barbituric acid derivatives and sulfonamides, do not affect the results of the creatinine assay because the potential interfering substances are not eluted from the ion exchange resin a t the specified pH. Additional analyses demonstrated recovery of known amounts of creatinine added to urine to be within 37,.
perienced frequently in obtaining duplicate checks, and analyses were repeated until satisfactory duplication was obtained. Results by the tlyo methods for 8 Of the 25 differed by more than 10% (11 to 46%). It-appeared that the-modified Jaffe procedure was subject to rather large errors due to incomplete removal of interfering substances by the Lloyd's
LITERATURE CITED
(1) Adams, W. S., Davis, F., Nakatsni, hf., Am. J . iwecz. 28, 726 (1960). ( 2 ) Barclay> J. Kenney~ '1. A , , Bzochem. J . 41, 5 (1947). (3) Carr, J. J., AKAL. CHEM. 25, 1859
(1953).
( 4 ) Dubos, R., Miller, B. F., J . Biol. Chem. 121, 427 (1937). ( 5 ) Gaebler, 0. H., Zbid., 89, 451 (1930). (6) Haugen, H. N., ~ l E, nf., ~ Scand. J . Clin. & Lab. Invest. 5, 67
REPRODUCIBILITY OF URINARY ANALYSES
Triplicate analyses, each on different days, were made by the ion exchange method on aliquots of 24-hour urine collections from 25 adults (Table 11). Absorption spectra of eluates from the resin columns for some of the urine specimens are shown in Figure 2. I n addition, each of the 25 urines were analyzed by the Haugen and Blegen modification of the Jaffe procedure (6). Considerable difficulty was ex-
variations Produced in the development and stability of the chromogen by external and internal influences. These factorsare not involved in the ion exchange method reported here, by which creatinine is not modified chemically, but is measured directly. ACKNOWLEDGMENT
The authors acknowledge the technical assistance of Ann Fein.
(laao).
(8) Langley, W. P., Evans, ill., J . Biol. Chem. 115, 333 (1936). (9) Owen, J. A., b o , B.,Scandrett, F. J.2 Stewart, C. p., Biochem. J . 58, 426
(1954). (10) Wollenberger, A., Acta Chem. Scand. 7 , 445 (1953). (11) Wollenberger, A., Yature 173, 205 (1954). RECEIVED for review February 1, 1962. Accepted April 16, 1962. Work supported by U. s. Public Health service Grant (CY-2433).
Spectrophotometric Determination of Titanium as Reduced Molybdotitanic Acid J. C. GUYON1 with M. G. MELLON Purdue University, lafayeite, Ind.
b The reduction product of a complex molybdotitanate is used as the basis of a spectrophotometric method for the determination of titanium(lV). An empirically devised method is described first. Factors affecting the color reactions are presented, and an application to the determination of titanium in glass is given. The useful range of the method for a 1-cm. cell is 6 to 30 mg. per liter. Also, using statistical methods, a regression equation is developed from which the concentration of titanium may be calculated from the absorbance at 755 rnp and the known concentrations of the reagents.
T
HE increased use of titanium especially in paint pigments and metallurgical products, indicates a need for accurate and sensitive methods for its determination. Although many chromogenic reagents have been proposed to determine the element spectrophotometrically, only one has been a heteropoly complex. Veitsman (9) measured titanium in the form of a complex, presumably the heteropoly anion molybdotitanophosphate. Present address, University of Missouri, Columbia, Mo.
