Anal. Chem. 1980, 52, 1855-1858 (12) Reichardt, C. I n "Organic Liquids"; Buckingham, A. D., Lippert, E., Bratos, S. Eds.; Wiley: New York, 1978; Chapter 16. (13) Reichardt, C. Angew. Chem., Int. Ed. Engl. 1979, 78, 98-110. (14) Gutmann, V. "The Donor-Acceptor Approach to Molecular Interactions"; Plenum Press: New York, 1978; Chapter 2. (15) Krygowski, T. M.; Fawcett, W. R. J . Am. Chem. SOC. 1975, 97, 2143-2 148.
1855
(16) Miller, J.; Parker. A. J. J . Am. Chem. SOC.1961, 83, 117-123. (17) Olekhnovich. L. P.; Minkin, V. I.; Panymhkin, V. T.; Kriul'kov, V. A. Zh. Org. Khim. 1969, 5 . 123-130.
RECEIVED for review March 24, 1980. Accepted July 18, 1980.
Spectrophotometric Determination of Micro Amounts of Trithionate via Mercury(I1) Thiocyanate Reaction Yasuyuki Miura' and Tomozo Koh Department of Chemistiy, Faculty of Science, Tokai University, Hiratsuka, Kanaga wa 259- 72,Japan
A method has been developed for the determination of trithionate. It is based on the formation of thiocyanate by the reaction of trithionate with mercury( 11) thiocyanate and on the spectrophotometric determination of the thiocyanate formed with iron(II1). The reaction of trithionate with mercury(I1) thiocyanate was elucidated in reference to a system containing the phosphate buffer, and tts chemical equation was balanced. The apparent molar absorptivity for trithionate at 460 nm is 7370 L mol-' cm-'. The present method can be applied to the to 1.4 X determination of trithionate in the range of 3 X l o 4 M (2.9-134.5 pg in 5 mL) and gives a standard devlation of 0.0044 absorbance unit and a relative standard deviation of 0.60% for 1 X M trithionate solution.
Polythionates are usually present as mixtures, because the individual polythionates are very similar in their chemical and physical properties. When 1 mol of polythionates reacts with cyanide, sulfite, sulfide, or hydroxide, the mole number of the thiosulfate formed is different in each instance. These reactions of polythionates must, therefore, be studied in detail in order t o determine a specific polythionate in the presence of the other polythionates. Many investigations have been made of the cyanolysis of polythionates (1-6), for the thiocyanate formed can easily be determined. T h e cyanolysis of polythionates has played a critical role in the development of methods for the analysis of mixtures of polythionates (7-12). We have found t h a t trithionate reacts with mercury(I1) thiocyanate t o yield thiocyanate according to the following equation: 4S30s2- + GHg(SCN),
+ HP04'Hg6S4(OH)2HP04 + 8S04'-
+ 10H20 = + 18H' + 12SCN-
(1)
A detailed examination of this reaction led to the development of a n accurate spectrophotometric method for the determination of trithionate. The purpose of this work is to establish the optimum conditions under which the reaction of trithionate proceeds t o stoichiometric completion according to eq 1 and t o clarify the reaction with special reference to the determination of trithionate. EXPERIMENTAL SECTION Apparatus. All spectrophotometric measurements were made a t 460 nm with a Hirama Model 6B spectrophotometer with 10-mm glass cells. The precipitate formed by the reaction of trithionate with mercury(I1) thiocyanate was collected on the interface between aqueous and organic phases with a Kokusan Model H-100B1 centrifuge. Desired temperatures were controlled 0003-2700/80/0352-1855$01 .OO/O
