coupling constant of CH3HgX on the chemical properties of X. Evans et ai. (12) and Scheffold (13) reported a linear decrease in the magnitude of the coupling constant for CH3HgOzCR complexes in deuterochloroform with increase in the aqueous solution PKA of the carboxylic acid. The coupling constants reported for the carboxylic acid complexes which were also studied in the present work (pivalic, acetic and mono- and dichloroacetic acids) are -22 Hz larger in aqueous solution. Similar solvent effects have previously been reported ( 4 ) . Scheffold also reported a n approximately linear decrease in the magnitude of the coupling constant for CH3HgXcomplexes in pyridine solution, where X was a series of inorganic anions, with increase in the logarithm of the aqueous solution formation constant. The chemical shift of the methyl protons in the CH3Hg02CR complexes is also dependent on the formation constant of the complex, increasing approximately linearly as the logarithm of K, increases. Probably the most useful correlation, however, is that shown in Figure 8 between the formation constant of the complex (12) D. F. Evans, P. M. Ridout, and I. Wharf, .I. Ckem. SOC.A , 1968, 2127. (13) R. Scheffold, Helc. Chim. Acta, 52,56(1969).
and the acid ionization constant of the carboxylic acid. From a least-squares treatment of the data PKA
=
1.7310gKI - 1.05
Studies are in progress to test the applicability of this relationship for predicting the formation constants of methylmercury complexes with carboxylic acid groups of known KA’S in peptides and proteins. The results reported in this paper demonstrate that P M R is a powerful and direct method for elucidating the solution chemistry of methylmercury complexes. Similar studies are in progress with model organic ligands containing other functional groups present in biological molecules and with several amino acids and peptides. Studies also are in progress on the solution chemistry of methylmercury complexes with inorganic ligands of interest in the environmental chemistry of methylmercury.
RECEIVED for review June 19, 1972. Accepted August 25, 1972. This work was supported, in part, by a grant from the National Research Council of Canada to D. L. R. and by a Postdoctoral Fellowship from the National Research Council of Canada to S. L.
Simultaneous Spectrophotometric Determination of Barium and Strontium Using Sulfonazo 111 Paul J. Kemp and Max B. Williams
Department of Chemistry, Oregon State University, Coroallis, Ore. 97331 An absorbance study of the barium and strontium complexes of sulfonazo Ill revealed that there is no pH sensitivity of the complexes in the pH range 2.6 to 7.7. The addition of the chelons EGTA and CDTA at pH 6 eliminates the interference of most cations except copper which has a light blue chelon complex. The use of EGTA at pH 6.1 and CDTA at pH 5.8 permits the determination of barium and strontium. Simultaneous equations derived from Beer’s law facilitate the analysis. The working range for the method is from 1.0 to 3.0 x 10-SM (1.4 to 4.1 ppm) for barium and from 2.0 to 8.0 x 10-5 (1.8 to 7.0 ppm) for strontium. An aqueous solution of sulfonazo Ill shows less than 1% decomposition in seven days. With periodic calibration, the analytical reagent can be considered stable for several months. Under these conditions, the determination is accurate to within 2% of the actual concentrations. The properties of the free reagent were also investigated, and it was found that sulfonazo Ill could be isolated at 99.7% purity by purification with ion exchange resins and carefully controlled drying. Pure solid sulfonazo Ill decomposes sharply at 222’ C, is soluble in water and water miscible alcohols, and is insoluble in nonpolar organic solvents.
THE QUANTITATIVE COLORIMETRIC DETERMINATION of small amounts of barium and strontium has generally been a difficult undertaking. Of the few methods which have been attempted, some involve reagents which are unstable and fade rapidly ( I ) . Some require the precipitation of barium or strontium as in(1) D. S. Russell, J. B. Campbell, and S. S. Berman, AM^. Cliim. Acta, 25,81 (1961).
