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.
1169
V O L U M E 2 8 , NO. 7, J U L Y 1 9 5 6 All of the aqueous solutions were stored in polyethylene bottles to prevent contamination from silica. EFFECT OF SOLUTION VARIABLES
The reduction of the yellow heteropoly molybdophosphoric acid, H8P(Mo3O10)4, to give the familiar heteropoly blue complex must be conducted under controlled conditions (3, 12). An excess of molybdate reagent must be present in order to shift the equilibrium to the heteropoly acid side ( I O ) . This excess reagent is reduced to a blue color if the reduction conditions are not carefully regulated. The heteropoly acid can be separated from essentially all of the excess molybdate reagent by estraction, after which it can be reduced by a strong reductant Lvithout any special precautions. The folloving procedure was used to stud)- the effect of solution variables upon the formation of molybdophosphoric acid and its extraction with isohutyl alcohol.
Reductant. If the acidity of chlorostannous acid is too low, reduction to the heteropoly blue does not occur. If the acidity is too high, the intensity of the blue color is decreased. Twenty-five milliliters of a 0.2yG chlorostannous acid solution, 2N in hydrochloric acid, was found to be satisfactory for reduction of the amounts of molybdophosphoric used in this study. This concentration is similar to that used by Berenblum and Chain ( 8 ) . The volume of reductant may vary from 15 to 35 ml. without affecting the final color intensity. Absorbance values of 0.353 and 0.357 were ohtained with 0.5 p.p.m. of phosphorus when 15 and 35 ml. of chlorostannous acid reagent were used
1
1001
-4 definite amount of the standard phosphate solution was transferred to a 50-1n1. volumetric flask and the desired amount of perchloric acid added. In the study of diverse ions, a definite amount of solution containing each diverse ion was also added. The solution was diluted to approximately 40 ml. and 5 nil. of the molybdate reagent was added. The solution was diluted to the mark with distilled Ivater, mixed, and allowed to stand for approximately 5 minutes. It was then transferred to a 150-ml. separatory funnel and extracted with 40 ml. of isobutyl alcohol. The extract vias w s h e d tn-ice with 25-ml. portions of distilled water. The aqueous phase 1%-asremoved and the desired amount of chlorostannous acid added. After shaking, t,he nonaqueous phase was drained int,o a 50-nil. volumetric flask. The funnel was rinsed with a small portion of isobutyl alcohol and the washings were drained into the volumetric flask. The solution was diluted to the mark with pure solvent and thoroughly mixed. The visible absorption spectra were recorded, or the absorbance was measured a t 725 mp using a reagent blank solution in the reference cell. Acidity. The optimum acidity for the formation of molybdophosphoric acid forination has heen shown by Boltz and JIellon ( 4 ) to be approximately 0.3A\r. Berenblum and Chain ( 2 ) selected an acidity of 0.5-V and varied it from 0.055 to 1,5.\- without any effect o n the ultimat,e color intensity. Large amounts of iron(II1) formed a precipitate with the molybdate reagent in solutions of lo\\- acidity. This precipitate did not form at higher acidities. Because it was desirable to eliminate the irori(II1) interference, higher acidities were used. When 0.5 p,p.m, of phosphorus was used with the final acidities in respect to perchloric acid as shown, the following absorbances were obtained: 0.47 0 . 7 0.93 1 . 1 6 1 . 4 1.86 dbsorbance 0.367 0.367 0.356 0.356 0.358 0.343 S,HClOi
A variation in acidity from 0.5Y to l.4\rdoes not affect the color intensity: therefore, an intermediate value of 1 . 2 s was selected for euiisequent esperimental work, Molybdate Concentration. -1large excess of molybdate ion is necessary for heteropoly acid formation (IO). With eolutions containing 0.5 p.p.m. of phosphorus and 3, 5, and 7 ml. of the molybdate reagent, absorbance values of 0.357, 0.353, and 0.356, respectively, n-ere obtained. Five milliliters of the 10% sodium molybdate solution in :L final volume of 50 ml. was selected as a suitable excess. Extractant. Isobutyl alcohol quantitatively extracts the yellow molybdophosphoric acid. One extraction with approximately 40 ml. of isobutyl alcohol is sufficient to extract the amounts of molybdophosphoric acid used in this study. Two extractions with 20-ml. portions of isohutyl alcohol do not increase the amount of molybdophosphoric acid extracted. Absorbance readings of 0.354 and 0.351, respectively, were obtained at 725 mp for 0.5 p.p.m. of phosphorus with one 40-ml. extraction and two 20-ml. extractions. Butyl alcohol and presumably other immiscible alcohols could also be used.
