947
V O L U M E 2 4 , N O . 6, J U N E 1 9 5 2 0.10 mg. of cobalt per ml. of solution. Adjust this solution until it is neutral to litmus. Desired Constituent. Transfer by means of a microburet or pipet a suitable portion of this prepared solution t o a 50-ml. volumetric flask. Add approximately 25 ml. of a saturated sodium bicarbonate solution and exactly 0.5 ml. 3% hydrogen peroxide, mixing the contents of the flask thoroughly. Adjust the meniscus t o the mark with the sodium bicarbonate solution and thoroughly mix the contents of the flask, Measure the absorbancy a t 260 m p in 1.00-cm. silica cells, using a freshly prepared reagent blank solution in the reference cell. LITERATURE CITED
ilyres, G. H., Rept. iTea England dasoc. Chem. Teachers, 42, 143-7 (1941). Rlanchetiere, A., and Piilot, 3. hI,, Compt. rend. aoc. biol., 101, 858 (1929). Durrant, C. H., J . Cheni. Suc., 87, 1781 (1905). Duval, C.,Anal. C h i m . Acta, 1, 201 (1947).
(5) Job, A., Ann. chim. phys., 20, 214 (1900). (6) Laitinen, H.A . , and Burdett, L. W., ANAL. CHEM.,23, 1268-70 (1951). (7) Metzl, G.,2. anal. Chem., 53, 537 (1914). (8) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., p. 283, New York, Interscience Publishers, 1950. (9) Snell, F. D., and Snell, C. T., “Colorimetric Methods of Analysis,” 3rd ed., Vol. 11, p. 376,New York, D. Van Nostrand Co., 1949. (10) Teiep, G.,and Boltz, D. F., ANAL.CREM.,22, 1020 (1950). (11)Ibid., 23, 901 (1951). (12) mi,,24,163 (1952). (13) Willard, H.H,, and Ayres, G. H., IND.ENG.CHEY.,ASAL. ED., 12,287 (1940). RECEIVED for review February 1, 1952. Bccepted April 7, 1952. Presented before the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 7, 1952, cosponsored by Analytical Division of Pittsburgh Section, ERICA^ CHEYICAL SOCIETY. and Spectroscopy Society of Pittsburgh.
Spectrophotometric Determination of Titanium with Ascorbic Acid EDW.4RD HINES’ AND D. F. BOLTZ, Wuyne Cmitrersity, Detroit, .VClich. A spectrophotometric study of the titanium-ascorbic acid reaction w-as made for the purpose of ascertaining the suitability of the yellow complex formed for the spectrophotometric determination of small amounts of titanium. With a large excess of ascorbic acid and a final pH of 3.5 to 6 a stable complex is formed which has a broad ahsorbancy maximum at 360 mp. Conformity to Beer’s law was found for
0.1 to 25 p.p.m. of titanium using 1-cm. absorption cells. The general procedure which was developed was tested by analyzing several standard samples and found to give satisfactory results. Ascorbic acid is a suitable reagent for the spectrophotometric determination of titanium, few ions causing interference. Fluoride, phosphate, and silicate ions are the main interfering ions.
OST organic compounds that have been reported to give a
This solution \vas standardized gravimetrically using cupferron as the precipitant, followed by ignition of the precipitate to titanium dioxide. The solutions of diverse ions used in the study of interferences were prepared from reagent, grade salts.
color with titanium have one structural feature in common, an ene-diol group, or a t least one or more hydroxy groups attached to an unsaturated carhon-to-carbon linkage. Thus, acid ( d ! 5, 9, I S ) , chromatropic acid, 1,8-naphthol-3,6-disulfonic disodium-1,2-dihydroxybenzene-3,5-disulfonate( 1 5 ) , salicylic acid ( 8 ) , gallic acid ( I ) , thymol ( 6 ) , dihydroxymaleic acid (4,7 ) , and catechol (10-12) have been used as reagents in the colorimetric determination of titanium. L-Ascorbic acid is another readily available compound that meets this specification of having one ene-diol group. Ettori (5)indicated that this acid gives a yellow hue with dilute solution? of titanium sulfate. T h e purpose of this investigation wae to make a spectrophotometric study of the tit,anic-ascorbic acid reaction and to determine the suitability of the reaction for a 5pect.rophotonietric method of determining titanium . APP.ARATUS ANI) SOLUTIONS
The absorbancy measurements were made with a Beckman ;\lode1 DV spect,rophotometer equipped with an ultraviolet accessory unit using 1.000-em. silica cells. Visual colorimetric measurements were made in 220-mm. Kessler tubes. -4Leeds & Korthrup Universal p H meter equipped n i t h a glass electrode was used for all p H measurements. The tascorbic acid reagent was prepared by dissolving 1.00 gram of reagent grade sodium bisulfite and 2.5 grams of L-ascorbic acid (Merck) in wat’er and diluting to 100 ml. T h e sodium bisulfite was added t o inhibit the oxidation of the ascorbic acid to dehydroascorbic acid. A standard titanium solution was prepared from potassium titanium oxalate by the method of Thornton and Roseman (14) 1
Present address, Detroit Edison Co., Detroit, Mich.
