.i166
ANALYTICAL CHEMISTRY
possible. They would also like to express their appreciation to Bernard Grushkin for permission to use some of his unpublished data in Table I. LITERATURE CITED
Bacharach. A. L.. Anahst 70. 394 (1945). (2) Bentley, 0. G., Snell, E. E., Phillips, P. ’H., J . B i d . Chem. 170,
(1)
343 (1947).
(3) B i &
6.
I.,’Ann. N . Y . Acad. Sci. 52, 877 (1950). (4) Doudoroff, P., Kats, AI., Sewage and Z n d . Wastes 25, No. 7, 802 (1953). (5) Gladhill, J. A , , hIalan, G. M,, Trans. Faraday SOC.48, 258 (1952). (6) Harris, D. A., ANAL. CHEM.27, 1690 (1955).
(7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)
Hastings, A. B., others, J . Biol. Chem. 107, 351 (1934). Ingols, R. S., Kirkpatrick, E. S., ANAL.CHEM.24, 1881 (1952). Lowrance, B. R., master’s thesis, University of Texas, 1955. Lowry, 0. H., Bessey, 0. A., J . B i d Chem. 155, 71 (1944). XIcLean, F. C., Hastings, A. B., Ibid., 107, 337 (1934). Kicholas, D. J. D., Analyst 77, 629 (1952). Owens, B. B , J . Am. Chem. SOC. 60, 2229 (1938). Pinkus, A., Hanrez, P., Bull. SOC. chim. Belges 47, 532 (1938). Schild, H. O., Analyst 75, 533 (1950). Shaw, W. H. R., J . Am. Chem. Soc. 76, 2160 (1954) Shaw, W.H. R., Science 120, 361 (1954). Shaw, W.H. R., Grushkin, B., unpublished data. Snell, E. E., Gyorgy, P., “Vitamin Methods,” vol. I, pp. 327505, Academic Press, New York, 1950.
RECEIVEDfor review October 7. 1955
Accepted January 10, 1956.
Application of Karl Fischer Water Method to Oxidants, Redwctants, and Amines AXEL JOHANSSON Royal Institute o f Technology, Stockholm, Sweden
Studies with the two-solution modification of the Karl Fischer method show that i t is applicable in the same cases as the original method and has special advantages with reducing and oxidizing substances and amines. In contrast to the original Karl Fischer reagent, the two solutions are rather stable. Substitutes for pyridine and methanol were tried, but none was found to give better results.
S
OME years ago a modification of the Karl Fischer method for the determination of water was published ( 2 ) . Instead of using a single solution containing pyridine, methanol, sulfur dioxide, and iodine, the use of two solutions was recommended, one containing pyridine, methanol, and sulfur dioxide (Solution I) and the other containing iodine and methanol (Solution 11). For titration accorjing to this modification the sample is dissolved or sispended in the first solution, Rhich is then titrated with the second. This paper deals with experience obtained in this laboratory using the method.
water equivalence was observed during the first 14 days. After that period, most of the solution was used and only a small volume left. In this case small amounts of absorbed water have a greater effect on the strength and the water equivalence is decreased. Sample B was of the same origin as A, but used only for the standardizations. After 0, 1, and 2 months the respective W-factors were 1.727, 1.732, and 1.705. Thus, by effective protection from atmospheric moisture Solution I1 is nearly as constant as the standard solutions used in alkalimetry. The iodine content of Solution 11, determined by thiosulfate titration, changed very little with time. Sample A changed from 0.2368N on the fourth day to 0.2367 and 0.2388N after 14 and 30 days, respectively; sample B rhanged from the same value on the fourth day to 0.2368, 0.2371, and 0.2374.V after 14 and 19 days and 2 months, respectively. This small increase in normality shows that the methanol slowly evaporates and that the effect was greatest for sample A when there was only a small volume left. Use of a stronger Solution I1 may be more convenient-e.g., 60 to 70 grams of iodine per liter-in order to limit the voluniee when the sample contains large quantities of water. The per-
KEEPING QUALITIES OF SOLUTIONS
In the normal Karl Fischer solution there is a rapid fall in the water equivalent, partly caused by absorption of atmospheric moisture by the hygroscopic reagents and partly caused by side reactions. In the two-solution modification, Solution I undergoes a slow change even when kept in stoppered bottles. Usually its water content decreases (2, 4 ) ; in one case the following water contents were determined after 0, 1, 2, 3, and 5 months: 0.027, 0.032, 0.019, 0.017, and 0.013%. This small change is, however, of no importance as a correction is automatically made in each titration. Of greater significance is the permanence of Solution 11. As Seaman, McComas and Allen ( 4 ) have shown, the standardization changes only because of absorption of moisture. It is therefore necessary to use effective drying tubes on the titration flask. Eberius ( 1 ) claimed, however, that the permanence of the Rolutions was not improved with the two-solution modification. This matter was therefore reinvestigated; in Figure 1the changes in two samples, A and B, of Solution I1 are compared with the change in a normal Karl Fischer solution of the same strength [figures are taken from Eberius (I)]. Sample A was a solution used every day i n routine work and was stored in a titration apparatus with large drying tubes (25 X 3 em.). No change in
3.0 L
0 c
w
t
2.5
. I -
I
3 2.0
1.5
‘
1.0 0
I
I
I
I
5
10
15
20
I
25 days
I
30
Figure 1. Comparison between stability of normal and modified Karl Fischer reagent 0. Normal X.
