ANALYTICAL CHEMISTRY
1322 Vrane, C . W., Forrest, J . , Step!ienson, O., and Waters, W. A , , Ibid., 1946, 837. Fieser, L. F., and Rajagopalan, S..J . Am. Chem. Soc., 71, 3935.
3938 (1949). Grob, C. A , , and Schmid, H. U., Ecperientia, 5 , 199 (1949). Grob, C. A , , and Schmid, H. U , Helu. Chim. Acta, 36, 1763 (1953). Hebbelynck, hl. F., and Martin, R. H.. Bull. S O C . chim. Relg., 60,54 (1951). Hebbelynck, AI. F., and Xartin, R. H., Ezperientia, 5 , 69 (1949). Hinsberg, O., Der., 23, 2902 (1890); 33, 3526 (1900). Kamm, 0.. "Qualitative Organic .lnalysis," 2nd ed.. p . 5 3 , Kern York, John Wiley 8: Sons, 1948. Kroller, E., Siiddeut. Apoth. Zty.. 90, 724 (1950). Lecomte, J., and Dufour, C., Compt. rurcd., 234, 1887 (1952). Lucas, H . J., J . Am. C'hrm. Soc., 52, SO2 (1930).
lleisenheilner, J., Bcr.. 46, 1145 (1913). Paolini, I. de, Gatz. chim. ital., 60,859 (1930). Paolini, I. de, and Hihet. G., I b i d . , 62, 1041 (1932). Price, C. C.. Pohland, d.,and \-elsen, B. H., J . Org. Chem., 12, 303 (1947). Reich, H., and Reichstein, T., H e h . China. Acta, 26, 562 (19431. Ritter, F. O., J . Chem. Educ., 30, 395 (1953). Sarett, L. H., J . Am. Chem. SOC.,71, 1185 (1949). Seliwauow, Th., Ber., 25, 3617 (1892). Sivadjian, J., Bull. SOC. chim., 2, 623 (1935). Wieland. P., and Miexher, K., Helr. C h i m , Acta, 30, 1876 (1947). R E C E I V Efor D review October 26, 1953. .Iccelited 3Iay 6, 1954. Prea3 part of a report a t t h e Joint Southwest-Southeast Regional Meeting, h X f E R I C A S C H E M I C A L S O C I E T Y , y e w Orleans, L a . , December 10, 1953. sented
Titration of Bismuth with Ethylenediaminetetraacetic Acid Spectrophotometric End Points A. L. UNDERWOOD D e p a r t m e n t o f Chemistry, Emory University, Emory University, G a .
Prior to the development of ethylenediaminetetraacetic acid, the volumetric methods for determining bismuth were indirect and unsatisfactory. Since this reagent offers a simple and direct titration but suffers from a lack of good indicators, an attempt has been made to extend its usefulness by application of the photometric titration technique. The progress of the titration may be assessed by following spectrophotometrically the disappearance of the yellow bismuth-thiourea complex. or the appearance of the blue complex formed by cupric ion with the titrant. Quantities of bismuth from 0.5 to 100 mg. can be titrated accurately when present in a volume of 100 ml. Large quantities of lead do not interfere with the titration, and bismuth can be determined rapidly in a mixture with tin, lead, arsenic, and antimony by a simple procedure.
B
ISMUTH is usually determined gravimetrically or colorimetrically. T h e volumetric methods previously available are indirect, based on precipitation of bismuth with oxalate, chromate, molybdate, etc., followed by titration of the anions. Such methods are not considered completely reliable ( 4 , IO). Thus a direct volumetric method, in xhich bismuth itself is tivated, is of interest. Pribil and IIatyska have recently shown that bismuth may be titrated amperometrically with ethylenediaminetetraacetic acid ( 6 ) . A visual titration based on the disappearance of the yellox bismuth-thiourea complex upon the addition of ethylenediaminetetraaceticacid has also been described (S), but the recommended quantities of bismuth are large (100 t o 200 nip.). Photometric titrations with ethylenediaminetetraacetic acid have recently been reported by Sweetser and Bricker ( 7 , 8), IIalmstadt and Gohrbandt (LT), and Cnderwood (9). I n the present paper, the photometric technique may be applied with advantage to the titration of bismuth with this reagent. -4s little as 1 mg. of bismuth can be titrated easily. Furthermore, the bismuth complex Lyith ethylenediaminet,etraacetic acid is very stable. Thus, moderate quantities of numerous other ions do not interfere, and in certain cases, separations prior to the actual titration need not be performed with great rigor. Two photometric methods for obtaining the end point are described: the disappearance of the bismuth-thiourea complex may be
followed, or cupric ion may serve as indicator. I n the latter case, the formation of the cupric complex of ethylenediaminetetraacetic acid (less stable than the bismuth complex) indicates the end point in the bismuth titration. The use of both indicators is described because they supplement each other in extending the useful range of the method. APPARATUS AND REAGENTS
