respectively (Figure 1). The NbOFb-2 coordination complexis dissociated when the AI :F mole ratio is 1.0. The slight increase in absorbance in the regions of an AI :F mole ratio less than 0.5 and greater than 1.0 is the direct result of measuring the absorbance against a blank that contained no Al+3. An order of stability of the fluoride coordination complexes of aluminum and of niobium is apparent from Figure 1-Le., A 1 P 2 > NbOFs-* > A1F;t. No value for the stability constant of NbOF5-2 could be found in the literature. However, Brosset and Orring ( 4 ) report pK values of 6.13 and 5.02 for AlFf2 and AIFz+, respectively. In the absence of Al+3, the NbOF6-2 complex prevents the formation of the yellou chromophore NbO(SCN),-; in the presence of Al+3, the chromophore forms. Polymer formation and use of the two-phase system prevent the determination of the chromopho1.e formula by the method of continuous variations, Figun: 2 gives the absorption spectra of the niobium thiocyanate cl-romophore in the presence of A P 3 when the niobium is added as the tartrate complex (Curve C) and as the fluoride complex (Curve 0).The fact that these spectra are identical inc icates that Al+3 completely removes fluorine from NbOF5-2. When the niobium is added initially as the fluoride complex and little or no A P 3 is present, the
(4) Cyril1 Brosset and Jonas Orring, Scensk Kern. Tidskr., 55, 101 (1943).
absorbance of the thiocyanate chromophore at the wavelength of maximum absorbance is negligible. The reaction NbOFS-’
+ 5A1+3 + 4SCN-
+
NbO(SCN)d-
+ 5AIF+’
(1)
is postulated as an expression for the formation of the chromophore from the NbOFb-2 complex. The order of reagent addition is critical. Maximum color stability is obtained when reagents are added in the order: Alf3-acid reagent, sample, stannous chloride, thiocyanate, ether, and stannous chloride. The Alf3-acid reagent contains enough Al+3 per 20 ml to complex about 9 mmoles of fluorine. Absorbance due to AV3 is appreciable at wavelengths below 360 mp (Figure 2). The molar absorptivity of the niobium thiocyanate chromophore in the final test solution is 32,400, which is in close agreement with that reported by Lauw-Zecha, Lord, and Hume (2). Beer’s law is followed over the range of niobium concentration from 0.2 to 1.0 pg/ml in the solution used for the absorbance measurement. In this range, the relative standard deviation is less than 2.3 %. RECEIVED for review June 27, 1966. Accepted January 6, 1967. Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation.
Spectrophotometric Determination of SiIver(1) as the 2-Amino-6-Mehylthio-4-PyrimidinecarboxylicAcid Chelate Okkung K. Chung and Clifton E. Meloan
Department of Chemistrj, Kansas State University, Manhattan, Kan.
CURRENTLY available methods for determining trace quantities of Ag+ in solution itre either inadequate or have serious disadvantages ( I ) . The common turbidimetric silver chloride method ( 2 ) requires a calibration curve to be determined with each batch of samples. The dithizone method (3) requires very pure solvents and a time-consuming extraction step. The method presented here is simple and direct, does not require an exceedingly le rge excess of reagent and the chelate is stable for a month. The structure of the reagent is shown in Figure 1. EXPERIMENTAL Apparatus. A Beckman Model DB spectrophotometer equipped with two matzhed 1.00-cm cells, a Beckman potentiometric recorder, and a Beckman Zeromatic pH meter were used.
Figure 1. Structure of 2-amino-6-methylthio-Cpyrimidine carboxylic acid
Chemicals. Solution of ions used for interference studies were prepared from analytical reagent grade compounds and deionized water. All other chemicals were reagent grade unless otherwise specified. Ligand. Ligand (0.463 gram) was dissolved in 110 ml of D M F and diluted to 250 ml with deionized water (4). Five pellets of solid NaOH were added to prevent precipitation at temperatures lower than 20” C. General Procedure. The sample containing no more than 50 mg of Ag per liter is dissolved in any acid except HCl or
(1) J. P. Dux and W. R. Feairheller, ANAL.CHEM., 33, 445 (1961).
(2) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” 2nd Ed., p. 549, Interscience, New York. (3) Ibid., p. 544.
(4) C. C. Cheng, G. D. Daves, F. Baiocchi, and R. K. Robins, J. Org. Clzern., 26, 2755 (1961). VOL. 39, NO. 3, MARCH 1967
383
r
1 mp
MOLE
FRACTION
OF
AG ( 1 )
Figure 2. Effect of pH on the color of the Ag(Ij2-aminodmethylthio-4-pyrimidine carboxylic acid system
Figure 3. Continuous variation study of the Ag(Ij2-amino-6methylthio-4-pyrimidine carboxylic acid system
HI04. Approximately a fivefold excess of ligand is added to the sample, the pH is adjusted to 10 =t0.1 with any base except NH4OH, and the absorbance is determined at 375 mp.
