Indirect Determination of Sulfate by Nonaqueous Titrimetry GERALD GOLDSTEIN, OSCAR MENIS,' and D. L. MANNING Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Tenn.
b A method was developed for the rapid determination of sulfate in the presence of ions such as uranyl and ferric, which interfere in most conventional methods. Sulfate is precipitated by the addition of excess barium acetate. The excess barium acetate is then determined by potentiometric titration with perchloric acid in an acetic acid medium. Both cationic and anionic interferences were evaluated. This method was applied i o the determination of total sulfate in solutions of reactor fuels which contained primarily uranyl and copper sulfates, and free sulfuric acid. A precision of better than 1% was obtained in replicate titrations. The results were in good agreement with the known values for synthetic samples, and with the values obtained by a cation exchange-barium chloride titration procedure for actual samples.
T
HE MAJORITY of methods for the rapid determination of milligram quantities of sulfate involve titration of the sulfate with a standard barium solution, using a visual indicator to detect the end point. However, numerous difficulties have been encountered in these titrations, including coprecipitation and solubility problems, slow equilibration rates, and indistinct end points (5). Moreover, most cations cause some error in the determination, either by coprecipitation or by forming a colored complex with the indicator, so that it is necessary to remove them, usually by cation exchange (3,4). A method developed in this laboratory is applicable to the indirect determination of sulfate in the presence of many cations without prior separations. It is also more easily adapted to remote handling operations because of the fewer manipulations required. Sulfate is precipitated in an acetic acid medium by the addition of an excess of barium acetate, which is a moderately strong base in this medium (1). The excess can, therefore, be determined by titrating i t potentiometrically with a
1 Present address, Nuclear Materials and Equipment Corp., ApoUo, Pa.
266
0
ANALYTICAL CHEMISTRY
strong acid. The method was applied specifically to the determination of total sulfate in solutions of reactor fuels which contain uranyl and copper sulfates, free sulfuric acid, and also corrosion products of stainless steel as minor components. EXPERIMENTAL
Apparatus. Precision-Dom Recordomatic Titrometer, equipped with glass- and fiber-type saturated calomel microelectrodes. A shielded lead was installed on the glass indicator electrode. Reagents. Perchloric acid, 0.0311; in glacial acetic acid. This reagent was prepared as described by Beckett and Tinley (2) and standardized by titration against potassium acid phthalate (9). Barium acetate, 0.05LV, in glacial acetic acid. This reagent was prepared by dissolving 3.2 grams of barium acetate in 500 ml. of glacial acetic acid. Procedure. Transfer an aliquot of the sample solution which contains from 1 to 6 mg. of sulfate to a 50-ml. beaker. Add 3 nil. of 0.05N liarium acetate solution with a pipet. Then add 6 ml. of acetic anhydride for each milliliter of water present, and 10 ml. of acetic acid. Boil the solution for about 2 minutes and allow it to cool. Dilute to about 25 ml. with acetic acid and then potentiometrically titrate the excess barium acetate with perchloric acid. I n addition, dilute and titrate 3 ml. of the barium acetate solution. Calculate the sulfate concentration. RESULTS A N D DISCUSSION
Effect of Water. The presence of water seriously decreases the magnitude of the potential break at the equivalence point of the titration of barium acetate with perchloric acid, making i t difficult to estimate the end point (Figure 1). When as little as 0.5 ml. of water was present in the 25-ml. volume, the potential break was decreased from 100 mv. t o about 60 mv. It is necessary, however, to use aqueous sample solutions because the solubilities of inorganic sulfates in acetic acid are limited ( I ) . Furthermore, the reaction of barium acetate with solid sulfate salts, in an acetic acid medium, is not quantitative, even on heating. It is necessary. there-
fore, to carry out the precipitation reaction in an aqueous medium. Then the water is eliminated by adding sufficient acetic anhydride to react with all the water (6 ml. of acetic anhydride per ml. of water) and boiling the solution for a fen- minutes to complete the reaction. Small amounts of excess acetic anhydride have no effect on the titrations. I n some cases nhen the aqueous solutions n-ere dehydrated by boiling with acetic anhydride, a precipitate of inorganic salts in addition to barium sulfate sometimes appeared, depending on the composition of the sample. When this occurred the recovery of barium acetate from these solutions was low, probably because of coprecipitation of the barium acetate. However, this difficulty can be avoided by adding, in addition to the acetic anhydride, about 10 ml. of acetic acid before dehydration, as recommended in the procedure. Effect of Cations, I n order to evaluate the effects of various cations on the determination of sulfate, 0.lX solutions of their sulfate salts were prepared, and 0.5-ml. aliquots mere titrated by the rrcommended procedure. Results of these titrations are summarized in Table I. The titration curves with a potential break of 100 mv. were identical with that of pure barium acetate. Typical titration curves of each type in Table I are shown in Figure 2. I n all titrations in which an end point could be detected they coincided with the equivalence point as shown in Table 11. These results can be interpreted in the following manner: MSOI
+ Ba(OAc)z
-+
M(O.lc)2
Bas04
+
+ excess Ba(OAc)2
(1)
In the reaction of the sulfate salt with barium acetate, a metal acetate is formed as well as BaS04. The end point in the titration of excess barium acetate will be satisfactory only if the basicity of the barium acetate is greater than the basicity of the metal acetate. The poor end points in the titration of the last group of sulfates in Table I are undoubtedly due to the fact that the acetates of their cations are only slightly less basic than barium
can be calculated. The total sulfate concentration is given by
-500,
hleq.