856
ANALYTICAL CHEMISTRY
Work in this laboratory has shown the possibility of employing what has been assumed to be simple heteropoly species for the determination of vanadium and niobium. The postulated species, before reduction, were molybdovanadate (11) and tungstovanadate (10) for vanadium, and molybdoniobate (3) for niobium. This paper reports the resuIts of a spectrophotometric study to ascertain the possibility of determining titanium in a similar way. The assumed species, before reduction, is molybdotitanate. Various workers (4-8) have postulated the existence of heteropoly compounds containing titanium. However, these compounds have not been isolated, nor has the stoichiometry in solution been established. The possible structure and formulation of heteropoly compounds containing titanium have been the subject of some controversy. No suggested analytical applications of such complexes were found. EXPERIMENTAL WORK
Apparatus. A Cary recording spectrophotometer, Model 10-11, having matched quartz absorption cells with a n optical p a t h of 1.000 0.002 cm., was used for the spectrophotometric
*
measurements. A Beckman Model G or Zeromatic meter was used for p H measurements. Reagents. A stock solution of sodium molybdate, approximately lo%, was prepared by dissolving 100 grams of Na2MoOc.2H20i n 500 ml. of distilled water and diluting t o 1 liter. A stock solution of sodium citrate, approximately 575, was prepared by dissolving 50 grams of NazCsHsO,.2H20 in water and diluting to a liter. A stock solution of chlorostannous acid, approximately lo%, was prepared by dissolving 110 grams of SnClz.2H20 in 170 ml. of concentrated hydrochloric acid and subsequently diluting to a liter. A few grams of mossy tin was added to the container. Dilutions of this solution were used as a reductant. A stock solution of titanium was prepared in two ways. Use of potassium titanyl oxalate: 4.00 grams of K2[TiO(C204)J.2H20 were dissolved in 25 ml. of hot concentrated sulfuric acid and then diluted to a liter. The solution contains 2.000 mg. T i per ml. Use of titanium dioxide: 1.000 gram of titanium dioxide, TiOz, was dissolved in 100 ml. of concentrated sulfuric acid containing 50 grams of ammonium sulfate and diluted to a liter. The resulting solution contained 0.5995 mg. Ti per ml. Either solution may be standardized by precipitation of the
~
~
hydrous oxide of titanium and ignition to T i 0 2 for weighing. RECOMMENDED METHOD
To obtain a system for the spectrophotometric measurement of titanium, t n o main chemical processes are concerned: preparation of a complex, presumably a molybdotitanate, and reduction of this complex to a n assumed heteropoly blue. Several variable factors affect each of these reactions. Based partly upon previous experience with heteropoly systems, and partly upon empirical experimentation with the molybdotitanate system, the following recommended procedure was evolved. Preparation of Calibration Curve. Prepare a calibration curve by transferring 0.0, 0.5, 1.0, 3.0, and 5.0 ml. of a solution containing approximately 0.6 mg. per nil. of titanium t o 100-ml. volumetric flasks. T o each flask add 5 ml. of 10% sodiun1 molybdate, adjust the pH to 3*8, a n d allow t o stand for 10 minutes may develop). Add, (some by llypodermic syringe, 5 of 1:4 sulfurlc acid, and, in the same Jvay and as quickly as possible, 1 ml. of 0.1% chlorostannous acid. Dilute to volume, miu, and measure the absorbance (transmittance) in exactly 3 minutes a t 755 mp ~ i t hdistilled !$'ater as a reference* an absorbance (transmittancei-concentration calibration curve. General Procedure. Dissolve the sample, if necessary, and treat t h e resulting solution t o remove any ions known to interfere (see Table V I I I ) . Concentrate the solution t o 25 to 50 nil. a n d continue preliaration of t h e solution and its measurement as described under Preparation of Calibration Curve. From the measured absorbance (transmittance) determine the amount of titanium. EFFECTS OF VARIABLES
As the effects of variables upon the processes of complex formation and reduction had t o be studied, the following tentative method was adopted. A solution containing about 10 mg. of titanium was adjusted t o p H 4.0. Three milliliters of 10% sodium molybdate mas added, and, after standing 10 minutes, the solution was reduced with 1 ml. of 0.1% chlorostannous acid t o obtain a blue hue. Then the problem was to determine the effect of each variable factor and t o control them, as far as possible, t o produce a measureable system. Turbidity develops in time in a titanate solution at pH 4 from hydrolysis of the T i O t 2 ion. At first citrate was used to prevent this reaction b u t serious interference was encountered in the coior reaction. Later this reagent was used only in the statistically de. signed method. Formation of a Molybdotitanate Complex. Upon addi.iion of ammo-