by a Taiyo Coolnit Model CL-15 thermoregulator. pH was measured with a Hitachi-Horiba Model M-7 pH meter. Reagents. All other chemicals used, besides trithionate and mercury(I1) thiocyanate, were of an analytical grade and used without further purification. Potassium trithionate was prepared as described by Stamm et al. (13). The trithionate so obtained was recrystallzied with redistilled wai er at a temperature below 35 "C and then dried in a desiccator containing concentrated sulfuric acid at room temperature, before storage at -10 f 2 "C. The water physically adsorbed on the trithionate was determined to be 0.14% by the Karl Fischer method. The trithionate was confirmed to be sufficiently pure for present purposes; its purity was calculated from the total potassium and sulfur contents ( 2 ) . Standard trithionate solution (1 X M) was prepared by dissolving 135.4 mg of the potassium trithionate (water content 0.14%) in redistilled water and diluting it to 500 mL, and it was stored a t 5 f 2 "C. A stock solution of thiocyanate was standardized by Volhard's method ( 1 4 ) , and working solutions were prepared by suitable dilution. These standards were used to confirm the stoichiometry and completion of the reaction of trithionate with mercury(I1) thiocyanate. Mercury(I1) thiocyanate was prepared by adding 250 mL of 0.4 M potassium thiocyanate to 250 mL of 0.2 M mercury(I1) nitrate in 1 M nitric acid, in small portions and with vigorous stirring. The precipitate of mercury(I1) thiocyanate formed was filtered off by suction with a sintered-glass filter, washed with distilled water and small amounts of methanol, and then air-dried at room temperature. Methanol solution of M) was prepared by dissolving mercury(I1) thiocyanate (3.5 X 280 mg of the mercury(I1) thiocyanate in methanol and diluting the mixture to 250 mL with methanol. Buffer solutions as required for adjusting the pH of reaction solutions to 2.7-8.1 were prepared by mixing phosphoric acid (0.11 M) with sodium dihydrogen phosphate (0.11 M), acetic acid (0.2 M) with sodium acetate (0.2 MI, sodium dihydrogen phosphate (0.11 or 0.2 M) with sodium monohydrogen phosphate (0.11M) or sodium hydroxide (0.2 M), and boric acid (0.2 M) with sodium tetraborate (0.05 M), in various ratios. The buffer solution of pH 7.4 used in the procedure section was obtained by mixing 50 mL of (2.2 M sodium dihydrogen phosphate with 39.5 mL of 0.2 M sodium hydroxide. A solution of iron(II1) nitrate in perchloric acid was prepared by dissolving 306.1 g of iron(II1) nitrate [Fe(N0J34H20]in 217.6 mL of 60% perchloric acid and diluting to 500 mli with redistilled water to give a 1.5 M solution of iron(II1) nitrate in 4 M perchloric acid. Procedure. Place 1 mL of phosphate buffer solution (pH 7.4), 5 mL of trithionate solution (up to 1.4 X M), and then 1 mL of methanol solution of 3.5 X M mercury(I1) thiocyanate in a 15-mL glass-stoppered tube. The pH of the solution is thereby brought to 7.7. Allow the mixture to stand at temperatures ranging from 20 to 35 "C for 10 min, to drive the reaction to completion prior to the spectrophotometric measurement. In this case, the reaction solution becomes turbid owing to the formation of a yellow precipitate. To this mixture, add 2 mL of 1.5 M iron(II1) nitrate in 4 M perchloric acid and about 1 mL of carbon tetrachloride. Shake the mixture vigorousi'y by hand. and centrifuge C 1980 American Chemical Societv
1856
ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980 1.2
-
1.0
-
oeC
oaC
m
e8 n
. 06-
a
0.L 01
O
2
t"
3
4
5
6
7
B
PH
0
10
05 Concn/lO-'M 0
'
10
20
Concn/lO-'M
14
Figure 2. Effect of pH on the reaction of trithionate with mercury(I1) thiocyanate at 22 OC in the presence of 0.11 mmol of phosphate: (0) 8X M S3Oe2'; ( 0 )2.4 X M SCN-; (0) reagent blank.
S30i30
42
SCN
Figure 1. Effect of amounts of phosphate buffer on the mercury(I1) S30e2- in the absence of thiocyanate reaction of trithionate: (0) phosphate buffer (tetraborate buffer was used for pH adjustment); ( 8 ) S306*with 0.05 M phosphate buffer: ((3) S30e2-with 0.1 M phosphate buffer; (0) S 3 0 t - with 0.15-0.3 M phosphate buffer; ( 0 )SCN- with tetraborate buffer or 0.3 M phosphate buffer (expected graph).
it in order to collect the precipitate formed on the interface between the aqueous and organic phases. Measure the absorbance of a clear aqueous solution of the iron(II1)-thiocyanate complex formed against distilled water a t 460 nm.