Sample A
0 . Sample B
1167
V O L U M E 2 8 , NO. 7, J U L Y 1 9 5 6 Determination of Water in Iodine-Consuming Substances
Table I. Compound
Method l a Sodium thiosulfate pentahydrate Potassium ethyl xanthate Ascorbic acid Method 2 b Sodium thiosulfate pentsli>-drate Ascorbic acid
Compd. Added, Mg.
Water Added,
105,s 106.2 127.8 97.6 104.9 111.5
..
.Mg.
..
40: 5 59.9 32.0
..
95.8 94.5 166.4
37: 8
Solution 11, hll.
RII. of Solution I1 for Oxidation
Water Found, Mg.
Water Calcd.. Mg.
24.61 24.76 29.52 26.38 38.40 2 2 , 08
1.70 1.73 2 49 2.55 2 98 5 08
37.9 38.1 43.9 40.5 60.8 32.9
38.4 38 5 46.4 40.5 59.9 32 0
21.90 21.63 32.30
1 64 1.62 8.04
:3? 6
34.8 34.3 37.8
34.2 38 1
Weighed amount of sample (column 1) and x a t e r (column 2) a a s dissolved in 10 ml. of methanol-pyridine solution (1 t o 1 ) and titrated with Solution I1 (iV = 0.2347, JF = 1.706, and,water content 0.409 mg. per nil.) (coluinn 4). Then 5 ml. of Solution I was added and t h e titratlon continued wi,tli Solution I1 (column 3). b Weighed amount of sample (column 1) and water (column 2 ) yas,dissolved in .? ml. of Solution I and titrated with Solution I1 (column 3 ) . This value should be corrected for iodine consumption of saiiiple (column 4). 0
centage decrease in the strength of such a solution from absorption of atmospheric moisture is minimized, but there is also a slight evaporation of iodine. As a net result the W-factor decreases about 2% during a month. REDUCIIVG SUBSTANCES
The original Karl Fischer method sometimes fails when the sample consumes iodine because the reaction between iodine and the iodine-consuming component is not quite stoichiometric and varies with titration conditions. When no precautions are used, the two-solution method suffers from the same limitations. Potassium xanthate gives, for example, varying results when the proportion of water to xanthate changes. In water-free conditions the iodine consumption is stoichiometric, but when the water content increases to over 3Oy0 the xanthate does not consume any iodine a t all. In these cases a modified procedure is useful. I n Table I some results with iodine-consuming substances are listed.
Procedure. The sample is dissolved in about 10 ml. of a mixture of equal parts of methanol and pyridine and then titrated with the iodine-methanol solution (Solution 11) until the color changes to yellow. Then a measured quantity of Solution I is added, and the titration is continued until the color changes to brown. The value obtained is corrected for the water content of the solvents of Solution I and of Solution 11. In this case it is convenient to use the normality instead of the TT’-factor of Solution I1 and to standardize it against standard sodium thiosulfate solution (a known amount of Solution I1 is added to an aqueous otassium iodide solution and titrated). The water content of olution I1 is calculated from the difference between the stoichiometric water equivalence (from the thiosulfate titration) and the real water equivalence (from a standardization with water). When the reaction between iodine and the iodine-consuming component of the sample is stoichiometric a shorter method may be used: An aliquot part of the sample is dissolved in methanolpyridine solution and titrated with Solution 11. The water content is then determined on another part of the Sam le, and this titration is corrected for the iodine consumption o f t h e sample obtained from the first titration. The value is also corrected for the water content of that part of Solution I1 ivhich is used for the oxidation.