0.2% chlorostannous acid. T o study the enhancement due t o titanium, 0.6 mg. of titanium was added. The total volume was 100 ml. The data in Table I1 show t h a t enhancement of the blue hue t o the 0.35 extent of 0.06 in absorbance occurs in f the pH range 2.60 to 4.25. No enhance$ ment was observed below pH 2.25. 3 Presumably the complex does not form 0.25 below p H 2.25 nor above pH 4.50. Considering these results, the optimum pH is between 3.50 and 4.25. For subsequent experiments, solutions !$-ere adjusted to a n intermediate value of 3.75. As the p H range in which 0.15 looo reduction of the control solution is 405 600 800 minimal is relatively wide, accurate Wavelength mp Figure 1. Absorption spectrum of control was unnecessary. Adjustment to p H 3.8 i. 0.2 was sufficient. This reduced molybdotitanic acid was readily accomplished without bufA. 24 p.p.m. of Ti 8. 18 p.p.m. of Ti fers. In an attempt t o eliminate the blank, nium molybdate to a titanate a pale 25 ml. of 1 : 4 sulfuric acid was added ye11oiv hue formed. hfore titanium just prior to reduction. This r\-ould gave a deeper hue. This color deprevent the reduction of the molybdate, velopment indicated some kind of but, if the reductant IT-ere added rapidly interaction. As t h e hue was not enough, not destroy the heteropoly sufficiently sensitive for analytical use, species before it could be reduced. the system \vas reduced. Using esXot only was the effect of the blank sentially t h e conditions of those of the removed, but the sensitil ity of the recommended procedure, described alsystem was increased about 25%. ready, the absorption spectra of Figure The addition of sulfuric acid was 1 were obtained for 18 and 24 mg. per incorporated into subsequent experiliter of titanium. The makimum ments. absorptionoCCursat7~5m~. EFFECTOF TIME. The rate of EFFECT OF MOLYBDATECONCEN- formation of heteropoly compounds is usually sufficiently sloa- that an apTRATION. The effect of the molybdate concentration was investigated by adding various amounts of 10% sodium molybdate to 2.4 mg, of titanium in a system otherwise identical to that Table I. Effect of Molybdate described in the previous section. The Concentration results are summarized in Table I. (2 4 mg. Ti/100 ml. solution) Sormally a n excess of one reagent is necessary to bring about complex 10% Sodium Absorbance, Molybdate, M1. 755 mp formation, and the molybdotitanate system is no exception. Approximately 0.00 0.0 0.21 a 300-fold excess of sodium molybdate 0.5 0.30 1.0 was necessary for optimum complex 2.0 0.38 formation. I n subsequent experiments 0.40 3.0 5.0 ml. of 10% sodium molybdate was 0.40 4.0 0.40 used. 5.0 EFFECT OF PH. To find the optimum pH for the formation of the heteropoly Table II. Effect of p H on Complex complex of molybdenum and titanium, Forma tion it ryas necessary to measure the reduc( 0 . 6 mg. Ti/100 ml.) tion of molybdate control solutions as a function of pH and then t o study the Absorbance, 755 mp enhancement of the color due t o the KO addition of titanium prior t o reduction. Titanium titanium PH This enhancement would be attributed 1.00 0.00 0.00 to the formation of molybdotitanic 1.50 0.08 0.08 0.21 0.21 2.00 acid. The optimum pH was then 0.38 0.37 2.50 selected on the basis of maximum 0.30 2.60 0.36 enhancement of the blue hue of the 0.31 0.25 2.iO reduction product by the addition of 0.20 2.80 0.26 0.15 3.00 0.21 titanium and minimum absorbance in 0.10 0.04 3.25 the control solutions. The control 0.08 0.02 3.50 solutions contained 25 ml. of 10% 0.08 0.02 4.00 sodium molybdate adjusted t o various 0.02 0.02 4.50 pH values and reduced with 1 ml. of 0.45
VOL. 34, NO. 7, JUNE 1962
857
preciable time interval is required to complete the reaction. I n addition, the stability of the reduced species is often a function of time. To study time of complex formation, systems identical with t h a t of the previous section were allowed t o stand for various periods of time prior t o reduction. The absorbance of the reduction product was taken as a measure of completeness of complex formation. The data of Table I11 show t h a t 10 minutes are sufficient for complex formation.