RESULTS AND DISCUSSION Calibration Graphs. A series of standard solutions of trithionate and thiocyanate were treated as in the procedure section. The resulting plots are shown in Figure 1. If trithionate is completely and stoichiometrically converted into thiocyanate according t o eq 1, the calibration graph for trithionate should coincide with that for thiocyanate when the molar concentration scale for trithionate is drawn to three times the scale for the thiocyanate concentration. These graphs proved t h a t the reaction had proceeded to stoichiometric completion. Positive deviation from a straight line in the lower concentration range is attributed to thiocyanate yielded as a result of slight dissociation of the mercury(I1) thiocyanate. T h e precision was estimated from 11 results for 5-mL aliquots of l X lo4 M trithionate solution. The present method gave a mean absorbance value of 0.737 against a reagent blank, with a standard deviation of 0.0044 absorbance unit and a relative standard deviation of 0.60%. Rate of the Reaction of Trithionate with Mercury(I1) Thiocyanate. Trithionate solution is unstable a t high temperatures but has been confirmed to be stable for a t least 30 min on standing a t 35 "C ( 2 ) . T h e rate of the reaction was therefore investigated a t various temperatures below 35 "C. T h e absorbance for 1 X M trithionate solution reached that for 3 X lo4 M thiocyanate solution which is the expected value, in 30 rnin a t 15 "C, in 8 rnin a t 20 "C, in 5 rnin a t 30 "C,and in 2 min a t 35 "C, respectively, and then remained constant. These results confirm that the reaction can proceed t o completion, as long as the reaction is carried out for 10 min a t temperatures ranging from 20 to 35 "C. Consequently, trithionate was allowed to react with mercury(I1) thiocyanate a t temperatures of 2G35 OC in this experiment. The expected values of absorbance against reagent blank increased as the temperature decreased, because the dissociation of the mercury(I1) thiocyanate in the absence of thiocyanate decreases as the temperature decreases.
Preliminary study revealed that the monohydrogen phosphate produced from the phosphate buffer added for p H adjustment participated directly in the reaction of trithionate with mercury(I1) thiocyanate. Therefore in the presence of the same amounts (0.11 mmol) of phosphate as those used in the Procedure, the rate of the reaction was studied a t various p H values. At lower pH, the fraction present as monohydrogen phosphate decreases due to its protonation; for instance a t pH 4.5 and 5.8, this fraction becomes very small, the pK,, of phosphoric acid being 7.2. Therefore, the absorbances obtained a t these p H values did not reach the expected value, though they were very reproducible. However, a t p H 7.0 and 8.1 where the percent fraction present as monohydrogen phosphate is much greater, the absorbance reached the expected value in 8 and 5 min, respectively, confirming that the trithionate was quantitatively converted into thiocyanate according to eq 1. On the other hand, poor and variable results were obtained a t p H 8.5 presumably because of some interaction between the trithionate and the hydrolysis product of mercury(I1) thiocyanate. The optimum pH range for the reaction of trithionate with mercury(I1) thiocyanate proved to be in the range of 7.0-8.1 under the conditions used in this experiment (Figure 2). A decrease in absorbance below p H 7.0 seems to be caused by a decrease in the fraction of monohydrogen phosphate. The optimum pH range could not be extended to a lower p H value by making the reaction time longer, because the reaction does not proceed any more a t lower p H values. Effect of Amount of Mercury(I1) Thiocyanate. In measuring the effect of the amount of mercury(I1) thiocyanate on the conversion of trithionate into thiocyanate, 1mL of each of methanol solutions of mercury(I1) thiocyanate of various concentrations was employed. In all cases, the trithionate was allowed to react with the mercury(I1) thiocyanate for 10 rnin in the solutions buffered to a p H of 7.0-8.1, which is the optimum pH range for the reaction. Figure 3 shows that the to 2 X reaction was incomplete with 1mL of 0.5 X M mercury(11) thiocyanate because of lack of mercury(I1) to 5 X thiocyanate. But when 1 mL each of 2.5 X M mercury(I1) thiocyanate was used, the absorbances for trithionate were in exact agreement with the values obtained for thiocyanate of three times the molar concentration of trithionate. Therefore, 1 mL of methanol solution of 3.5 X M mercury(I1) thiocyanate was used in the Procedure. Effect of Amount of Phosphate Buffer. As mentioned during the discussion of pH, the monohydrogen phosphate yielded from the phosphate buffer used for p H adjustment was found to participate directly in the reaction of trithionate with mercury(I1) thiocyanate. In order to establish the optimal amounts of the buffer, we treated trithionate with mercury(I1) thiocyanate in the presence of various amounts of phosphate
ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980
I
0
1 1
2
Concn/lO-'M
3
L
5
Hg(SCN)?