1. At 375mp 2. At 400 mu
RESULTS AND DISCUSSION Figure 2 is the spectrum of the chelate at various pH's. Even though the system has the greatest sensitivity at pH 11, pH 10 is actually preferred because at pH 11 Ag20 slowly forms destroying the color. The nature of the species present at pH 10 was investigated. Job's method (5) was used. Two wavelengths, 375 and 400 mp were used and the results are shown in Figure 3. Since each series studied had a maximum absorbance at a Ag+ mole fraction of 0.6, it is apparent that there is only one species formed. With a ligand-to-metal ratio of 2 to 3 there is a real problem as to what the structure really is. Two possible structures are shown in Figure 4. However, the authors are not able to find a technique that would distinguish between them. The ability of this system to follow Beer's law in aqueous solution was studied. The system follows Beer's law well up to about 5 X mole of Ag+ per liter. The e is 2.09 X 103 over this range. Interferences. Forty-two cations and 12 anions were studied as possible interfering substances. The metal ions were obtained mostly from the nitrates and partly from the sulfates and oxides. The chlorides and the ammonium salts could not be used because of the formation of either AgCl or Ag(NH&+. A typical solution was prepared by mixing 1 ml of 0.01M AgN03, 5 ml of 0.01M ligand, and 25 ml of deionized water. The absorption curves obtained between 480 and 320 mp using the various ions were compared with the curves obtained with none of the interfering ions present. Using concentrations of foreign ions equal to the Agf ion, three groups of interferences were obtained. ( 5 ) P.Job, Ann. Chim., 10 (9), 113 (1928).
384
ANALYTICAL CHEMISTRY
H , C S A Ag
OR
Figure 4. Possible chelate structures GROUPI. No interference: Li(I), K(I), Si(IV), W(VI), Tl(I), V(V), U02(II), Na(I), Se(VI), Te(VI), Ge(IV), AI(III), Be(II), and Re(VI1). GROUP11. Destroyed the chelate to some degree: As(III), Cr(VI), Bi(III), Fe(III), Au(III), Se(IV), 23(II), Cu(II), TI(III), and Co(II1). GROUP111. Formed insoluble precipitates that absorbed the chelate to some degree: As(V), Pb(II), Ni(II), Sr(II), Ba(II), La(III), Ir(III), Mn(II), Mg(II), Cd(II), Yt(III), Ca(IV), Th(IV), Ti(IV), Co(II), Fe(II), Ca(II), and Ce(1II).
The anions that did not interfere were: OAc-, NOz-, SOa-2, S03-2, S20s-2, .BrOs-, Io3-, HzPOd-, C03-2, and C4H406-2. IO4- formed a browri precipitate of AgIOd. The presence of Br03- did not interfere immediately but after standing for 1 hour it formed a white precipitate of AgBrOs which destroyed the chelate. Extraction Studies. Thirteen organic solvents were purified (6), generally by distillation to remove peroxides, and
employed as extracting agents. They were benzene, hexane, cyclohexane, n-heptane, decalin, benzyl alcohol, isoamyl alcohol, n-hexyl alcohol, xylene, diethyl ether, methyl ethyl ketone, chloroform, and Skelly C. Not one extracted the chelate nor did mixtures of 2 or 3 of the solvents have any effect. RECEIVED August 24, 1966. Accepted January 3, 1967. (6) C. E. Meloan, Ph.D. Thesis, Purdue University, 1959.
