V O L U M E 24, NO. 8, A U G U S T 1 9 5 2
1247
of 8 ml. of 1.371 S hydrochloric acid plus 72 ml. of water with 1.439 N sodium hydroxide. The capacitance changes a t this concentration were very small. The theoretical end point for this reaction was a t 7.64 ml. The value read from a large scale plot of the data was 7.62 ml. Formation of Thorium Oxalate. Figure 6 shows the end point obtained in the standardization of a thorium nitrate solution with oxalic acid. This raction has been studied in detail by Blaedel and Malmstadt (3) by a high-frequency method. Correspondingly good end points have been obtained with many other precipitation reactions.
LITERATURE CITED
(1) Alexander, F. C., Jr., Electronics, 18, No. 4, 116 (1945). (2) Bender, P., J. Chem. Education, 23, 179 (1946). (3) Blaedel, W. J., and Malmstadt, H. V., ANAL. CHEW, 23, 471 (1951). (4) Fischer, R. B., Ibid., 19, 835 (1947). (') Hall, J. L., Ibid., 24,1236 (1952). (6) Hall, J. L., and Gibson, J. A., Jr., Ibid., 23, 966 (1951). (?, \ I I Reynolds, A. E., and De Vries, T., J . Am. Chem. Soc., 72, 471 (1951).
RECEIVED for review November 8, 1951. Accepted May 28, 1952.
Titration of Sulfates With Aid of a High-Frequency Oscillator OSCAR I. MILNER Research and Development Department, Socony- Vacuum Laboratories, Paulsboro, N. J . The gravimetric determination of small amounts of sulfate is subject to considerable error. Where the sulfate is in the form of sulfuric acid, it may be determined by titration with standard base, but other acidic components interfere. To provide an alternative to titration with barium chloride conductometrically or with tetrahydroxyquinone as an indicator, a high frequency oscillator has been adapted to the determination. The method is based on the titration of the sulfate using a titrator of the tuned-plate tuned-grid type. The instrument and titration technique are suitable for the determination of sulfate in aqueous solutions containing a few tenths of a milligram of sulfate.
T
H E conductometric titration of sulfates based on precipitation as barium sulfate has been known for years (12, 17). Hoaever, somewhat erratic results are caused by what is apparently an induction period in the formation of the precipitate (11). Kolthoff and Kameda ( 1 8 )studied the precipitation from various alkali sulfate solutions and found that by carrying out the reaction in dilute alcohol solution, equilibrium could be attain'ed much more rapidly. Although they obtained somewhat imperfect results which were attributed to adsorption on colloidal barium sulfate, the titration was nevertheless considered preferable to a gravimetric or nephelometric method for determining sulfate in fairly dilute solutions. hfore recently, a conductometric apparatus and technique have been described, by means of which adsorption effects were minimized and small amounts of sulfate in sea water could be determined with an accuracy of +1% ( 5 ) . Preliminary work showed that by conductometric titration with barium chloride in a seeded alcoholic medium, quantities of sulfate as low a s 0.5 mg. could be determined accurately. However, electrolysis of the solution with accompanying meter instability, and electrode contamination by bits of precipitate caused some dissatisfaction with the method. To overcome these disadvantages, a titration terhnique using a high frequency 09cillator as a titrator waa devised. The theory of high frequency titrations has been reviewed ( 7 , 16, 20). In the instrument used for the procedure described, the changes in total ionic conductance cause an increase in the plate current; under the conditions of the titration, the relationship is practically linear. The titration has been found useful for the determination of sulfur in a variety of petroleum products, and also in combustible gas mixtures by a modification of the Hakewill and Rueck method (14). APPARATUS
A number of forms of titration apparatus which should prove suitable for the purpose have been reported in the literature
( 4 , 6-9, 21). The particular apparatus used in the described work employs a tuned-plate tuned-grid circuit similar to that described by Jensen (15,16),and is illustrated in Figures 1 and 2. Figure 1 shows the instrument with the test solution in place. A 5-ml. microburet and stirrer are mounted on an adjustable sleeve, so that they may be conveniently lowered into position when the titration is begun. The titration cell is a 200-ml. test tube, held firmly in place within the field by means of a spring clamp fitted to the shield. Stirring must be continuous and smooth, and any changes in orientation of the cell during the titration must be carefully avoided. The temperature of the solution need not be controlled, other than to ensure its having reached equilibrium before the titration is begun. Under suc h conditions, there is no drift in the instrument. A complete wiring diagram is shown in Figure 2. C1 is initially set so that the oscillator operates a t a frequency of approximately 18.5 megacycles per second. S1 is a meter-reversing switch, by means of which the range covered by M z is doubled. On the whole, the instrument is rugged and compact, and, although simple, entirely adequate for the purpose described. PROCEDURE
After any preliminary processing necessary to convert sulfur to the form of sulfate, the sample solution is adjusted so that it is between 0.01 and 0.0001 N with respect to this ion. If carbonate is present, it is first eliminated by making the solution acid and boiling for a few minutes. The solution is then transferred to the titration cell, adjusted to contain between 30 and 40% ethyl alcohol by volume, and just neutralized to methyl red with carbonate-free sodium hydroxide. [The alkali is freed from carbonate by centrifuging ( 1 9 ) or by ion exchange ( I O ) . ] The solution is cooled to room temperature and a few milligrams of barium sulfate are added. After the instrument controls have been adjusted to the setting previously determined to yield maximum sensitivity and stability, the titration is begun. Either 0.1 N or 0.02 N barium chloride is used, depending on the quantity of sulfate present. Increments are added in the usual manner, a few moments being allowed for the system to come to equilibrium after each addition before the meter reading is recorded. (In general, the size of the sample and of the increments should be such that no more than 5 ml. of reagent solution will be required and no more than a total of 10 to 15 increments need be added.)