124
soluble complexes (2) or chromates (3) with the subsequent separation and redissolving of the precipitates. Others involve the decrease in absorption of the complexing agent as the barium or strontium complexes or precipitates ( 4 , 5 ) . In addition, most of the methods require prior separation of interfering ions such as calcium, magnesium, and some of the heavier metals (I, 5 ) . The most promising of the complexing agents for barium and strontium analysis are the sulfonazo dyes developed by Savvin and his coworkers (6, 7 ) and investigated by Budesinsky and others (8-12). Of particular interest js the dye 2,7bis (2‘-sulfonophenylazp)-1,8-dihydroxy-3,6-naphthalenedisulfonic acid (Figure 1) which was given the trivial name by Budesinsky and Vrzalova of sulfonazo 111. These workers ~~
(2) E. R. Caley and C. E. Moore, ANAL.CHEM., 26. 939 (1954). (3) I. M. Kolthoff and P. J. Elving, “Treatise on Analytical Chemistry,” Part 11, Volume 4, Interscience Publishers, New York, N.Y., 1966, p 185. (4) P. J. Lucchesi, S. Z. Lewin, and J. E. Vance, ANAL.CHEM., 26,521 ( I 954). I I 110 ) , (1959). (5) J. R. Dunstone and E. Payne, Analyst ( L o I ~ ~ o84, (6) S. B. Savvin, Ju. M. Dedkov, and V. P. Markova. 2li. A17nl. Khim., 17,43 (1962). (7) S. B. Savvin, Ju. M. Dedkov: and V. P. Markova, Z. A m l . Cliem.. 194,286 (1963). (8) B. Budesinsky and D. Vrzalova, ibid., 210, 161 (1965). (9) B. Budesinsky, ANAL.CHEM., 37, 1159 (1965). (10) Z . Slovak, J. Fischer, and J. Borak, Talanra, 15, 831 (1968). (11) B. Budesinsky, D. Vrzalova, and A. Bezdekova, Actrr Cliim. Acad. Sci. Hung., 52, 37 (1967). (12) B. Budesinsky and D. Vrzalova, Talanta, 13, 1217 (1966).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
found the reagent to be an excellent indicator for use in the titration of sulfate with barium (9) and also reported conditions under which barium might be determined colorimetrically although no analytical data were given (8). Later they similarly investigated the behavior of barium and strontium with dimethylsulfonazo I11 and rebated compounds (11). Since sulfonazo I11 has now become commercially available and seems to be the most promising reagent for barium and strontium determination, we have attempted t o use this in a simple, direct simultaneous method for barium and strontium determination in the presence of diverse metal ions masked by the chelons, EGTA (ethylene glycol bis(P-aminoethyl ether)-N,N'-tetraacetic acid) and CDTA (trans-l,2diaminocyclohexane-N,N'-tetraaceticacid). EXPERIMENTAL
Apparatus. Absorbance spectra were made on a Cary 15 recording spectrophotometer, using 1-cm matched silica cells. Reagents. Distilled water was deionized by passage through an ion exchange monobed prepared by mixing Amberlite IR-120 cation exchange resin (Hf form) and Dowex 1-X8 anion exchange resin (OH- form) together and then through Dowex 50W-X4 cation exchange resin (H+ form) followed by collection and storage in a borosilicate glass carboy with minimal access to the atmosphere. The water was found to be at pH 7.00 with a spec fic conductance of 4.0 x lo-' ohm-', and was used for all solutions. The sulfonazo I11 was obtained from Aldrich Chemical Company, Inc. and was purified by the method described below before use. Standard barium, strontium, and calcium solutions were prepared from their anhydrous carbonates (ACS reagent). The magnesium solution was prepared from magnesium carbonate trihydrate (CP) and solutions of other metals were prepared directly from their reagent chloride salts. Purification of Sulfonazo 111. The commercial sulfonazo 111 proved to be quite impure and since attempts to follow the purification procedure of Budesinsky and Vrzalova ( 8 ) gave inadequate purity and low yields, a more successful method was devised. Five grams of raw sulfonazo I11 were dissolved in 80 ml of water and filtered through Whatman 42 filter paper. The filtrate was treated with a little activated carbon and filtered again through another piece of paper. The second filtrate was passed slowly through a 30-mm i.d. X 260-mm column prepared from freshly recharged 20-50 mesh Dowex 50W-X4 cation exchange resin (H+ form). The resin eluate was evaporated to dryness using a water ispirator to evacuate the drying flask at 65" C. The dried material was dissolved in 30 ml of water, mixed with carbon, and filtered as before. The resin was recharged and the .hird filtrate was passed through the resin and water bath, .hen