Figure 3. Effect of amounts of mercury(I1)thiocyanate on the reaction of trithionate: (@) 1 X lo-' M S306'-; ( 0 )3 X M SCN-; (0)
reagent blank. buffer (pH 7.4) prepared by mixing equimolar sodium dihydrogen phosphate with equimolar sodium hydroxide in a volume ratio of 50:39.5 mL. The resulting graphs are shown in Figure 1. In the absence of the phosphate buffer where 1mL of tetraborate buffer solution (pH 7.8) was used for pH adjustment, the graph for trithionate gave the greatest negative deviation from the expected values. When 1mL of 0.05 or 0.1 M phosphate buffer was used, the reaction did not go to completion because of an insufficient amount of phosphate. But the graph for trithionate was in exact accordance with that for standard thiocyanate when 1mL each of 0.15-0.3 M phosphate buffer was used. Therefore, the buffer solution (pH 7.4) prepared by mixing 50 mL of 0.2 M sodium dihydrogen phosphate with 39.5 mL of 0.2 M sodium hydroxide was employed for p H adjustment in our procedure. Reaction of Trithionate with Mercury(I1)Thiocyanate in the Presence of Phosphate. In order to determine the stoichiometric relationship of trithionate t o mercury(I1) thiocyanate, we investigated the reaction by the method of continuous variations. For application of this method, 4x (x = mole fraction of trithionate) mL of 5 X M trithionate solution and 4(1 - x ) mL of methanol solution of 5 x M mercury(I1) thiocyanate were used with a varying mole ratio of trithionate to mercury(I1) thiocyanate. In this case, 4x mL of methanol and 4(1- x) mL of distilled water also were added in order t o keep the volumes of water and alcohol constant, since the alcohol gives an increase in absorbance. In this experiment, 1 mL of a 0.2 M phosphate buffer solution (pH 7.0) was employed to adjust the pH to the optimum range of 7.0-8.1. Then the absorbances of a series of solutions were measured a t 460 nm against water. Figure 4 shows the plots of absorbance vs. mole fraction of trithionate, in which Hg(SCN), blank in the region x < 0.4 and Hg(SCN),-free blank in the region x > 0.4 were subtracted, respectively. The two curves intersected a t a point indicating 0.4 mol fraction of trithionate. This signifies that 4 mol of trithionate apparently reacted with 6 mol of mercury(I1) thiocyanate in the recommended procedure. A yellow precipitate which is soluble only in aqua regia was formed in addition to the thiocyanate as a result of the reaction of trithionate with mercury(I1) thiocyanate. We attempted to elucidate the reaction by determining mercury, sulfur, and phosphorus contents in the precipitate. Some amounts of the precipitate obtained in the Procedure were dissolved in a small volume (3-4 drops) of aqua regia by heating, and then the solution was diluted with distilled water as required. The solutions so obtained were offered for the determination of mercury by atomic absorption spectrophotometry and for the determination of phosphorus by a spectrophotometric method
0
v
-
l
02
J
C6
C4
Hg( SC N )z
08
1857
10
S,O{
X
Figure 4. Continuous variation plot for S30B2--Hg(SCN)2 reaction in the presence of phosphate.