l o t I
I
I
550
600
650
I
I
700 750
WAVE LENGTH, rnp
Figure 1. Absorption spectra of heteropoly blue of phosphorus in isobutyl alcohol
Color Development. The time allowed for complex formation before extraction is not critical. With no diverse ions added, identical absorbance values were obtained when the molybdophosphoric acid solutions were allowed to stand 3 minutes and 30 minutes after the addition of molybdate reagent before being extracted. Stability. The blue color is stable for a t least 20 minutes provided the solution is kept in a stoppered flask. There is a slight but constant decrease in color intensity after 20 minutes; the amount of fading after 1 hour is approximately 370. When 0.5 p.p.m. of phosphorus was used. the following absorbance values were obtained: Time, minutes 5 10 20 30 45 60 90 dbsorbance 0.359 0.359 0.358 0.356 0 . 3 5 3 0.3.50 0.344 Phosphorus Concentration. The absorption spectra for various concentrations of phosphorus are shown in Figure 1. Beer's law applies a t 625 and i25 mp, with greater sensitivity being obtained by making spectrophotometric measurements at 725 mp. The absorption spectrum for the heteropoly blue complex obtained in aqueous solution with a different reductant and different conditions of reduction has its absorbance maximum a t 830 mp with an inflection point a t 650 mp ( 3 ) . The molar absorptivity for phosphorus at 725 mp is 22,700
1170
A N A L Y T I C A L CHEMISTRY
liter per mole centimeter, and a t 625 mp 19,200 liter per mole centimeter. Effect of Diverse Ions. These studies were made with solutions containing 0.6 p.p,m of phosphorus. An error of less than 2y0 of the phosphorus present was considered negligiblc. One thousand parts per million (50 mg.) of the following ions did not interfere. (The concentration of diverse ion is expressed as parts per million in solution prior to extraction). Acetate, bromide, carbonate, chloride, citrate, dichromate, fluoride, iodate, nitrate, nitrite, oxalate, permanganate, sulfate, ammonium, aluminum, barium, bismuth( 111), cadmium, calcium, chromiuni(III), cobalt(II), copper(IT), iron(II), iron(III), lead(I1), lithium, magnesium, manganese(II), nickel(II), potassium, silver, sodium, thorium( IV), uranyl, and zinc. M7hen 50 mg. of nitrite was present, 50 ml. of the chlorostannous acid solution were required to reduce the molybdophosphoric acid. The ions which were found to interfere are listed in Table I, along with the tolerance for adherence to control limits. The tolerance with respect to a diverse ion is often much lower in solutions of higher ionic strength.
Table I.
Interfering Ions
Amount Form 70 Tolerance,& Added, P.P.M. P.P.M. Error Ion Added +l5 60 As+++ 100 20 Over 100 0 As +5 - 50 0 10 C e +4 Os16 0 20 Over 100 G e +i n 20 20 Au+++ 12 1000 60 I20 600 - 6 ng 1000 10 300 500 30 +25 1000 60 -35 SCN0 20 - 5 5103 - n 20 -32 Sn++ 40 1000 10 Sn + I 5 8 0 w0,-v +l 0 20 Time variable Causiri g less than 2% error using 0.6 p .p.m. of phosphoi“US.
3: +
(5-7, 11). Arsenic is best removed as the bromide. Both quinquevalent and trivalent arsenic ITere distilled from a perchloric acid solution after the addition of sodium bromide. Five milliliters of 35% aqueous sodium bromide solution are sufficient for quantitative removal of a t least 25 mg. of arsenic. Germanium is readily volatilized either as the bromide or the chloride. Twenty-five milligrams of germanium is quantitatively removed by the addition of 5 nil. of 35% sodium bromide solution and subsequent evaporation to perchloric acid fumes. Thus, by the addition of sodium bromide the interference caused by germanium and arsenic is eliminated in one operation. The silicon present is also made innocuous in the same operation when the solution is evaporated to perchloric acid fumes. It is not necessary to remove the dehydrated silica by filtration because in the extraction process the precipitate remains in the aqueous layer. The germanium can also be removed by adding 10 ml. of concentrated hydrochloric acid and heating. This treatment quantitatively removes up to 25 mg. of germanium. However, arsenic( 111) and arsenic( V) are not completely volatilized, and are best removed as the bromides. The following procedure n as used for the removal of arsenic and germanium from samples containing small amounts of iron. Add 5 ml. of 72% perchloric acid to the solution of the sample. Add 5 ml. of 35% aqueous sodium bromide and evaporate to dense fumes of perchloric acid. It is important that the solution be evaporated to dense fumes in order to ensure complete elimination of the interfering ion. Cool the solution, add approximately 40 ml. of water, and determine the phosphorus as just described
--
S o loss in phosphorus was detected using this step with up to 25 mg. of arsenic and germanium.