I
I
-8001
+--I--$
,
I
I
0.03 pl ncio,.mi
Figure 1 . Effect of water on titration of barium acetate
0 0.5 1
acetate, and i t is not possible to differentiate between these acetates and barium acetate. This is in agreement with previous work on the titration of inorganic acetates by Pifer, Wollish, and Schmall (8). The cations in the middle group form weakly basic acetates, and, although the potential break a t the end point is somewhat smaller, do not interfere. The cations in first group form stable acetates which do not affect the titration of barium acetate. The alkali metals represent a special case, because in an acetic acid medium their sulfates are basic salts. The direct titration of potassium sulfate with perchloric acid gives an inflection point after 1 mole of perchloric acid per mole of potassium sulfate has been consumed (6) and, therefore, the reaction can be written as follows: KzSOa
+ HClO,
+
+ Kclo4
KHSOa
(2)
For the sake of simplicity the titrant is expressed as HC104 rather than the acetonium ion. Advantage can be taken of this in the analysis of mixtures of alkali and other sulfates, by titrating first the alkali sulfate with perchloric acid, then adding the barium acetate, and titrating again with perchloric acid. For example, if a mixture of potassium and uranyl sulfate is titrated first with perchloric acid, KpSOI
+ UOzSOa + HClOa KHS0.j
S 0 4 - 2meq.
+
+
+
+
+ +
-- I
+
and the solution is again titrated with perchloric acid, the reaction being, Excess Ba(OAc)2 KOAc 3 HC104 3 HOAc Ba(C1O4)2 KC104 ( 5 ) From the first titration, Equation 3, the concentration of alkali sulfates
-
w
HC104,ml
Figure 2. Titration end points in determination of sulfate in various salts
+
(3)
UO2SOa Ba(OAc)*-, KH80, BaSOl U O Z ( O A ~ ) ~KOAc HOAc excess Ba(0Ac)z ( 4 )
+
meq. Ba(0Ac)p added HClO,, - HClOa,
0.03
therefore, increases the apparent sulfate concentration by an equivalent amount. KC1 does not interfere, since a n y reaction of KC1 with barium acetate would release an equivalent amount of potassium acetate. NITRATE."03 reacts with barium acetate t o precipitate insoluble barium nitrate, thus increasing the apparent sulfate concentration by a n equivalent
Table I. Effect of Cations on Determination of Sulfate, Shown b y Potential Break
(Conditions: volume, 25 ml.; titrant, 0.03N HC104; B a ( 0 h ) added, ~ 0.15 meq.; sulfate, 0.05 meq.) 50 Mv. or 100 Mv.; 75 hlv.; Less; End End Pt. Well End Pt. Pt. Poorly Defined
Adequate
uozso,
Defined CdSO, COSOL
cusoa VOS04
Table II.
MgS0.j MnSOl NiSO, ZnSOa
Determination of Sulfate in Various Salts SO4+ Found,
Sulfate Salt
Meq."
0.0500 0,0494
n .n489 0.0491 0,0481 0,0502 0.0519
0.0500 meq. added.
