COMPLEX SllANOLS
Table 111.
Effect of Gas Volume
OH. CT, of Theory; after Gas Evolved, MI. Blank Theory Observed Correction 1.76 0.41 102.9 3.68 2.29 100.3 5.56 4.13 99.5 7.47 6.02 99.3 9.43 7.99 99.6 99.4 10.76 9.30 12.71 11.23 99.4 14.18 12.78 100.0 99.9 16.03 14.62 99.9 17.77 16.36
ml. (absolute) for a single determination, independent of the gas volume. The least squares slope of the observed us. calculated volume is 0.998 =t 0.006, which includes the theoretical value of 1. The intercept (blank value) determined statistically is -1.40 =!z 0.01 ml. The blanks determined a t the start and end of the experiment mere -1.43 and -1.34 ml.; their average, -1.39 ml., agrees with the calculated value. This negative blank is added to the volume of gas observed.
The method was successfully applied to complex hydrolysis products from alkyl and aryl trialkoxysilanes, as well as to commercial silicone resins. Mass spectrometric analyses of the gases from the complex as well as the simple silanols showed that only methane was produced. This is convincing evidence that no gas-producing side reactions occurred. Reaction was fast and apparently complete, as no more gas was observed after heating a t 95” C. The standard procedure was follon-ed, except for silicone resins, where the major portion of solvent was stripped under vacuum (1 mm.) at room temperature, before dilution with butyl ether. This avoided errors from the vapor pressure of solvents. ACKNOWLEDGMENT
The author is indebted to AI. hI. Sprung for many helpful suggestions, t o P. D. Zemany and F. J. Norton for mass spectrometer analyses, and to L. S. Nelson for assistance with the statistical design and analyses.
LITERATURE CITED
(1) Burkhard, C. A,, J . Am. Chenz. SOC.
67.2173f 1945). DiGibrgio,‘ P. A,,Sommers, L. H., Whitmore, F. C., Ibid., 68, 344 (1946). Fuchs, W., Ishler, T.H., Sandhoff, A. G., ISD. EKG.CHEU., AXAL. ED.12,507 (1940). Gilman, H., Miller, L. S., J . A m . Ghem. SOC.73,2367 (1951). Grubb, W.T., I b i d . , 76, 3408 (1954). communicaGrubb, W. T.,. Drivate tion. (7) International Critical Tables, Vol. 111, p. 3, PrfcGraw-Hill, Sew York, 1928.
(8) LU&;G. R., llartin, R. IT., J . ~ m . Chem. Soc. 74,5225 (1952). (9) Sauer. R. O., Ibid., 6 6 . 1T07 (1944). (10) Siggia, S., “Quantitative Organic
analyses Via Functional Groups,” p. 43, Wiley, Sew York, 1948. (11) Sommer, L. H., Pietrusza, E. K., Whitmore, F. C., J . Am. Chem. Soc. 68, 2282 (1946). (12) Sprung, %I. M., Guenther, F. O., Ibid.. 77. 3990. 3996. 6045 (1955). (13) Wright; G. F:, “Organometallk Compounds for the Determination of Active Hydrogen In Organic Analysis,” Vol. I, p. 155, Interscience, Xex York, 1953. RECEIVEDfor review August 29, 1956. Accepted February 15, 1958.
Determination of Fluoride Ion by Turbidimetric Titration WARREN W. BRANDT and ALLEN A. DUSWALT, Jr. Department o f Chemistry, P urdue University, Lafayette, Ind. ,Milligram quantities of fluoride may be determined by a rapid, convenient method using a turbidimetric titration. The simple apparatus is easily and quickly assembled from materials that are generally available. It permits the use of cells varying in size from glass tubing 1 cm. in diameter to large beakers, depending upon the volume of solution and type of precipitate obtained. With calcium ion as the titrant, concentrations of 0.01 to 0.09M fluoride ion may b e determined with an average relative error of &2%. Thorium ion may b e used as the titrant to determine fluoride ion from 0.04 to l.OM, with an average relative error of f. 2%. As little as 0.4 mg. of fluoride ion in 2 ml. of solution may b e determined with a relative error of f 4%. The time for a single determination is generally less than 10 minutes.