Table 111.
Effect of Time on Complex Formation ( 2 . 4 mg. Ti/100 ml. solution)
Time, minutes
Absorbance, 755 mp
2 4
0.20
0.30 0.35 0.39
6 8
10
0.40 0.40 0.40
12
60
Table IV.
Effect of Sulfuric Acid
( 2 . 4 mg. Ti/100 ml. solution) 1:4 Sulfuric
Acid, M1.
6 15
Table V.
Absorbance, 755 mfi 0.30 0.33 0.36 0.36 0.36
Effect of Concentration of Reductant
( 2 , 4 mg. Ti/100 ml. solution) Absorbance, Reducing Agent 0 , 1% HzSnCl,, MI. 755 mp
0.0 0.25
0.50 1.0
3.0
5.0
Table VI.
0.0 0.12 0 35 0 36 0 36
0 36
Stability of Reduction Product
( 2 4 mg. Ti/100 ml. solution) Time, Absorbance, Minutes 755 mp
2.0 2.5
3.0 3.5 4 0
5 0
60
7 8 9 10 20 60
858
0 0 0 0 0 0
0.395 0.399 0.400 0 400 0 400 0 395
0,390 0 380 0,370 0.360 0 360 0 300
0.180
ANALYTICAL CHEMISTRY
EFFECTOF TEMPERATURE. Temperature had little effect on the time or extent of complex formation. Solutions placed in boiling water for 10 minutes gave absorbances little different from solutions allowed to stand at room temperature. Reduction of Molybdotitanate Complex. As already stated, i t is necessary t o have the p H about 4 for formation of the molybdotitanate complex, with sufficient time for the process and a n adequate excess of molybdate. If reduction t o the heteropoly blue is made a t this point, both the complex and the excess molybdate will contribute. Sufficient sulfuric acid largely prevents reduction of the molybdate, but a t the same time such excess acidity destroys the molybdotitanate complex, if given sufficient time. Consequently, i t was necessary t o determine the concentration of sulfuric acid required to eliminate a blank correction without destroying the heteropoly species before it could be reduced and measured. To solutions containing 2.4 mg. of titanium and 5 ml. of 10% sodium molybdate, adjusted t o p H 3.8, and allowed to stand 10 minutes, different amounts of 1 : 4 sulfuric acid were added from a hypodermic syringe to obtain rapid efficient mixing. This was followed a t once, and in the same way, with 1 ml. of 0.1% chlorostann'ous acid, and the solution diluted to 100 ml. and mixed. The results (Table IV) show that with 3 or more ml. of acid the absorbance is independent of the amount used. CHOICE OF AND HANDLING REDUCTANT. The reductant must be added as quickly as possible after the sulfuric acid. Otherwise, the complex partially decomposes before there is opportunity for its reduction. For this reason, a hypodermic syringe was used. Several reductants were tried. Ascorbic acid and a sulfonic acid-sulfite reductant gave a pale green product. Hydrazine hydrochloride gave a deeper blue hue than the previous substances, but the hue \vas still not very intense. Ferrous ammonium sulfate and chlorostannous acid gave the best results, the latter yielding a somewhat greater absorbance for the same amount of titanium. Consequently, this reductant was selected. The effect of concentration of reductant in a system similar to that described in the previous section is summarized in Table V. One milliliter of 0.1% chlorostannous acid was chosen as the most satisfactory concentration. STABILITYOF COLOR. T o study the stability of the reduction product, the absorbances of the systems described under Effect of Time were followed as a function of time. The solutions 'rvere transferred quickly to the spectropho-