2,4,6-Trichlorophenol in a Constant Ionic Strength Buffer System for pH Range 5.5-6.5 Albert L. Caskeyl Department of Chemistr y , Iowa State University, Ames, Iowa
BUFFER SOLUTIONSof varying pH but fixed ionic strength, prepared from monoba dc acid and monoacidic base systems, have been described by Bates ( I ) . While determining the naphtholic acid dissociation constant of 2-nitroso-1-naphthol4-sulfonic acid (2), the need arose for such a buffer usable at pH 6. Hydroxy1amin:-hydroxylammonium chloride, the only available one, wa!; eliminated for use because it might react with the quinont:-oxime tautomer. Work delimiting 2,4,6-trichlorophenol for use as such a buffer is reported here; it is appreciably water soluble, it has been studied more extensively than others, and the acid ionization constant is of the correct order. Reported pK, values ranged from 7.59 (3) to 5.50 ( 4 ) . Many values fell in between (5-9); a recently reported one is 15.2(IO). A simple acid base indicator was not available for titration of 2,4,6-trichlorophenol with standard base as a simple, rapid method of determination. A suitable indicator, briefly described by Csanyi, (;], I2), 2,2‘,3,3 ’-tetramethylphenolphthalein [3,3-bis(4-hydroxy-2,3-xylyl)phthalide], was prepared. E YPERIMENTAL
Reagents. Reagent ,gade chemicals meeting ACS specifications were used whenever commercially available. The 2,4,6-trichlorophenol (Eastman Kodak 1469) was sublimed, dried, and stored over anhydrous Mg(C10&: neutralization 1 Present address, Department of Chemistry, Southern Illinois University, Carbondale, Ill. 62901
(1) Roger G. Bates, “Elcctrometric p H Determinations,” Wiley, New York, 1954. (2) A. L. Caskey, Dept. of Chem., Iowa State University, Ames, Iowa, unpublished work, 1966. (3) N. T. Crabb and F. E Critchfield, Talanta, 10,271 (1963). (4) A. G. Ogston, J . Chen.8. SOC.,134, 1713 (1936). (5) L. F. Fieser and M. Fieser, “Organic Chemistry,” 3rd ed., D. C. Heath and Co., Eloston, 1956. (6) A. Hantzsch, Ber. Deht. Chem. Ges., 32, 3066 (1899). (7) J. A. A. Ketelaar, H. R. Gersmann, and M. Beck, Rec. Trau. Chim.,71,497 (1952). (8) G. J. Tiessens. Rec. Trau. Chim.. 48. 1066 (1929). (9) G. J . Tiessens, Zbid.,50, 112 (1931). (10) E. C. Steiner and J. M. Gilbert, J. Am. Chem. Soc., 85, 3054 (1963). (11) W. Csanyi, Austrian Datent80633 (15 October 1919). (12) W. Csanyi, Z. Elektrochem., 27, 64 (1921).
equivalent, before sublimation, 196.5, after sublimation 197.7 + 0.2 (potentiometric titration in 50z ethanol with standard NaOH); theor., 197.46. The 2,3-dimethylphenol (K and K Laboratories, Inc.) was sublimed: m.p. 74” C, b.p. 213” C ; lit. m.p. 74” C, b.p. 213°C (13). The 2,2’,5,5’tetramethylphenolphthalein was prepared by a reported method (14) and recrystallized from tetraline and ethanolwater: m.p. 285.5-7” C ; lit. m.p. 276” C (14, 286-7” C (15). The hydroxylammonium chloride, NHzOH-HCI,was recrystallized [from dilute, aqueous HCl, washed with ethanol, dried 30 minutes at 110” C, and stored over anhydrous Mg(C104)2]: assay, 100.15z (rel. std. dev. 0.30z, 4 trials) by titration with 0.1009NAgN03(rel. std. dev. 0.23 6 trials) using dichlorofluorescein adsorption indicator. The 0.100M potassium hydroxide in 0.1000M potassium chloride was prepared by dissolving dried KC1, 7.4555 grams, in deionized water, converting it into KOH using an anion exchange column containing Amberlite IRA-410 in the OHform, adding KC1, 7.4568 grams, and diluting to 1 liter. The solution was 0.09942N in KOH; 43.82 ml neutralized 0.8897 gram potassium acid phthalate. The buffer solution of NH,OH.HCl was prepared 0.05000M in both KCl and NH20H.HC1, with an ionic strength of 0.1000, following the practice of Bates ( I ) . The stock solutions of 2,4,6-trichlorophenol were prepared 0.1000M in KC1. The weight, grams of 2,4,6-trichlorophenol, the volume, ml, of 0.100M KOH (in 0.1000M KCl), and the weight, grams, of KC1, respectively, in 1 liter of each of the solutions were: for the study at 312 mp, 0.3871, 20.00, and 7.3069; for the study at 286.5 mp, 0.8294, 42.00, and 7.1425; for the study at 246 mp, 0.2106, 11.00, and 7.3733. Solutions used in the measurement of the pK, were prepared by pipetting into each of several 100-ml volumetric flasks, 25.00 ml of stock solution of potassium 2,4,6-trichlorophenolate, 10.00 ml of NH20H.HC1-KC1 buffer mixture and either 0.0971MHCl or 0.100MKOH (in 0.1000M KCl). Deionized water was added to each strongly basic solution and each solution was diluted with 0.1000M KCl. Equipment. The pH meter used was a Beckman Model G equipped with a general use glass electrode and ground sleeve
z,
(13) “Handbook of Chemistry and Physics,” 40th Ed., D. Hodgman, ed., Chemical Rubber Publishing Co., Cleveland, Ohio, 1958. (14) A. Thiel and L. Junger, 2. Atiorg. Allgem. Chem., 178, 49 (1929). (15) M. Dominikiewiez, Roczniki Chem., 11, 113 (1931). VOL. 39, NO. 3, MARCH 1967
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