A
AMOUNTS of fluoride are often determined gravimetrically. These methods involve lengthy procedures t o ensure pure, filterable precipitates, but low results are obtained. PPRECIABLE
1120
ANALYTICAL CHEMISTRY
Recently, several articles have described the determination of milligram quantities of fluoride ion. Curry and PIIellon (3) have distilled fluoride as silicon tetrafluoride and reacted the hydrolyzed silicon to form the heteropoly blue compound. This color is then measured spectrophotometrically and related to original fluoride content. Onstott and Ellis (6) have determined fluoride b y titrating with samarium ion containing europium carrier-tracer. The end point is determined by measuring the excess titrant by a radiometric procedure. Grant and Haendler ( 5 ) have determined macro quantities of fluoride by titrating with thorium nitrate, using a high frequency oscillometer to detect the end point, and Chilton and Horton ( 2 ) have titrated fluoride acidimetrically with aluminum ion. An excellent general review is available on the various analytical methods for the determination of fluoride through 1952 (6). The method proposed here is a precise, rapid, and simple determination of fluoride ion within the concentration range of 0.01 to 1.OM. Many problems and difficulties inherent in the gravi-
metric determination of fluoride are not met in this analysis. 30 filtering, drying, or weighing is necessary. Low values are not obtained because of loss of fluoride as occurs in the gravimetric methods. The method of turbidimetric titration is based upon the light absorbing and scattering properties of precipitates in solution. The amount of precipitate formed by the titrant with the sample is measured by the decrease in light transmitted through the cell. A photocell in series with a potentiometer is used t o measure the change in transmitted light. The end point is obtained from a plot of log Zo/lus. milliliters of titrant. lois the intensity value for the clear solution and I , the value for the intensity after the titrant is added. The intersection of the lines forming the slope and plateau of the plot is the end point. Under the most favorable concentration conditions of fluoride ion, a relative error of =tl%was obtained for the rapid determination of fluoride by turbidimetric titration. For the poorest conditions the error was =t4%. The method of turbidimetric titration is not new, although it is underdeveloped
as a rapid, versatile method of analysis. The analytical work, methods, and scope of turbidimetric titrations are reviewed by Bobtelsky (1). This method is potentially a very useful tool. EXPERIMENTAL
Reagents. Analytical grade sodium
fluoride weighed, dissolved in distilled water, and diluted t o the desired concentration. Analytical grade calcium carbonate dissolved in lOy0 hydrochloric acid, heated to expel carbon dioxide, and diluted to the proper concentration. Analytical grade thorium nitrate tetrahydrate dissolved in distilled water and diluted to the desired volume. Apparatus. An easily constructed apparatus for turbidimetric titrations may be assembled quickly from materials t h a t are generally available (Figure 1). Kecessary items are: potentiometer, photovoltaic cell (approximately 1.5 pa. per foot-candle), microburet, magnetic stirrer, lead storage battery, flashlight reflector, and a dashboard light bulb (G.E. 46). Constant current is obtained from the lead storage battery after 1 hour of supplying the light bulb with current. Titration cells can be made from beakers or glass tubing, depending upon the size of cell desired. For slight precipitates and/or large volumes of solution, beakers of almost any size can be used. For very thick precipitates and/or small volumes, glass tubes as small as 1 cm. in diameter are practical. A 1.00-cni. Beckman cell with a stirring bar 0.8 cm. in length was used successfully to titrate 2 ml. of solution. I n this case the stirring bar was covered with a thin protective film of Duco cement because a glass covering was too heavy for proper stirring. Procedure. Introduce the appropriately prepared sample of fluoride into the cell and dilute if necessary. Align the cell in the light path so as to permit only the light transmitted through the cell and sample solution to fall on the photovoltaic cell. Add the stirring bar and rotate a t a speed
I
I
,i , , 1
03[,,,7.L 0.2 ‘
0.7 0 9
1.1
13
1.5 1.7 1.9
MILLILITERS
Calculated end point, 1.334 mi.
A.
1.324 ml. 1.345 ml. 1.345 rnl.
E. C.
1.346 mi.
D.
E. Thorium titrant
DISCUSSION
Calcium chloride is preferred as the precipitant for fluoride concentrations between 0.01 and 0.09M. Below 0.01M the precipitation occurs a t too slow a rate to be practical. Above about 0.1N the galvanometer does not reach a steady reading, but creeps along slomly. The value of the reading becomes uncertain and the procedure yery time consuming. -4 coagulation takes place near the end point and obscures it. For the range of 0.05 to 0.09-$1 fluoride ion, the relative error is slightly less than 1% in a large number of runs (Table I, Figure 2). As the concentration is decreased, the smaller amount of the precipitate results in decreased sensitivity; the relative error increases to 2% a t a concentration of 0.03M and to 47, a t a concentration of 0.01M (Table 11, Figure 3). Thorium nitrate is applicable t o the determination of fluoride in the range from 0.04 to 1,OM. Below 0.04M the time of a determination lengthens considerably and the precipitate be-
Table l. Titration of 10 MI. o f 0.05M Sodium Fluoride with 0.2M Calcium Chloride Rrh-.