ANALYTICAL CHEMISTRY
1248
Table 11. Determination of Sulfur % S (Osoillimetrio)
3'% S (Gravimetric)
Vertiohl Tube Method (IS) % S (Alkalimetric) 0.28 0.28 1.09 1.01 0.33 0.29 0.05 0.05 0.54 0.53 Lamp Method (SI
% S (Alkalimetric)
n ,023
0.019 0.81
0.82
0.035 n.ni2 0.014
0.040 0.013
0.017 n.oa n on9
0.030
n.on 0.28 0.24
0.27 0.2%
solution diquoted, one aliquot being titrated with the titrator, the other being comploted by alkalimetric titration or gravimetrically. These results are summarized in Table 11. DlSCUSSION
Agreemcnt between methods is generally within acceptable precision limits. There seems to be a tendency for the values obtained by alkalimetric titration t o run somewhat higher than
Figure 1. Titration Assembly
At the completion of the titration whioh normally requires not more than 10 minutes, the current is plotted against volume of reagent. The paint of intersection (end point) is then determined in the conventional manner. A typical titration curve is shown in Figure 3. RESULTS
The method was initially tested against a number of hydrogen peroxide solutions containing measured amounts of standard sulfurio acid. These solutions simulated the end products of the determination of sulfur in light petroleum products by the lamr method (I), in which the sulfur i8 burned t o sulfur dioxide ancI oxidized to sulfate in a hydrogen peroxide solution. The result. are shown in Table I. The standard deviation, calculated from these values according t o the equation r =
e$,
is 2.2%.
To test the titration further, a number of determinations were made on various samples of petroleum products of the type mutinely encountered. In each case the sample was decomposed b j combustion 8 6 required by the method, and the final sulfate
Figure 2.
Oscillator Cirouit
c1. oto100n G . 50 mmc
Table I. Determination of Sulfur as Sulfuric Acid
cx.
0.01 mf m
Sulfur Present,
R,.
ino,woot
ME.
0.085 0.17 0.34
0.68 1.02 1.70 3.40
6.80
Sulfur Found, Mg. o.ogo 0.17 0.34.0.33.0.34 0.66.0.69, 0.68 1.02
1.67 3.38 6.72
Deviation.
%
+5.8
0.0 0.0 -2.8 0.0 -2.8: +1.5:o.n 0.0 -1.8
-o.3 -1.2
G. oto150n
Rs. 680ohms R ~ . in,nno ohms
R&. 50Wohrna Xa. 5ohms
M , . otoinma. M ~ .ntosope. L I . 2.5 mh. R.F. choke SI. On-off switch Sa.
Meter-reversing rwifch iturns No. 10 wtee, 1.'
Coila.
Titration cell in soil 2
V O L U M E 24, N O
8, A U G U S T 1 9 5 2
1249
thz values obtained by titration with barium chloride. This may be due t o the presence of small amounts of halogens or nitrogen n.hich would yield acidic decomposition products to be titrated along with the sulfuric acid when alkali is used as the titrant. The barium chloride titration, on the other hand, is as specific as the gravimetric method for solutions of the type described. This specificity, relative speed, and the fact that small amounts of sulfate can be determined with an accuracy unobtainable by the gravimetric method, constitute the chief advantages of the procedure.
tions of sodium chloride. Blaedel and Malmstadt ( 7 )have shown that the maximum tolerable electrolyte concentration is directly proportional to the frequency at which the instrument operates, and have recently developed a stable oscillator operating at much higher frequencies (8). With this instrument it appears that concentrations up to almost 1 Jf can be tolerated. This development should permit the use of high frequency titrators for certain titrations which, until now, have been unsuccessful. INTERFERENCES
S o comprehensive investigation of interferences was made, However, for other applications, the sulfate solution must be free from ions that would also be precipitated by the titrant-for example, phosphate, chromate, oxalate, and silver.