Table I. Determination of Mercury, Sulfur, and Phosphorus in the Precipitate o/c element amt of pre% element calcd in cipitate, amt element found in Hg,S,mg found, mg precipitate (OH),HPO,
mercury 11.06 9.33
9.00 7.68
81.4 82.3
13.46 9.60
sulfur 1.192 8.86 0.845 8.80
15.05 16.05
0.319 0.338
82.34
8.77
phosphorus 2.12 2.11
2.12
(15). For the determination of sulfur contents, the precipitate was mixed with tin(I1)-strong phosphoric acid, and the mixture was heated a t about 280 "C for 15 min (16). The evolved hydrogen sulfide, equivalent to the amount of sulfur in the precipitate, was absorbed into a 1M zinc acetate solution. The fixed sulfide was determined by iodimetry. By dividing the percent compositions of mercury, sulfur, and phoshorus listed in Table I by each atomic weight, the ratio of the number of atoms for Hg, S, and P in the precipitate was calculated to be 5.96:4.02:1.00. The precipitate was, therefore, identified as possibly Hg6S4(0H)2HP04, because the compositions of Hg, S, and P were extremely close to those calculated for this particular formula (see Table I). On the basis of the following findings: (1) 4 mol of trithionate react with 6 mol of mercury(I1) thiocyanate (Figure 4), (2) 1 mol of trithionate produces 3 mol of thiocyanate (Figure 3), (3) the monohydrogen phosphate yielded from the phosphate buffer participates directly in the reaction, and (4) the precipitate formed has a chemical formula of Hg,S,(OH),HP04; it was concluded that the reaction proceeded to completion according t o eq 1 under the conditions used for the determination of trithionate. Effect of Diverse Ions. A 5-mL solution containing 76.9 Fg of trithionate and various amounts of diverse ions was treated according to the procedure presented in the Experimental Section. The foreign ions present for the determination of 76.9 Fg of trithionate with an error of below 3.3% are shown in Table 11. Ammonium, chloride, bromide, iodide, cyanide, and sulfide gave positive interferences even when present in small amounts, because they reacted with mercury(I1) thiocyanate to yield thiocyanate. Metal ions such as Al(III), Zn(II), Cd(II), Mg(II), Ca(II), Cu(II), Pb(II), and Mn(I1) gave negative interferences caused by a decrease in the fraction of monohydrogen phosphate owing to the for-
1858
ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980
T a b l e 11.
A detailed examination of this reaction led to the conclusion
Effect of Diverse Ions amt
s,o,*-
ion
added
none Na+ K' NH,+
NaNO, KNO, ",NO,
Mg2+
Mg(NO3 ) z .6H,O
Ca'+
Ca(NO 3 ) 2 .4H, 0
Mn2+
MnSO;XH,O
CU"
CU(NO 3)2.3H2 0
Zn2+
ZnSO;7 H, 0
CdZ'
CdSO;XH, 0
PbZ+
Pb( No
~13'
Al( NO 3 ) ,,9H,O
F-
3 )?.
NaF NaCl BrKBr 1KI CNNaCN NaNO, KNO, Na,S.9H2 0 HSO ,NaHSO, Na,CO, co32so*,Na2S0, HA-~O,2Na, HAs0,.7H, 0 CH,COO- CH,COONa borate ion H,BO, In 5 m L of sample solution. phate buffer solution was used.
c1-
amt,a found, error, bg Pg 76
76.9 77.1 76.9 78.3 75.0 1000b 74.7 500 75.0 lOOOb 74.6 2500 75.4 5000b 74.6 500 74.7 l O O O b 74.6 50 75.2 lOOb 74.4 50 75.4 lOOb 74.4 500 75.4 1OOOb 74.8 50 74.8 l O O O b 74.9 5000 76.9 1 78.3 1 77.8 5 78.3 1 78.6 5000 76.9 5000 78.6 1 79.3 50 74.6 2500 75.2 5000 76.9 5000 76.4 5000 76.6 5000 76.9 1 mL of 0.5 M 5000 5000 5 500
+0.1 0 +1.8 -2.5 -2.9 - 2.5 -3.0 -2.0 -3.0 -2.9 .- 3.0 -2.2
--3.3 -2.0 -3.3 -2.0 -2.7 -2.7 -2.6 0 +1.8 +1.3
+1.8 +2.2 0
+2.2 +3.1
-3.0 -2.2 0
-0.7 -0.4 0
phos-
mation of their insoluble phosphates. The interferences of these metals could be eliminated by increasing the amount of phosphate buffer; when 1 mL of 0.5 M phosphate buffer solution was used, Mn(I1) did not interfere in amounts up to 5000 Kg, and Al(III), Mg(II), Ca(II), Cu(II), and Pb(I1) did not interfere up to 1000 pg. Thiosulfate and tetrathionate produced strong interference by reacting with mercury(I1) thiocyanate to yield thiocyanate.
