for the Examination of Water and Sewage,” 9th ed., pp. 76-9, 1946. (2) Lambert, J. L., ANAL.CHEX,23,1247 (1951). (3) Snell, F. D., and Snell, C. T., “Colorimetric Methods of Analysis,” Vol. 11, 3rd ed., pp. 743-53, Kew York, D. Van Nostrand Co., 1949. RECEIVED for review August 4, 1952. Bcoepted October 16. 1952. Contribution C 480, Department of Chemistry, Kansas State College, Manhattan, Kan.
Coulogravimetric Determination of the Halides A New Type of Indirect Analysis WILLIAM M. MAcNEVIN, BERTSIL B. BAKER’, AND RICHARD D. MCIVER Department of Chemistry, The Ohio State University, Columbus, Ohio
IhS
TANCES often arise in both coulometric and electrogravimetric analyses in which the deposition potentials of two elements, occurring together in a mixture, lie too close to allow their separate determination. It is the purpose of this paper to present a new method of indirect analysis whereby simultaneously obtained coulometric and electrogravimetric data are combined to permit the analysis of such mixtures. The principle of this new type of double measurement, conveniently called coulogravimetric analysis, is best explained by an example. Lingane and Small (3) have determined coulobrometrically-by precipitation on a silver anod-hloride, mide, and iodide when present individually, and also, by controlling the anode potential, mixtures of iodide and chloride or iodide and bromide. They were unable to determine chloride and bromide when present together, since a t the potential required 1 Present address, Southern Research Institute, 917 South 20th St. Birmingham, Ala.
for the deposition of bromide some chloride codeposited. n their work the authors have deposited together all of the chloride and bromide and have measured both the total number of coulombs required and the weight of the combined deposits. These data give the following information: Increase in weight of anode = x-eight of chlorine
+ weight of bromine + + ‘i9.92
Coulombs required = total equivalents C1 Br = 96 ,500 wt. of C1 wt of Br 35.46
(1)
(2)
From these two equations, by appropriate substitution and rearrangement, the individual weights of chlorine and bromine may be calculated. Good accuracy will be obtained from indirect analyses of this type only when the equivalent weights of the two elements differ