taken to dryness in a vacuum oven 50 " C under 130 to 250 mm Hg pressure. The solid so obtained was dark purple :almost black) with a metallic maroon iridescence. DTA and TGA of this product revealed that the material contained t.4 moles of water per mole of sulfonazo 111, and that the ;ulfonazo 111 decomposed exothermally at 222 OC. The solid was observed to be very soluble in water, soluble in water niscible alcohols, somewhat soluble in acidified aqueous ;elutions, and insoluble in nonpolar organic solvents such as 2enzene or chloroform. The hydrated material was placed in a small open vial Nhich was in turn placed inside a vacuum flask containing VaOH pellets. The vacuum flask was heated to 120 to 125 T in a drying oven and connected to a vacuum pump which -educed the pressure below 0.1 rnm Hg. The sulfonazo 111 rYas dried in this manner for 3.5 hours. The material so dried was assayed by direct titration with standard base to an end point at pH 6.80 after titration of four hydrogen ions. The
Figure 1. 2,7-bis(2 '-sulfonophenyIazo)-1,8-dihydroxy-3,6napthalenedisulfonic acid ; formula C2?H18N1O1&; formula weight 688.628; trivial name sulfonazo I11 equivalent weight of sulfonazo I11 in the assay is 172.16, and vacuum dried material was found to be 99.7 sulfonazo 111 by weight. This purified sulfonazo 111 was used to prepare all standard wlfonazo 111 solutions. EGTA Buffer Solution at pH 6.1. EGTA (ethylene glycol bis(p-aminoethyl ether)-N,N'-tetraacetic acid), 7.6 grams, obtained from K and K Laboratories, were added to about 500 ml of water containing 5.4 grams of NaOH. When dissolved, 7.0 grams of maleic acid were added to complete the buffering action and the solution was diluted to 1,000 ml with water. If the EGTA is not added first with the NaOH, it is extremely slow to dissolve. CDTA Buffer Solution at pH 5.8. CDTA (trans-1,2diaminocyclohexane-N,N'-tetraacetic acid), 7.3 grams, obtained from K and K Laboratories, was added to about 500 ml of water containing 5.4 grams of NaOH. Seven grams of maleic acid were added and the solution was diluted to about 950 ml. The pH was monitored while enough additional maleic acid was added, if necessary, to lower the pH to 5.8. Final dilution to 1000 ml and mixing were then completed. Analytical Procedure. Solutions for spectrophotometric measurements were prepared in 100-ml volumetric flasks. Into flasks to be used as barium and strontium standards, appropriate volumes of 2.00 X 10-*M stock solutions were pipetted to give a final concentration range from 0.5 x 10-5M to 10.0 X 10-"M. Into flasks used for analysis, 10.0-ml aliquots of the synthetic samples were pipetted. Blank solutions were prepared without cations. Into each solution were pipetted 5.0 ml of the appropriate chelon-buffer solution and 5.00 ml of 2.00 x 1 0 - 3 M ~ u l f o n a z ~ I11 stock solution. The flasks were then diluted to their marks with water and mixed thoroughly. The differential absorbance of each solution was then determined at 640 nm. Due to the high color intensity of sulfonazo 111, it is difficult to pipet the stock solutions under normal laboratory conditions. However, since sulfonazo I11 is fairly transparent to red light, viewing the meniscus of a sulfonazo 111 solution in a pipet is relatively easy when held against a red lamp or similar light source, such as an ordinary photographic darkroom safelight. RESULTS AND DISCUSSION
Spectral Characteristics. The absorption spectra of sulfonazo I11 and its barium and strontium complexes cs. H 2 0 are shown in Figure 2. The absorbance of free sulfonazo I11 follows Beer's law. The differential absorption spectra of the barium and strontium complexes of sulfonazo 111 are presented in Figure 3. Stoichiometric Ratio. The ligand-to-metal ratio of the barium and strontium complexes of sulfonazo 111 was determined by the method of continuous variations. An isomolar concentration of 1.000 X 10-4Mwas employed, and the resultant curves are plotted in Figure 4. Blank solutions with sulfonazo I11 at appropriate concentrations were used to obtain differential absorbance measurements, the barium com-
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
*
125
Z
Figure 2. Absorption spectra of sulfonazo I11 and its barium and strontium complexes A . 2.00 X l O - 5 . M sulfonazo 111. B. 2.00 X 10-3.W barium in 2.00 X 10-5M sulfonazo 111. C. 2.00 x lO-3M strontium in 2.00 X lO-5M sulfonazo 111
"0; 0.5
0.3
W a v e l e n g t h in nm