E ’
OXIDIZIXG SUBSTANCES
It is also more convenient, when oxidizing substances are present, to use the normality of Solution I1 for the calculation, rather than the W-factor, and then to correct for the water content of the solvents (including that of Solution 11) and for the iodine formed in the reaction between iodide ion and the oxidizing agent. However, many substances such as hydrogen peroxide require an acid medium for the oxidation HzOi
+ 2 H + + 21-
11
+ 2H20
In pyridine solution this equilibrium is thus displaced to left and the main reaction is oxidation of sulfur dioxide to sulfuric acid ( 6 ) . Therefore, in these cases the water can be titrated without interference f r o m t h e oxidizing agent. Copper (11) and iron (111) salts are e x a m p l e s of s u b stances which require correction. AMINES
In the original Karl Fischer method, amines strotiger than benzylamine ( K = 2.4 X 10-6) must be neutralized by dissolving in acetic acid ( 3 ) . iJ7ith the modified reagent no such neutralixation is necessary if the proportion of Solution I to amine is sufficiently large. As a rule, 10 ml. of Solution I can take about 0.5 gram of amine without interference. For example, a sample of 98.2y0 ethanolamine (determined by titration with standard hydrochloric acid) was found to contain 1.79 i 0.02y0 water; when 43.7 mg. of water was added, 43.3 mg. were recovered. Other amines such as methylamine, ethylamine, dimethylamine, diethylamine, and ethylenediamine gave similar results. When the water content is low it is necessary to weigh a gieater amount of amine, and it is then advisable to add acetic acid, as otherwise the end point is not stable. The most convenient procedure is to add 10 ml. of acetic acid to 20 ml. of Solution 11, titrate to the first end point, add the sample, and titrate again to the final end point. In this way an automatic correction for the water content in the acetic acid is obtained. With this procedure the followingvalues were obtained with about 1.5,1.5,2.0,3.0,5.0, and 7.0 grams of the 98,2y0 ethanolamine: 1.80, 1.79, 1.78, 1.76, 1.74, and 1.767, water. Other amines tried included ethylenediamine, aniline, diphenylamine, and dimethylaniline. The end point with aniline is not entirely stable, and the brown color fades in about 1 minute, but this is not a serious limitation. SUBSTITUTES FOR PYRIDIIVE AND METHANOL
The reasons for searching for substitutes for pyridine are its comparatively high price and the difficulty in observing the end point change from yellow to brown. Smith, Bryant, and Mitchell ( 5 ) have tried a variety of organic bases for the normal Karl Fischer reagent, but none was found to give stable solutions. As side reactions, which may be considered to be the cause of the poor stability, are of less importance in the two-solution modification, some experiments were made to find a better substitute for pyridine. Aniline and ethanolamine were tried but did not give stable end points. The only base tried which gave reproducible values was hexamethylenetetramine. The reaction here is the same as in the original Karl Fischer titration, except that the hexamethylenetetrammoniuni iodide is only slightly soluble and is precipitated (it is, however, not sufficiently insoluble to permit an acidimetric determination of water by filtration and titration with standard alkali), The determinations were performed in the following way: Oven-dried hexamethylenetetramine ( 5 grams) was dissolved in 25 ml. of methanol, and the remaining water was titrated with a solution of 30 grams of iodine and 10 grams of sulfur dioxide per liter in methanol. Then a weighed amount of water was added, and the titration continued. The end point was sharp and stable, and the color of the solution changed from colorless to iodine yellow. Three such titrations gave W-factors of 1.789, 1.786,
1168
ANALYTICAL CHEMISTRY
and 1.791, for another solution W-factors of 1.702, 1.695, and 1.690 were obtained. The solution of sulfur dioxide and iodine was not stable, however, and after 14 days it was only pale yellow with a precipitate of sulfur on the bottom of the bottle. Titrations where solutions of sulfur dioxide and hexamethylenetetramine were placed in the titration flask and titrated with an iodine-methanol solution did not give good results. The solutions turned yellow before the end point was reached when sulfur dioxide was in excess, and the results were very variable when hexamethylenetetramine was in excess. As substitutes for methanol, other alcohols or glycols may be used in the two-solution modification as in the original method. None gave better results than methanol. With higher alcohols the pyridinium iodide is rather insoluble, which gives a possibility of acidimetric determination of water. In this case a small excess of an iodine-octanol solution was added to a weighed amount of water in a sulfur dioxide-pyridine-methanol solution, and the precipitate formed was filtered through a funnel with a sintered disk and mashed with carbon tetrachloride. The precipitate was then dissolved in water and some ethanol to give a clear solution and then titrated with a standard sodium hydroxide solution. In a typical experiment the precipitate corresponding t o
34.0 mg. of water consumed 49.30 ml. of 0.1105N sodium hydroxide solution, which gave 49.1 mg. of water. After correction for water in the solvents, the amount of wat.er found was 36.9 mg.-about 10% too high. ACKNOWLEDGMENT
The financial support given by Statens Naturvetenskapliga Forskningsrld is gratefully acknowledged. The author also wishes to thank Karin Lindgren and Eivor Ljungqvist for their experimental assistance. LITERATURE CITED (1)
(2)
(3) (4) (5) (6)
Eberius, E., “Wasserbestimmung mit Karl-Fischer-LGsung,” p. 31, Verlag Chemie, Weinheim, 1954. Johansson, A., Svensk Papperstidn. 50, 124 (1947). Mitchell, J., Jr., Smith, D. M., “iiquametry,” p. 126, Interscience, New York, 1948. Seaman, W., McComas, W. H., Allen, G. A., ANAL.CHEM.21, 510 (1949). Smith, D. M., Bryant, W. &I. D., hIitchel1, J., Jr., J. Am. Chem. SOC.6 1 , 2 4 0 7 (1939). Zimmermann, A,, Fette u . &&fen 46, 446 (1939).