I
ACKNOWLEDGMENT
Thanks are expressed to R. J. Zahner, who collaborated in making preliminary conductometric titrations, to C. S. Tegge who constructed the titrator, and to E. T. Scafe and Perry Swanson for advice and encouragement.
END POINT, 1.02 ML EQUIVALENCE POINT, 1.035 ML
\
L
I
0.2
I
1
I
LITERATURE CITED I
0.4 0.6 0.8 VOLUME OF REAGENT (ML 0.1 N SOLN.)
1.0
(1) Am. SOC. Testing Materials, Method D 90-50T, “Sulfur in
I
1.2
Figure 3. Titration of Sodium Sulfate with Barium Chloride I
‘“I
0
z0 Q W
d d W
I
W
I
tJ
Petroleum Products by the Lamp Gravimetric Method.” (2) Am. Soc. Testing Materials, Method D 129-50, “Sulfur in Petroleum and Lubricants by the Bomb Method.” (3) Am. SOC. Testing Materials, Proposed Method for “Sulfur in Petroleum Products by the CO?-Oz Lamp Method,” Appendix VI, “ASTM Standards on Petroleum Products,” 1949. (4) Anderson, K., Bettis, E. S., and Revinson, D., ASAL. CHEM., 22, 743 (1950). ( 5 ) Anderson, L. J., and Revelle, R. R., Ibid., 19, 264 (1947). (6) Bever, R. J., Crouthamel, C. E., and Diehl, H., I o w a State College J . Sci., 23, 289 (1949). (7) Blaedel, 7V. J., and Malmstadt, H. V.,ASAL. CHEW.,22, 734 (1950). (8) Ibid., p. 1413. (9) Blake, G. G., Australian J . Sci., 10, 80 (1947); 11, 59 (1948). (10) Davies. C. W,. and Kancollas. G. H.. ’Vature. 165. 237 (1950). .~..., ( l l j Duke, F . R., Bever, R. J., and Diehl: H., Iowa State College J . Sci., 23, 297 (1949). (12) Dutoit, P., B u l l . soc. chim.,7, 1 (1910); J . chim. phyls., 8, 2 (1910). (13) Hagerman, D. B., 4 s . 4 ~ CHEM., . 19, 381 (1947). (14) Hakewill, H., and Rueck, E. hl., Am. Gas Assoc. Proc., 529 (1946). (15) Jensen, F. TV., paper presented a t 2nd Annual Symposium on Modern Methods of Analytical Chemistry, Louisiana State University, Baton Rouge, La., March 2 to 5 , 1949. (16) Jensen, F. W., and Parrack, A . L., Agricultural and Mechanical College of Texas, Bull. 92 (1946); IND. E m . CHEM., ASAL. ED.,18, 595 (1946). (17) Kolthoff, I. M., Z . anal. Chem., 64, 433 (1923). (18) Kolthoff, I. M., and Kameda, T., ISD.ENG.CHEY.,ANAL.ED., 3, 192 (1931). (19) Kolthoff, I. M., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” rev. ed., p. 551, S e w Tork, Macmillan Co., 1943. (20) E. H. Sargent and Co., Chicago, “Scientific Apparatus and Methods,” Vol. 4, p. 34, 1951. (21) \Vest, P. W.,Burkhalter, T. S . , and Broussard, L., ANAL. CHEM.,22, 469 (1950).
“r
0.03 M N a C l
J u
2 10 .-.-. I
-.-.-.-
0.01 M N a C l
I 0.10
I 0.20
//
’
/
/
i-
FEQUIVALENCE POINT I I I
I I I 0.20 0.40 0.50 0.60 0.70 0.80 VOLUME OF B A R I U M C H L O R I D E (ML 0.1 N S O L N ~
0.90
I 1.0
Figure 4. Titration of Sodium Sulfate in Presence of Sodium Chloride
The principal disadvantage is that large amounts of foreign ions decrease the sensitivity and accuracy. Hydrogen ions, being highly mobile, are particularly troublesome in this respect, and it is to decrease this effect that free acid is neutralized before beginning the titration. The total electrolyte concentration 13 hich can be tolerated in the desrribed system is of the order of 0.03 M; honever, this will vary nith the particular electrolj te present. Figure 4 shons the effectof several different concentra-
,
RECEIVEDfor review 11arch 26, 1952. Accepted June 2 , 1952. Presented a t t h e Sleeting-in-Miniature, Philadelphia Section, . ~ I I E R I C A N CHEMICAL SOCIETY, January 18, 1981.