tometer and measured at 755 mp, The data of Table VI show t h a t measurement 3 minutes after reduction is satisfactory. The absorbances decreased when the products were maintained at the temperature of boiling water. CONFORMITY TO BEER'S LAW. The straight line calibration curve showed conformity to Beer's law for the concentration range studied. The useful range for a 1-cm. cell is 6 t o 30 mg. per liter. EFFECTSOF DIVERSEIONS. The results of the effects of selected diverse ions are summarized in Table VII. A 2% error in the determination of the titanium was considered tolerable. Ag+, Ba+2 Bi+3, Pb+2, and Sr+2 form insolublk precipitates under the conditions of the experiments. As+a, As+5, citrate, CN-, C20*-2J C O + ~ , Cr+3, Cr+6, C U + ~ ,F-, Fe+2, IO4-, P01-3, Sb+3, SCN-, Se0,-2, SiOa-2, tartrate, V03-, and W04-2interfere seriously. The ones listed in the table can be tolerated in the concentration range reported. To introduce the ions studied, the substances used added ions such as K+, Xa+, Br-, C1-, and NOa-. These did not interfere under the conditions reported. APPLICATION OF METHOD
To check the applicability of this new method to an industrial product, NBS glass No. 93 was selected. The elements in most borosilicate glasses are Al, As, B, C1, Fe, K, Mg, Na, S, Si, Ti, and Zr. On the basis of the study of diverse ions, As, Fe, Si, and Zr will interfere in the deter-
Table VII. Effects of Diverse Ions ( 2 . 4 mg. Ti/100 ml. solution)
Added As
1on5 ~
1
~
100 -~ 100 100 100
3
~
NH4 BrB+3
Cd + 2 ClO3-
100 100
c10,-
c u +2 Fei3 I-
Amount Permitted, P.P.M.
Fe( N08)2 KI
100 40 10
100 100
pi + 2
100 100 100
100
SZOB-2 Zn + 2
Zr + 4
4
100 p.p.m. added.
100
100 25
mination of titanium. The following procedure will avoid these interferences. Weigh a dried sample of the glass, of particle size small enough to pass a 200mesh screen, and containing between 0.1 and 3.0 mg. of titanium. Transfer the sample to a platinum dish, treat with 5 ml. of hydrofluoric acid, and evaporate to dryness. Repeat this treatment, if necessary, to ensure complete removal of silicates and a large portion of the arsenic. Add 10 ml. of concentrated hydrochloric acid and take to dryness to complete the removal of arsenic. Dissolve the residue in 25 ml. of 6hf hydrochloric acid and transfer with 6M hydrochloric acid to a separatory funnel. Extract the iron with three 25-ml. portions of diethyl ether. The amount of zirconium present was tolerable, and, therefore, was not removed. Add 5 ml. of concentrated sulfuric acid, and heat to fumes. Cool the solution, develop the color as directed under Preparation of Calibration Curve, and read the absorbance a t 755 mp, with distilled water as a reference. From the calibration curve determine the amount of titanium. The results of nine determinations of titanium in NBS Glass No. 93 follow: 0.025, 0.024, 0.027, 0.029, 0.024, 0.025, 0.031, 0.032, and 0.030%. The certified value is 0.027’%. The mean of the values is 0.0274%) with a 95% confidence limit of +0.007% and a standard deviation of 0.003%.
T o carry out the statistical analysis, the concentrations of all the reagents in question were varied in a systematic manner. The data were then treated in a two-way classification analysis of variance ( I ) , the Wherry-Doolittle technique was applied ( l a ) , and finally a regression analysis was carried out (8). The result of these operations was a regression equation of the form, A
=
0.0609
+ 0.0042T - 0.0664R + 0.0026TC + 0.01416TR
where A is the observed absorbance a t 755 mp, T is the titanium concentration in parts per million, R is the reducing agent concentration in milliliters of 0.1% chlorostannous acid, and C is the citrate concentration in milliliters of a 5% sodium citrate solution. This regression equation was checked in two ways. First, the data used to obtain the regression equation were recalculated, and second, a series of knowns was prepared and the actual absorbance compared with that calculated from the equation. The results of this work are given in Tables VI11 and IX. Confidence limits on the observed value of T were calculated a t f 0.6 p.p.m. titanium a t the CY = 0.05 level. The useful range of the equation is from 6 to 18 p.p.m. titanium.