I
1
I
I
I
I
I
i
I
I
0.8
I.,
I\
I 1
1
: 0.4 n a 0.2 0.6
E
-0 0
@
0
.-c ? L
0---
-
r
-
4
-
Figure 3. Differential absorption spectra of the barium and strontium sulfonazo I11 complexes
-
Blank. 2.00 X 10-5M sulfonazo 111. A . 2.00 X 10-aM barium in 2.00 X 10-5M sulfonazo 111. B. 2.00 X lO+M strontium in 2.00 x 10-~Msulfonazo111
-
i -0.2I
I
I
I
Table I. Absorbance Depression Effect of Various Salts on the Barium Sulfonazo I11 Complex, Measured at 640 nm Salt Depression, Salt Depression, (1.0 x 10-2M) (1.0 x IO-ZM) Yo None 0 .O (C2H;)eNHSCl 22,O LiCl 16,O (C2HJ4NBr 26.6 NaCl 17.8 NaBr 19.2 KC1 31.9 NaC10, 17.8 NHJCl 22.7 NaN03 19.2 CHsNHsCl 23.4 NaOzCH 17.8 (CH&NHKl 22.0 Na02CCH3 17.8 (CH3)4NCI 22.0
z
plex was measured at 640 nm, and the strontium complex was measured at 643 nm. The ligand-to-metal ratio was found to be 1 :1 for both complexes. Stability of Sulfonazo I11 and Its Complexes. The free sulfonazo I11 in aqueous solution showed some decomposition on standing at ordinary laboratory conditions, the absorbance decreasing less than 1 % in seven days to 6 % decrease in 44 days. A sample standing in strong sunlight for 44 days showed a decrease of 14% in absorbance, while one placed in darkness in a refrigerator for 44 days showed 4 decrease. The differential absorbance of the barium complex L'S. sulfonazo I11 showed a slight increase on standing a t ordinary laboratory conditions (3x in 97 days) while a similar strontium complex decreased 4z. Effect of Sodium Ion and Nonreactive Salts. Solutions of sulfonazo 111 and its barium and strontium complexes were prepared in the presence of a series of common cations and anions at constant concentration to determine their relative effects on the absorbance of the complexes. Another series was prepared using sodium acetate at varying concentrations to determine the concentration effect on the absorbance.
z
126
e
I
A series of solutions, 2.00 x 10-5M in sulfonazo I11 and 2.00 X 10-4M in barium was prepared. To each 1.0 X 10-2M nonreactive salt was added and the differential absorbance of the complex was measured. A comparison of the influence of the salts tested measured at 640 nm is made in Table I. It is evident that the common univalent cations greatly depress the absorbance of the complexes, lithium having the least effect of those tested. It is interesting to note that all of the ammonium salts, independent of the degree of substitution or ionic size, have virtually the same effect on the complex. Common anions seem to have little effect. A series of solutions, 2.00 x lO-5M in sulfonazo 111 and 2.00 x 10-jM in either barium or strontium was prepared. Sodium acetate was added in concentrations from zero to 3.5 x 10-2M. The effect of the barium complex was measured a t 640 nm, the effect on the strontium complex was measured a t 643 nm, and the effect on the blank was measured at 634 nm. The variation in the absorbance of the complexes and of the blank with the concentration of sodium acetate is shown in Figure 5. Effect of pH on Sulfonazo I11 and Its Complexes. Solutions of sulfonazo 111 prepared for p H effect studies were adjusted to the desired pH value by the dropwise addition of dilute HC1 or dilute NaOH. A Thomas p H combination electrode was used to measure the p H since it could be inserted into the solution inside the volumetric flask. The p H of the solution was adjusted before the final dilution to the mark, and the findl pH measurement was made after dilution and mixing. Solutions of 2.00 x 1 0 - 5 ~sulfonazo I11 were prepared from p H zero to p H 14. The significant absorption spectra recorded for the solutions are presented in Figure 6. The absorbance curves are practically p H independant in the pH range 2.6 to 7.7. Solutions 2.00 x IO-SMin sulfonazo 111 and 2.00 X 10-4M in either barium or strontium were prepared from p H zero to pH 14, and their differential absorption spectra were recorded
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
Table 11. Response of Sulfonazo I11 to Selected Cations No. 1 2 3 4 5 6 7 8 9
Cation Ba(I1) Sr(I1) Ca(1I) Mg(I1) Pb(I1) Pd(l1)
IO 11 12 13
HdI)
14 15 16
17 18 19
Cu(I1)
Ni(I1) Al(II1) Fe(1I) Co(I1) Hg(II)
Zn(11) Fe(111)
A€&PPQ
40,050 24,050 5,100 200 35,850 31,500 28,500 10,100 5,400 4,300 3,350 2,700 2,150 2,100 1 ,200
Xrnax
AA1340
640 643 645
0.801 0.464 0.096
...
0.004
650 617 603 646 606 647 648 647 649 645 655
0,188 0.060 0.076 0.058 0.049 0.034 0.039 0.017
&(I)
(25
. .
0.000