End Point, >La 1.345 1.325 1 340 1 355 1 335 1 325 1.340 1.345
I
a
*
M.S .
I
I
Turbidimetric titration a p p a -
A. Buret B. Battery, 6-volt storage C . Cell DC. 10-volt direct current supply L. Lamp M.S. Magnetic stirrer P. Potentiometer PC. Photocell
tive
Abs. Error
Error,
70
Foundb
t-0.010 -0.010 +0 005 +O 021 +o 001 +O 010
0.8
10.48 10.34 10 45 10 56 10 42 10 31 10.44 10.47
0.i
0 4 16 0 1 0 7 +0.005 0.5 -0,010 0.8 .4v. 0 . 8 Calculated, 1.335 ml. Calculated, 10.4‘2 mg.
Mg.
Table II. Titration o f 10 MI. o f 0.01M Sodium Fluoride with 0.2M Calcium Chloride I
Figure 1. ratus
Determine the end point for precipitation with calcium ion by extrapolating the sloped line before the end point and the horizontal line after the end point to form an intersection. For the titration with thorium, in B-hich the plot is a rapidly ascending curve just prior to the end point, the end point is a t the sharp break which begins the horizontal line.
Figure 2. Titration curves for 10 ml. o f 0.05M fluoride ion
IA
I\/
I
adequate for suspension of the precipitate. Place the tip of the buret below the surface of the liquid. Set the potentiometer so that no galvanometer deflection is obtained. Record the potentiometer setting for 0 ml. of titrant added. Then add increments of titrant and, after allowing a few seconds for “optical equilibrium” to be reached after each addition, bring the galvanometer back to zero and record the new reading. The end point has been passed \Then the galvanometer no longer deflects upon addition of titrant. Plot a graph of log (Io,/Iz) us. milliliters, where I o and I , are potentiometer readings for 0 and 2 ml. of titrant added, respectively.
.I6
I ! 1 .20 .24 .2 8 .32
i
i
.3 6 .40
MILLILITERS
Figure 3. Titration curves for 10 mi. of 0.01M fluoride ion
X1.a 0.254 0.254 0.271 0.256 0.257
8. C.
0.257 ml. 0.271 ml. 0.254 ml.
D. E.
0.256 ml. 0.254 ml.
Abs. Error -0.013 -0.013 +0.004 -0.011 -0.010
Error,
yo
4.9 4.9
1.5 4.1 3.8 3.8
Av.
Calculated end point, 8.266 ml.
A.
Relative
End
Point,
a
*
Mg.
Foundb 1.98 1.98 2.10 2 02 2.01
Calculated, 0.267 ml. Calculated, 2.08 mg. ~~~
VOL. 30, NO. 6, JUNE 1958
1121
qomes too slight for good sensitivity. (The calcium titrant is superior for 0.04 M fluoride and below.) At a concentration of 1.OM fluoride and above, the thorium fluoride precipitate is voluminous and large amounts of titrant must be added in order to obtain a practical response from the galvanometer. T h e relative error is about hl% for the range of 0.05 t o 0.12M. At 1.OM fluoride ion the error is about =t4% and increases with increasing concentration. Unlike calcium fluoride, which is a finely divided precipitate, the thorium fluoride precipitate is gelatinous. It fills the titration cell with a semisolid, opalescent mass which increases the absorbance readings as titrant is added. As little as 0.4 mg. of fluoride in 2 ml. of mater can be determined in a 1-em. cell with a relative error of *4%. The titration curves show different behavior near the end point. The calcium fluoride titrations show , a marked decrease in absorbance that is not found in curves for the thorium salt. The phenomenon suggests the following possibilities. At low concentrations of fluoride the precipitation of calcium fluoride may be very slow. At or near the end point the concentration of fluoride is extremely low and the precipitate may come out over a n appreciable period of time. The precipitation a t the time of the reading would, therefore, be incomplete and the absorbance reading too low. The other possibility is that size and shape of a
crystal forming at the end point may differ from that formed in more concentrated solutions. Shinzo (8) has shown that, in the case of barium sulfate, crystal shape and surface area may change depending upon concentration of the reactant at precipitation. The end point determination for the calcium fluoride precipitate is therefore accomplished by extrapolation of the straight lines to an intersection. The end point for the thorium fluoride precipitate does not show a loss in absorbance and may be taken from the break beginning at the horizontal line. Shain (7) has investigated cerous ion as a titrant in the turbidimetric titration of fluoride ion. Satisfactory results are reported.