The titration cell was similar to that described by Goddu and Hume (2), with a light path of about 2.2 em. Such a cell can be mounted in the test tube attachment supplied for use with the Beckman Model DU spectrophotometer. The cell is thus entirely enclosed within the sample compartment) minimizing difficulties due to stray light. Holes drilled in the cover of the compartment to admit st'irrer and buret were fitted with felt gaskets. The stirrer and the 5-ml. Esax buret were painted black for a short distance on either side of the level where they entered the compartment. pH measurements were made with a Beckman Model G p H meter equipped with a glass electrode. Standard bismuth solutions (generally 0.01M) Tvere prepared by dissolving Baker's analyzed bismuth metal (99,8'%) in the minimal quantity of 1 to 1 nitric acid with gentle heating, followed by dilution with distilled water containing sufficient nitric acid to make the final solution about 0.5M in acid (to prevent hydrolysis of the bismuth). -411 the titrations were carried out at a pH of about 2, although the pH is not extremely critical. A chloroacetate buffer is appropriate because of the pK value of chloroacetic acid. However, chloroacetate solutions slowly liberate, on long standing, sufficient chloride ion to form a precipitate with bismuth (BiOCli. Thus, buffer solutions were not kept on hand, but rather, apprnpriate quantities of solid chloroacetic acid were added as desirel. folloJYed by p H adjustment with ammonia or sodium hydroside in the final solutions to be titrated. Eastman Kodak Co., Rochester, S . Y., practical grade, monochloroacetic acid, distilled to remove dark-colored impurities, was employed. The disodium salt of ethylenediaminet'etraacetic acid (Benworth Chemical Co., Framingham, Mass., disodium versenate. analytical reagent) was dissolved in distilled water to prepare solutions of the titrant (0.1 or 0.01M) which were standardized by photometric titration against the st'andard bismuth solutions. 411 other materials were reagent grade or the equivalent. TITRATIONS WITH CUPRIC ION AS INDICATOR
The titration of a bismuth-copper mixture with ethylenediaminetetraacetic acid is very similar to the titration of a n ironcopper mixture previously reported from this laboratory (9). Figure 1 shows the type of titration curve obtained a t a wave
V O L U M E 26, NO. 8, A U G U S T 1 9 5 4
1323
length where the cupric complex of the titrant absorbs strongly. The end points may be obtained for both bismuth and copper in a single titration. In this study, however, the emphasis has been placed on bismuth titrat,ions alone, with the copper merely serving as indicator. Procedure for Pure Bismuth Solutions. To a bismut,h solution rontaining from 5 to 100 mg. of bismuth, add 2 grams of solid chloroacetic acid and 1 ml. of a 1Ji' cupric nitrate solution. .idjust the volume of the solution to about 100 ml., and adjust its pH to about 2, using 1 to 1 ammonia or 5M sodium hydroxide. (The pH requirement is not extremely critical: satisfactory titrations having been performed a t p H values of 1.5 to 2.4; below 1'1% 1.5, the end point becomes less sharp, and above pH 2.5 there is a danger of precipitating bismuth.) Transfer the solution to the titration cell, and position it in the spectrophotometer. Srst the instrument to zero absorbance a t 745 mp, using the slit width and sensitivity controls in the u s u d manner. Titrate with a 0.1JI ethylenediamine tetraacetate solution previously standardized against a known bismuth solution. Read the absorhance after addition of appropriate increments of titrant, obtaining as many points as desired to define the t x o straight lines {Those irrtrrswtion givrs thr rntl point.
0.12
t-
Hi-i Cu End
0.09
Q
0.06
P Bi End Point 0 1
2
4
3
ML. OF EDTA
Figure 1. Titration of Bismuth-Copper Jlixture w-ith 0.1M Ethylenediaminetetraacetic Acid 41.8 mg. Bi, 13.1 mg. Cu; 745 mp
Results. The results shown in Table I were obtained n i t h pule bismuth solutions. The errors are within the range reasonably expected in good volumetric usage. In obtaining these results, the absorbance readings were corrected for dilution before the titration curves were plotted, although this affected the end points only very slightly.
Table I.
Titration of Pure Bismuth Solutions
Copper Indicator Bi taken. Bi found, Error, nip. mg. parts/1000 4.18 4.18 10 5 10.5 20 9
20.9 41.8 62.7 83 6
105 0
4.16
4.15 10.5 10.6 20.9 20.9 41 9 62.6
-
Thiourea Indicator Bi found, Error, Ing. parts/1000
Bi taken, mg.