-
Adjust the sample size so that the amount of phosphorus is within the limits of this method (0.01 to 0.06 mg.). Weigh the steel sam le and transfer to a conical flask. Dissolve the sample with 3 m?. of nitric acid and 5 ml. of hydrochloric acid. The amounts may be varied according to sample size. Add 6 ml. of 72% perchloric acid and evaporate to dense perchloric acid fumes. Fuming removes nitric acid which, if present, would oxidize the bromide. Cool, wash the sides of the flask, and add a t least 5 ml. of 35% sodium bromide solution for every 15 mg. of arsenic resent in the sample. Continue heatinguntil the dark brown ferric bromide complex decomposes, liberating bromine Again fume for several minutes. Cool, add approximately 40 ml. of water, and determine the phosphorus according to the general procedure.
G E N E R i L PROCEDURES
A suitable sample solution or aliquot, after proper preliminary treatment, should contain 0.01 to 0.06 mg. of phosphorus as orthophosphate. If nitric acid is used to dissolve the sample, or if sodium bromide has been used to remove arsenic or germanium (as described later), add 5 ml. of perchloric acid and evaporate until dense fumes are evolved. Add 5 ml. of perchloric acid to the solution. Precise duplication of acidity IS not required; hence, exact neutrality of the original solution is unnecessary. Dilute to ap roximately 45 ml. and add 5 ml. of molybdate reagent. Mix an%allow to stand for several minutes. Transfer to a glass-stoppered 125-ml. separatory funnel. Rinse the vessel with a small portion of distilled water and add the washings to the separatory funnel. Extract with 40 ml. of isobutyl alcohol by shaking for 60 seconds. Drain and discard the lower aqueous layer. Shake the isobutyl alcohol extract with two successive 25-ml. portions of distilled water. Discard the lower aqueous layers. Swirl the solution in the funnel t o collect droplets of water into one globule and discard. Add 25 ml. of chlorostannous acid reagent and shake for 15 seconds. Discard the lower aqueous la er and drain the alcohol phase into a 50-ml. volumetric flask. %ash the funnel with 10 ml. of isobutyl alcohol and add the washings to the 50-ml. volumetric flask. Dilute to the mark with.iqobuty1 alcohol. The solution should be a clear blue. After mixing thoroughly, measure the absorbance a t 725 mfi using a reagent blank as reference. Determine the amount of phosphorus present by referring the absorbance reading to a standard curve obtained under the same conditions from standard phosphate solutions. Elimination of Arsenic and Germanium Interference. The interference caused by germanium and arsenic can be eliminated by volatilizing these elements in the form of the proper halide
PROCEDURE FOR P L i I N CARBON STEELS
Table 11. Determination of Phosphorus in S t a n d a r d Steels Phosphorus, % Type of Steel BOH BOH BOH AOH AOH Bessemer High silicon Cr-Mo Mo-Xi
NBS NO. 14c
1 Id 13d 21c
35, 1 Od 125 72d llla
Certificate value 0.012 0.006 0.016 0.062
0.037 0.088 0.008 0.017 0.017
Found 0.011 0.0066 0.014 0.060 0.035 0.083 0.005 0.014 0,014
Diff. -0.001 $0,0006 -0.002
-0.002 -0,002 -0,005 -0.003 -0,003 -0,003
Inasmuch as the precise duplication of acidity is not necessary for reliable results, the quantity of perchloric acid used in dissolving the sample or in oxidizing the excess sodium bromide is not a critical solution variable. Germanium is not completely volatilized from a steel sample as either the bromide or chloride when the above procedure is used. Presumably, the volatilization of germanium halides is inhibited by the high concentration of iron( 111) halide complexes. However, up to 1 mg. of germanium may be present in the steel without causing interference if phosphorus is deter-
1171
V O L U M E 28, NO. 7, J U L Y 1 9 5 6 mined according to the general procedure. The addition of 20 ml. of hydrochloric acid and 6 ml. of perchloric acid after dissolution of the sample and heating to perchloric acid fumes removes 5 mg. of germanium. When a larger amount of germanium is present, prolonged fuming leaves a residue which dissolves with difficulty. Table I1 lists the results when the recommended procedure was used to determine the phosphorus content of several plain carbon steel samples. The results listed in Table I1 are the average of two tleterminations with a maximum deviation of 0.002%. In general the experimental values are slightly below the certified value. For steel samples containing no arsenic, higher results Lvere obtained when the addition of sodium bromide nas omitted. However, no loss of phosphorus was observed when sodium bromide was used on synthetic samples.