Spectrophotometric Study of Modified Heteropoly Blue Method for Phosphorus CHARLES H. LUECK and D. F. BOLTZ W a y n e State University, Detroit, M i c h .
A spectrophotometric study has been made of the modified heteropoly blue method in which the yellow molybdophosphoric acid is extracted with isobutyl alcohol and subsequently reduced with chlorostannous acid to the heteropoly blue. The absorption spectrum of the heteropoly blue of phosphorus in isobutyl alcohol is different from that obtained in aqueous medium; characteristic absorbance maxima are found at 625 and 125 mfi. The system conforms to Beer’s law with an optimum concentration range from 0.1 to 1.3 p.p.m. of phosphorus when measurements are made at 725 mp in 1-cm. cells. The effect of solution variables, especially diverse ions, was investigated. The interference from arsenic and germanium can be eliminated by a preliminary volatilization as the bromides. The modified method has been applied to the determination of phosphorus in plain carbon steels.
T
HE heteropoly acid of phosphorus, in both the unreduced
and reduced form, is commonly utilized to determine small amounts of phosphorus. Some of the more commonly occurring elements which also form similar heteropoly acids are arsenic, silicon, and germanium. In an effort to eliminate interferences and make the heteropoly method more specific, selective extraction of the heteropoly acids has been utilized. Most of the early work on liquid-liquid extraction of heteropoly acids has been summarized by Wadelin and Mellon (IO). Berenblum and Chain ( 2 ) extracted the yellow molybdophosphoric acid with isobutyl alcohol and reduced it to the blue heteropoly complex by shaking the extract with a solution of chlorostannous acid. 411en ( I ) found this method useful for the determination of phosphorus in highly colored or turbid solutions.
Schaffer, Fong, and Kirk (9) varied the procedure by extracting with n-octyl alcohol and thus determined micro and submicro amounts of phosphorus in biological samples. Pons and Guthrie applied the method to the determination of phosphorus in plant materials (8). Because the method as reported by Berenblum and Chain was not sensitive to variations in acidity and reductant concentration-conditions which ordinarily must be rigidly controlledfurther investigation of this method seemed desirable. This spectrophotometric study was undertaken to determine the effect of solution variables, especially diverse ions, and to apply the modified method to the determination of phosphorus in steel and mater samples. APPARATUS AND REAGENTS
Absorbance measurements were made in 1.000-em. matched cells with a Beckman Model DU spectrophotometer, The initial spectrophotometric curves were obtained with a Warren Spectracord. A reagent blank was used in the reference cell. However, in most cases the reagent blank did not differ in absorbance from the pure solvent. The following reagent solutions were prepared. Standard phosphate solution (0.025 mg. of phosphorus per ml.), Dissolve 0,1098 gram of reagent grade potassium dihydrogen phosphate in distilled water and dilute to 1 liter. Sodium molybdate solution, 10%. Dissolve 25 grams of sodium molybdate, NarMoOl. 2H.20, in distilled mater and dilute to 250 ml. The solution must be clear. Chlorostannous acid solution, 0.2%. Dissolve 2.38 grams of stannous chloride, SnClz 2H20, in 170 ml. of concentrated hydrochloric acid and dilute to 1 liter with distilled water. Add several pellets of metallic tin. The isobutyl alcohol used for extraction was Matheson Co., Inc., No. 2858. The perchloric acid used mas 72y0double vacuum distilled (G. F. Smith Chemical Co.). All other chemicals and acids used were of reagent grade, with the exception of several salts used in the diverse ion study.