Titrations may be run in neutral or acidic conditions down to and beyond p H 1. Carbonate interference is eliminated by acidifying and heating the solution to be analyzed. Lead chloride was investigated as a possible titrant for fluoride ion. The useful range of the lead titrant is severely limited, however, by the slight solubility of lead chloride in water. A nephelometric arrangement was attempted to determine whether any increase in sensitivitity could be obtained over t h a t found by measuring the transmitted light. -4 decrease in sensitivity was obtained from the measurement of the scattered light, however, and the nephelometric arrangement was discarded. LITERATURE CITED
INTERFERENCES
Positive interferences were observed with solutions 0.1111 in sulfate, phosphate, oxalate, tartrate, aluminum, iron, lead, magnesium, and hydroxyl ion. Because the interferences are due to reaction of the titrant or fluoride ion with the interfering ion before they react with each other, the amount of interference corresponds to the amount of interfering ion present. When the reaction of the interfering ion with either the titrant or fluoride ion is affected by pH, the amount of interference would be expected to vary with acidity. No interference was observed from solutions 0.1M in borate, chloride, bromide, acetate, nitrate, or zinc ion.
Bobtelsky, M., Anal. Chim. Acta 13, 172 (1955). Chilton. J. M.. Horton. A. D.. ANAL. CHEM. 27,842 ( m j . Curry, R. P., Mellon, M. G., Ibid., 28, 1567 (1956). Elving, P. J., Norton, C. A,, Willard, H. H., in “Fluorine Chemistry,” Simons, J. H., ed., Vol. 11, pp. 51-211. Academic Press. New York. 1954. ‘ i954. (5) Grant, C. L., Haendler, H. M., ANAL.
CHEY.28, 415 (1956). CHEY (6) Onstott, E. I., Ellis, W. P., Ibid., 28,393 (1956). ( 7 ) Shain, I., private communication. (7) (8) Shinzo, 0. H., ANAL.CHEnr. 27, 1481 (1965). RECEIVEDfor review May 10, 1957. Accepted January 27, 1958. Division of Analytical Chemistry, 131st meeting, ACS, Miami, Fla., April 1957.
Determination of Small Amounts of Arsenic in Selenium JAMES
F.
REED
Westinghouse Research laboratories, Pittsburgh 35, Pa.
It is sometimes necessary to determine trace amounts of arsenic in rectifier grade selenium. Arsenic is determined by the molybdenum blue color method, but it is necessary to remove the selenium quantitatively without loss of arsenic. Thus selenium is precipitated with sulfur dioxide. When about 20 p.p.rn. of arsenic is present, the standard deviation is 1.2 p.p.m. As little as 2 y of arsenic may b e determined. The matrix, selenium, was precipitated without significant loss of arsenic.
R
grade selenium sometimes contains traces of arsenic which must be determined quantitatively for certain applications. The most attractive method appeared to be the molybECTIFIER
1122
ANALYTICAL CHEMISTRY
denum blue colorimetric method (8, 9), which precipitates selenium as the free element when the molybdenum-arsenic complex is reduced to produce the blue color. A search of the literature revealed no method for the quantitative separation of selenium from arsenic. Distillation as the trichloride is inapplicable because the chlorides of selenium are also volatile. Coprecipitation of arsenic with magnesium ammonium phosphate (4) failed because selenium also coprecipitated. Extraction of arsenic trichloride ( 2 ) with benzene was unsuccessful because selenium was also extracted. When arsenic trichloride is extracted with diethyldithiocarbamate (6) the organic reagent reduces the selenium to the free element. Selenium does not
interfere in the method described by Gullstrom and Mellon (S), but it is not sensitive to traces of arsenic. The precipitation of selenium with sulfur dioxide is described by Hillebrand and Lundell (4) as a method for the determination of selenium, but not for the separation of small amounts of arsenic from selenium. Admittedly, this involves precipitation of the matrix from the impurity with possible coprecipitation and loss of the impurity. However, it is shown here that arsenic can be quantitatively separated from selenium by this approach and that any coprecipitation of arsenic is negligible. EXPERIMENTAL
Reagents. Sulfurous acid. Saturate a bottle of distilled water with