LITERATURE CITED
(1) Allen, R. T. L., Biochem. J . 34, 858 (1940).
(2) Berenblum, I., Chain, E., Ibid.,32, 286, 295 (1938). (3) Boltz, D. F., llellon, M. G., ANAL.CHEX 19, 873 (1947). (4) Ibid., 20, 749 (1948). ( 5 ) Dennis, L. M., Johnson, E. B., J . Am. Chem. SOC.45, 1380 (1923). (6) Luke, c. L., Campbell, 11.E., -%BAL. CHmf. 25, 1588 (1953). ( i )Magnuson, H. J., Watson, E. B., IXD. ENG.CHEM.,-4x.k~.ED. 16, 339 (1944). (8) Pons, W. A,, Guthrie, J . D., Ibid.,18, 184 (104G). (9) Schaffer, F. L., Fong, J., Kirk, P. L., ABAL. CIimr. 25, 343 (1953).
(10) TVadelin, C., JIellon, R I . G., Ibid.,25, 1668 (1953). (11) Weissler, .I.,ISD. EXG.CHEX.,AXAL.ED. 16, 311 (1944). (12) Woods. J . T., AIellon, 11. G., Ibid.,13, 760 (1941). RECEIVED for review October 12, 1955. Accepted April 16, 1956. Division of Analytical Chemistry, 128th meeting, ACS, Minneapolis, Minn., September 1955.
Interpretation of 10.3-Micron Infrared Absorption Band in lubricating Oils S. A. FRANCIS Beacon Laboratories, The Texas C o , Beacon,
N. Y.
Infrared absorption by lubricating oils in the 10.3micron region is due to two different types of structural groups. trans-Olefins produce a band at 10.35 microns which is removed by hydrogenation and is not concentrated by thermal diffusion. Another type of structure, probably polycyclic naphthenes, produces a band at 10.27 microns which is not removed by hydrogenation and is concentrated in the bottom fractions by thermal diffusion.
that the 10.35-micron band is associated with olefinic groups and that the 10.27-micron band is associated with some other structure. Another sample similar to oil A was separated into 10% cuts by thermal diffusion. Figure l , b , shows spectra of two of these cuts. The 0 to 10% cut from the top of the column shows only the 10.35-micron band. The 80 to 9O$i” cut from near the bottom of the column has its strongest band a t 10.27 microns and a shoulder near 10.35 microns. This result suggests that the 10.27-micron band is associated with polycyclic naphthenes,
T
HE work of Fred and Putscher ( I , 5 ) and Haak and van
Nes ( 2 ) strongly indicates that olefinic structural groups present in Pennsylvania oils produce an infrared absorption band near 10.3 microns. However, Lillard, Jones, and Anderson ( 4 ) have shown that absorption in the 10.3-micron region is not due solely to olefinic groups, but may be produced by other structures, of which polycyclic naphthenes are most likely. The work reported here confirms these previous results and, in addition, indicates that these ttvo types of structural groups, when present in lubricating oils, can be distinguished by means of the infrared spectrum. I t is concluded here that trans-olefinic groups produce an absorption band a t 10.35 microns and that some other structure, probably associated with polycyclic naphthenes, produces a band a t 10.27 microns. Evidence for this conclusion is that the 10.35-micron band is removed by hydrogenation and isnot concentrated appreciably by thermal diffusion] while the 10.27micron band is not removed by hydrogenation and is concentrated in the bottom fractions from a thermal diffusion column. EXPERIMENTAL
Results are reported here for two oil samples. Oil A was a fraction obtained from the silica gel percolation of a Mid-Continent distillate. It was collected near the end of the paraffinnaphthene portion, was essentially free of aromatics, and had an enhanced absorption in the 10.3-micron region. Spectra in the 10.3-micron region are shown in Figure lla, before and after hydrogenation. The original oil has two bands of about the same intensity a t 10.27 and 10.35 microns. Hydrogenation preferentially reduced the intensity of the 10.35-micron band, thus indicating
10.0
10.2
10.4 10.6 10.0 10.2 10.4 WAVELENGTH ( M I C R O N S 1
Figure 1. Infrared spectra of oil fractions
10.6