STATISTICALLY DESIGNED METHOD CONCLUSIONS
As already stated, a t p H 4 the TiO+2 ion hydrolyzes slowly to form a hydrous oxide which results in turbidity and ultimately precipitation. If this should happen before completion of the formation of the molybdotitanate complex there would be an interference with the reduction reaction. Early attempts to avoid the hydrolysis reaction led to the use of sodium citrate to form a titaniumcitrate complex. However, as noted under Effect of Diverse Ions, citrate interferes seriously. Some time was spent in a statistical study of the possibility of developing a method involving the various interacting variables, namely titanate, citrat,., and reductant. Time, pH, and molybdate are important, too, but previously determined optimum conditions for these variables were readily controllable. The study of the interacting variables involved a statistical analysis of data from a series of experiments in which the concentrations of the interactants were systematically varied. This technique normally leads to a predictior: equation by which the concentration of the unknown can be calculated from the absorbance and the concentrations of the reactants.
Perhaps the most important item about this method is the fact that presumably a reducible heteropoly complex is formed between molybdate and titanate. It is more sensitive than the molydotitanophosphate method, but both are subject to a number of interferences. Although applied only to a glass, it should be generally applicable if provision is made for possible interferences.
Table VIII. Recalculation of Data Used to Obtain Regression Equation
T
C
6.0 60 6.0 6.0 6.0
6.0 12.0 12.0 i2.0 12.0 12.0 12.0 18.0 18.0
18.0
18.0 18.0 18.0
1.0 2.0 3.5 1.0 2.0 3.5 1.0 2.0 3.5 1.0 2.0 3.5 1.0 2.0 3.5 1.0 2.0 3.5
Table IX.
T 6.0 6.0 ... 12.0 12.0 18.0 18.0
R 0.5
0.5 0.5
1.5
1.5 1.5
0.5
0.5
o5
1.5 1.5 1.5 0.5 0.5
0.5 1.5
1.5 1.5
Arrlo.
Aob..
0.111 0.102 0.150 0.129 0.145 0.169 0.194 0.226 0.273 0.298 0,329 0.376 0.278 0.325 0.395 0.466 0.513 0.584
0.10 0.12 0.i6 0.14 0.19 0.15 0.20 0.23 0.28 0 28 0.29 0.36 0.27 0.35 0.37
0.44 0.57 0.59
Data Used as Check on Prediction Equation
C
2.0
3.0
2.0 3.0 2.0 3.0
R 1.0 1.0 1.0 1.0 1.0 1.0
-4-10
Aoti.
0.14 0.15 0.28 0.31 0.42 0.47
0.15 0.17 0.28 0.32 0.46 0.48
( 6 ) Klein, V., Bull. SOC. chim. France 36,
17 (1881). (7) Pechard, T., Compt. rend. 117, 788 (1893). (8) Souchay, P., Ann. chim. [12], 1, 248 (1946). (9) Veitsman, R. M., Zavodskaya Lab. 25, 408 (1959). (10) Wallace, G. W., Mellon, M. G., ANAL.CHEM.32,204 (1960). (11) Wallace, G. W., Mellon, M. G., Anal. Chim. Acta 33, 355 (1960). (12) Wherry, R., Psychometrika 11, 239 (1935). ReRECEIVED for review May 8, submitted April 2, 1962. Accepted April 18, 1962.
ACKNOWLEDGMENT
The authors gratefully acknowledge the financial support of Eli Lilly and Co. LITERATURE CITED
(2) Duncan, A. J., “Quality Control and Industrial Statistics,]’Cha .33, Richard I). Irwin, Inc., Homewoot Ill., 1959. (3) Guyon, J. C., Wallace, G. W., Mellon, M. G., ANAL.CHEM.34, 640 (1962). (4) Hallapeau, R., Ann. chim. phys. [7], 19, 92 (1900). (5) Illingworth, J. W., Keggin, J. F., J . Chem. SOC.1935, 575.
Correct io n Volumetric Assay Method ium Using Spectro-
I n this article by C. E. Caldwell, L. F. Grill, R. G. Kurtz, F. J. Miner, and N. E. Moody [ANAL.CHEM. 34, 346 (1962)], on page 347, column 1, paragraph 3, line 5, 6N H2SO4 should be 1N H&04. VOL 34, NO. 7, JUNE 1962
859