+
+ KC104 + UOzSOa +
=
where HC104, and HC104, refer t o the milliequivalents of perchloric acid utilized in the first and second titrations, respectively. An application of this scheme is shown in Figure 3, where a mixture of 0.046 meq. of uranyl sulfate and 0.060 meq. of potassium sulfate was titrated to evaluate the total sulfate content. The first portion of the curve represents the titration of potassium sulfate, after which 0.15 meq. of barium acetate was added and the potential dropped by about 150 mv. The second portion of the curve is the titration of the excess barium acetate plus potassium acetate. The total sulfate found was 0.108 meq. compared t o 0.106 meq. added. Effect of Anions. I n evaluating t h e effect of anions on t h e determination of sulfate three factors must be considered : t h e cation associated with t h e anion, t h e reaction of t h e anion with barium acetate, and t h e reaction of t h e anion with perchloric acid. Since the first consideration obviously depends on t h e specific composition of t h e sample, a complete evaluation cannot be made here. Tests were made, however, by titrating different anions as their acid and alkali salts. These tests were made by preparing solutions which contained 0.1 meq. of barium acetate and 0.1 meq. of the anion in a volume of 25 ml. of acetic acid, and titrating these solutions with perchloric acid. The results, presented in Table 111, are summarized below. CHLORIDE. HC1 is a strong acid which reacts with barium acetate and,
?'hen a known quantity of barium acetate (excess) i.r added,
+
meq. Ba(0Ac)Z added meq. of excesa Ba(OAc)z
I n the second titration, Equation 5, both the excess barium acetate and potassium acetate are titrated. However, the potassium acetate produced in Equation 4 is equivalent t o the perchloric acid used in Equation 3. Therefore,
Solution volume, 25 ml. Water, ml.
A. B. C.
=
-*""I
I
Table 111. Effect of Various Anions on the Titration of Barium Acetate
Anion -600
1
Compound Added
Ba(0Ac)z Found,
c1-
hleq." 0 0,100 0,0994
Koa-
n
0.185
O.03N HCIO,,ml
Figure 3. Determination of sulfate in mixed K2S04 and UOZSOI
F-
I,. 102 0.0984 0.195
0.100 meq. added.
VOL. 33, NO, 2, FEBRUARY 1961
267
Table IV.
Determinations of Sulfate in Synthetic Samples
(Six replicates used) SO,-*,Mg. per M1. Coeff. of Variation, % Added Founda
Sample 0.8 HzS0, Synthetic fuel A 0.5 Synthetic fuel B 0.7 a By recommended procedure. By cation exchange-BaCL titration.
Table V. Analyses of Seven Fuel Milligrams per Samples for Milliiiter
Cation Exchange- NonBaClz aqueous Titration Titration 8.4 8.3 8.4 8 4 8.2 8.5 8.5
8.07 8.36 8.71 8.66 8.54 8.65 8.66
Difference % Mg. -0.33 +0.06 +0.31 +0.26 f0.34 +0.15 +0.16
3.9
0.7 3.7 3.1 4.1 1.8 1.9
amount. KNO3 also reacts with barium acetate, but an equivalent amount of potassium acetate is formed so t h a t no interference is observed. FLUORIDE. I n the presence of HF the correct titration value was obtained, but the potential break a t the end point was decreased t o about 60 mv. Apparently, HF reacts with barium acetate to produce barium fluoride. Barium fluoride. however, is a sufficiently strong base to be titrated although i t is a weaker base than barium acetate ( 7 ) . N a F m-as titrated quantitatively with perchloric acid along with the barium acetate to give high titration values. PHOSPH~TE. H3P04did not interfere. No precipitate of barium phosphate was formed. Na2HP04 was titrated with perchloric acid so t h a t when this salt was present the titration values were high. DICHROMATE. K2Cr20,reacted with barium acetate to yield a precipitate of barium chromate.
+
K2Cr207
+
2 Ba(OAc)z H 2 0+ 2 BaCrOc 2 KOAc 2 HOAc
+
+
I n this reaction 0.5 meq. of potassium acetate is produced for each milliequivalent of barium acetate consumed. Therefore, one half of the barium acetate was recovered in the subsequent titration with perchloric acid. I n general, the cation associated with the anion governs the degree of interference of the anion. Anions which form precipitates with barium, such as nitrate, will interfere in the determination of sulfate unless the cation acetate also formed is a sufficiently strong base to be titrated. Also, some compounds, such as fluorides and 268
e
ANALYTICAL CHEMISTRY
5.22 6.72 2.08
5.23 6.60 2.08
Foundb
...