COLOR REACTION
,Iscorbic acid reacts ivith titanium ions to give a yelloiv-colored product exhibiting masimum absorbancy in the near ultraviolet region. The at,tainment of maximum rolor intensity is dependent upon the concentration of reactants :tnd the p H of the solution. .Ilthough the exact nature of the reaction product has not yet been determined, it is highly probable that it is a complex ion containing a conjugated resonator due to thc asrorbic acid. SOLUTION VARIABLES
Titanium Concentration. The nlisorption spectra obtained with 0, 5 , 10, and 20 p.p.m. of titanium arc shown in Figure 1. Cdnformity to Beer‘s law \vas found at 360 nip for 0.1 to 25 p,p.m. of titanium. , Conformity to Beer’s law in the near visible (400 t o 420 mp) region also indicates the pract.icabilit>-of using Sessler tubes on a color comparator for routine colorimetric analysis. The color is stable for at least 12 hours. Ascorbic Acid Concentration. Thc intensity of the color produced by the titanium-ascorbic acid complex increases as the ascorbic acid-titanium ratio increases, but the rate of increase is almost negligible when this molar ratio is larger t,han 50 to 1. Therefore, a concentration of 5000 p.p.m. of ascorbic acid is recommended for samples containing up to 25 p.p.m. of tit,aniuni. pH. T h e effect of varying pH was studied and it) was found
948
ANALYTICAL CHEMISTRY
that the maximum color development is dependent upou pH. The solution should have a p H of less than 1.5 before t,he addition of the ascorbic acid and a final pH of 3.5 to 6.0. Variance between 3.5 and 6.0 in the h a 1 p H , however, has no effect on the absorption charact,eristics of the complex. Diverse Ion Concentration. The effect of various ions on the titanium-ascorbic acid complex was determined using solutions having a concentration of 10 p.p.m. of titanium, 2000 p.p.ni. of sodium bisulfite, and 5000 p.p.m. of ascortiic acid. One thousand parts per niillion of the following ions did not interfere a t 360 mp: aluminum, barium, calcium, cobaltous, chromic, cupric, ferrous, feri,ic, potassium, magnesium, manganous, sodium, animonium, nickelous, strontium, tungstate, chloride, nitrate, and sulfate. All ions that exhibit color folloa-ing the addition of the reagent would interfere in visual color comparisons. Presumably some of the elements in the higher state of valence, such as iron, would be reduced by the sodium bisulfite. Fluoride ionP interfere seriously, undoubtedlj- because of forniation of the niore stable titanium fluoride complex. Fluorides are seldom encountered, inasmuch as they are volatilized 13 ith the silicon upon treatment with hydrofluoric acid in the analysis of ores and other siliceous materials. Phosphate, molybdate, vanadate, and silicate ions interfered, provided they were present in concentrations exceeding 100 p.p.m. Table I.
Determination of Titanium in NBS Samples TI ( N B S ) ,
NBS Sample Lime stone. l a Cast iron, 5f Plastic clay, 98 Silica brick, 102
% 0 096 0 036
0,859 0,096
Ti Found, % 0.092 0.040
0.846 0.102
Standard Deviation, "0
0.0014 0.0036 0,0190 0 0067
Absorption Spectra of Titaniuni-iscorl,ic Acid Complex
Figure 1. 1.
2.
Reference cell, distilled water
0 p.p.m. Ti 5 p.p.m. Ti
3. 4.
10 p.p.m. Ti 20 p.p.m. Ti
RECORiMEh-DED GENERAL PROCEDURE
The following general procedure is recommended for deterniining titanium using the ascorbic acid reagent. Sample. Weigh, or measure by volume, a sample containing a sufficient amount of titanium so that the acidic solution obtained following the necessary preparative treatment contains 1 to 50 p.p.m. of titanium. Desired Constituent. Transfer a 20-ml. aliquot to a 50-nil. flask, or to a Sessler tube, and add 10 ml. of the ascorbic acid reagent. Add 5 ml. of glacial acetic acid and mix thoroughly. Keutralize to a p H of about 5 with 5 .V sodium hydroxide and dilute to the mark. Measure the absorbancy a t 360 mp using 1cin. cells, or compare the color with the color of a standard solution similarly prepared. APPLIC.kTION O F hIETHOD
The general procedure that was developed was tested b>-applying it to the determination of titanium in several Sational Bureau of Standards standard samples. The resulk of these determinations are summarized in Table I. In analyzing the liinestone,,plastic clay, and silira hrick samples, a preliminary removal of silica was effwted by volatilization as silicon tetrafluoride in the presence of sulfuric acid. The residue was fused with sodium carbonate, dissolved in hydrochloric acid, and diluted to volume. The cwt iron sample was dissolved in hydrochloric acid and in a platinum crucible. After treatment with .drofluoric acid to remove the silica, thereniained with sodium carbonate. The melt \vas dissolved in h\-droc.hloi,ic.acid and the solution cliluted to constant volume. The results obtained with the standard samples indicate the possible applicability of t,he ascorhic acid niet,hod to a large variety of samples. This spectrophotometric- study has ahoan that under the proper cmtiitions titanium ions give with ascort>ic acid a stable y e l l o ~
color which is suitable for the deterniination of 0.1 to 25 p.p,in. of titanium. The exact adjustment of pH i3 not critical, although the control of p H to within certain limits is important. Very few of the substances usually associated with t,itaniunicontaining materials interfere with the method, and niost of those ions that do interfere can be eliminated in the treatment of the sample prior to measurement. The color-forming miction is carried out with a reducing reagent as compared to : i n oxidizing reagent in the case of the hydrogen peroxide method. LITERATURE CITED Das-Gupta, P . N., J . Indian C h e m Soc., 6, 355 (1929). Endredy. A , and Brugger, F.,2. anorg. a / / g e m . ( ' / I ( ' J I , . , 219. P ( i X (1942). Ettori, J., Compt. r e d . , 202, 852 (1936). Fenton. H . J. H . , J . Chem. Soc.. 93,1061 (1!408). Hofmann, K. A , Ber., 45, 2480 (1912). Lenher, V.. and Crawford. IT. G., J . A m . C ~ U J iSo(,.. ~ . 35, 141 (1913). hlellor, J. TT., Trans. Eng. C e m m Soc.. 12, Pt. I. :