that thiosulfate reacts with mercury(I1) thiocyanate stoichiometrically but tetrathionate reacts nonstoichiometrically; 1 mol of thiosulfate produces 3 mol of thiocyanate. We have previously developed a method (17) for the determination of tetrathionate, thiosulfate, and sulfite in their mixtures. T h e method is based on the spectrophotometric measurement of excess iodine for thiosulfate and sulfite and the thiosulfate formed from tetrathionate by its sulfitolysis. Tetrathionate reacts with sulfite as follows:
S4062-+ S 0 3 2 - = Sz032-+ S3062-
(2)
The method consisted of three procedures, and the three sulfur compounds in the mixtures gave t h e following equivalents: procedure I corresponds to S4062- SzOi'-; procedure I1 corresponds to S2Oa2-;procedure I11 corresponds to Sz032ZS032-. We need four procedures for the analysis of tetrathionate, trithionate, thiosulfate, and sulfite in a solution. When the present method is applied t o a mixture system of the four sulfur compounds treated according to the previous method ( 171, the following other equivalent can be obtained: procedure IV corresponds to 6S4062-+ 3S302' 3S2032-.The proposed method will therefore play an important role in the development of methods for the analysis of mixtures of sulfur species containing trithionate and thiosulfate.
+
+
+
LITERATURE CITED (1) Urban, P. J. 2.Anal. Chem. 1961, 179,422-426. (2) Koh, T.; Wagai, A.; Miura, Y. Anal. Chim. Acta 1974, 71, 367-374. (3) Nietzel, 0. A.; DeSesa, M. A. Anal. Chem. 1955, 27, 1839-1841. (4) Koh. T.; Iwasaki, I. Bull. Cbem. SOC. Jpn. 1966, 39,352-356. (5) Koh, T. Bull. Chem. SOC. Jpn. 1965, 38, 1510-1515. (6) Koh, T.; Iwasaki, 1. Bull. Cbem. SOC. Jpn. 1965, 38, 2135-2138. (7) Urban, P. J. 2. Anal. Chem. 1961, 780, 110-116. (8) Koh, T.; Iwasaki, I.Bull. Cbem. SOC. Jpn. 1966, 39, 703-708. (9) Kelly, D. P.; Chambers, L. A.; Trudinger, P. A. Anal. Chem. 1969, 4 1 , 898-90 1. (IO) Mizoguchi, T.; Okabe, T. Bull. Chem. SOC.Jpn. 1975, 48, 1799-1805. (11) Koh, T.; Taniguchi, K. Anal. Chem. 1973, 45, 2018-2022. (12) Koh, T.; Aoki, Y.; Iwasaki, I.Analyst (London) 1979, 104, 41-46. (13) Stamm. H.; Goegring, M.; Feldmann, U. 2.Anorg. Allg. Chem. 1942, 250, 226-228. (14) Kolthoff, I. M.; Sandell, E. B.; Meehan, E. J.: Bruckenstein, S. "Quantitative Chemical Analysis", 4th Ed; Macmillan: New York, 1969; Chapter 40. (15) Boltz, D. F.; Mellon, M. G. Anal. Chem. 1948, 2 0 , 749-751. (16) Kiba, T.; Akaza, I.; Sugishita, N. Bull. Chem. SOC. Jpn. 1957, 3 0 , 972-975. (17) Koh, T.; Taniguchi, K.; Miura. Y.; Iwasaki, I.Nippon Kagaku Kaishi 1979, 348-353; Cbem. Abstr. 1979, 90, 179574.
RECEIVED for review December 21, 1979. Accepted July 8, 1980.