6.80 2.11
phosphates, form basic salts in an acetic acid medium. These salts, depending on the associated cation, may be sufficiently basic to react with the perchloric acid titrant and interfere in the determination of sulfate. Pifer and Wollish (7‘) list some of the basic inorganic salts which can be titrated with perchloric acid. Precision and Accuracy. The procedure was applied t o t h e determination of sulfate in sulfuric acid and in two solutions of synthetic reactor fuels. Solution il consisted of 0.065.V uranyl sulfate, 0.0302V cupric sulfate, 0.014hT nickel sulfate, and 0.036S sulfuric acid. Solution B had the following composition: 0.018;V uranyl sulfate, 0.0094N cupric sulfate, 0.0049N nickel sulfate, and 0.012N sulfuric acid. Results of these titrations are shown in Table IV. The precision was better than 1% in all cases, and the results are in good agreement with the known values. Application to Samples. Seven fuel samples submitted for analysis were analyzed by t h e pi oposed method and the cation exchange-BaC12 titration procedure. A typical sample contained 10 mg. of uranium, 1.3 mg. of copper, and 0.01 mg. of nickel per ml., and was 0.06N in sulfuric acid. The inflection point of the titration curve resembled the curve shown for cupric sulfate in Figure 2. The results are shown in Table V. Duplicate determinations were made by the recommended procedure and a single determination by the cation exchange-BaC12 titration method. The coefficient of variation based on duplicates was 0.8%. A statistical analysis was made of the data whereby it was demonstrated that the two methods are not significantly biased with respect to each other. CONCLUSION
The application for lkhich this method is best suited is the analysis of liquid samples where the components of the sample are generally known. I n these cases the nonaqueous method is rapid and precise. Because the determination is carried out in acetic acid, many of the difficulties encountered in the aqueous titration of sulfate with barium ion are avoided.
The titration itself is an acid-base reaction and not one which depends upon the formation of a precipitate. No insoluble product is formed when the excess of barium acetate is reacted with perchloric acid. The reaction reaches equilibrium rapidly, and the end point is easily detected. Interference and coprecipitation phenomena are also substantially different in aqueous and and nonaqueous sulfate titrations. In aqueous sulfate determinations many of the polyvalent cations interfere because they form stable sulfates which tend to coprecipitate with barium sulfate. I n the acetic acid the polyvalent cations form stable acetates which minimizes coprecipitation. Anions, such as phosphate, can interfere in aqueous barium sulfate precipitations because of coprecipitation of barium phosphate I n acetic acid the coprecipitation of barium salts is unlikely because very few compounds are sufficiently acidic to react with barium acetate. Difficulties in the nonaqueous method do include, however, coprecipitation of barium acetate when insoluble metal acetates are formed. I n addition, many salts are sufficiently basic in acetic acid to react with the perchloric acid titrant. I n general, many cations which interfere seriously in aqueous titrations may not interfere at all in the nonaqueous method and rice versa. The effect of many anions is also inverted when changing from one medium to the other. ACKNOWLEDGMENT
The authors acknom-ledge the assistance of H. P. House in the preparation of this report. LITERATURE CITED
(1) .4udrieth, L. F., Kleinberg, J., “Non-
Aqueous Solvents,” Chap. 1711, Wiley, New York, 1953. (2).Beckett, A. H., Tinley, E. H., “Titration in Non-Aoueous Solvents.” 2nd ed., p. 19, Briti’sh Drug Houses, Ltd., Poole, England, 1957. (3) Fritz, J. S., Freeland, hl. Q., AXAL. CHEM.26,1593 (1954). (4) Fritz, J. S., Yamamura, S. S.,Ibid.,
27,1461 (1955). (5) Kolthoff, I. M., Stenger, V. A., “Volumetric Analysis,” Vol. 11, p. 306, Interscience, New York, 1947. (6) Pifer, C. W., Wollish, E. G., AXAL. CHEM.24,300 (1952). ( 7 ) Ibid., p. 519. ( 8 ) Pifer, C. W., Wollish, E. G., Schmall, hl., Ibid., 26, 215 (1954). (9) Seaman, W.,Bllen, E., I b i d . , 23, 592 (1951).
RECEIVEDfor review Rlay 5, 1960. Accepted August 29, 1960. Division of Analytical Chemistry, 138th Meeting, ACS, New York, N. Y.] September 1960. Work carried out under Contract KO. W-7405-eng-26 a t Oak Ridge National Laboratory, operated by Union Carbide Corp., for the U. S. Atomic Energy Commission.