0.418 1 05 2 09 4 18
6 27 8 36 10 50
0 412 1 06 2 09 4 16 6 26
8 38 10 4 0
14
10 0 5 7
I
9
83.6 105 0
TITRATIOh S WITH THIOUREA AS INDICATOR
Khere the quantity of bismuth is sufficient t o permit convenient titration with a 0.1M ethylenediamine tetraacetate solution, cupric ion is a better indicator than thiourea, because it is subject to fewer interferences. However, for titration of
smaller quantities of bismuth with a 0.01JJ reagent solution, thiourea indicator is recommended for this reason-using copper ion as indicator, the slope of the titration curve beyond the bismuth end point decreases by a factor of 10 when the concentration of the titrant is changed from 0.1 t o 0.01144. Because the molar absorbance index of the copper complex is rather small, this tenfold decrease in slope results in absorbance readings which are attended by considerable photometric error. The bismuththiourea complex absorbs so strongly that the above considerat,ions are unimportant for the bismuth concentrations involved in this study. One difficulty which is encountered with the thiourea indicator should be pointed out, although it is easily circumvented. The intensity of the yellow color formed when this reagent reacts Rith bismuth is dependent, upon t,he p H of the solution, decreasing as the pH is raised. At room temperature, the extent of the decrease is also a function of time---i.e., if the pH is raised, the color very slowly decreases t o its equilibrium value. Thus, if an acid bismuth solution containing t,hiourea is adjusted to p H 2 preparatory to a tit,ration, there follows a troublesome drifting in absorbance i,eadings throughout the titration, and a poor titration curve is obtained. However, if the bismuth solution is heated to about 70" C. for 10 to 15 minutes, after its pH has been adjusted t o 2, there is 110 drifting whatsoever after the addition of thiourea. When the pH of an acidic bismuth solution is raised, there occurs an hydrolysis reaction which is slow a t room temperature, and that t,he bismuth hydrolysis product has less tendency to react with thiourea than does the simple bismuth ion. The wave length a t which the titration is performed may be varied depending on the intensity of the bismuth-thiourea color. The absorbance maximum of the thiourea complex actually occurs in the ultraviolet (about 340 to 350 mp, shifting somewhat as the ratio of bismuth to thiourea changes), but the yellow color is sufficiently intense to permit measurements a t 400 mp except near the extreme lower limit of the method. Because there is no reference solution with which t o compare the solution being titrated, once the tit,ration has been etarted, it is necessary to guard against the possibility of the absorbance readings' falling below zero during the titration. This is the reason for setking the instrument before the addition of thiourea. Procedure for Pure Bismuth Solutions. To the bismuth s o h tion. containing from 0.5 to 10 mg. of bismuth, add 0.5 gram of solid chloroacetic acid and dilut'e the solution to about 100 ml. Adjust the pH to a value of about 2, using 1 to 1 ammonia or 5-11 sodium hydroxide. (Satisfactory titrations are obtainrd a t pH values between 1.5 and 2 . 4 . ) Heat the solution to about i o " C., holding it a t this temperature for 10 to 15 minutes. Cool the solution to room temperature, and transfer it to the titration cell. Position the cell in the spectrophotometer, and adjust the slit Iridth and sensitivity controls to obtain an absorbance reading of zero. Then add 5 ml. of a 111 solution of thiourea in xvster. The absorbance will increase immediately to its maximal vnluc, after which the titration may be commenced. Results. A titration curve obtained in this manner is shoivn R number of titrations of pure bismuth solutions are also compiled in Table I. iilthough the results are not so accurate as thoPe shown i n Table I for the copper indicator, they are satisfactory in view of the small quantities of bismuth. i n Figure 2. The results of
IVTERFERENCES
The study of interfering ions was centered about the elements tin, lead, arsenic, and antimony, with which bismuth is commonly associated in samples encountered in practical analysis. A few other ions were also studied. simple procedure prior t o the actual titration permits the determination of small quantities of bismuth in lead-tin alloys containing arsenic and antimony. Pribil and Matyska (6) pointed out t h a t tin interfered in their amperometric titrations of bismuth with ethylenediaminetetraacetic acid. Such interference was also noted in the present study. Removal of tin as hydrous stannic oxide from a nitric
ANALYTICAL CHEMISTRY
1324 acid solution of the sample is not satisfactory because of the tendency for bismuth to coprecipitate. Volatilization of the tin by fuming its hydrobromic acid-bromine solution with perchloric acid has been found very satisfactory. The standard procedure presented in the ASTM manual ( 1 ) can be used with only the minor modification of omitting phosphoric acid from the recommended solutions. (This is necessary because of the tendency of bismuth t o precipitate in the presence of phosphate when the pH is raised to 2 prior to the titration.) Because arsenic and antimony are also removed during the volatilization of tin, the interference of these elements required no further study.
0.1 6
: Y
0.19
6
d
5
0 0.08 4
0.04
~
______
~
Table 11. Determination of Bismuth in Mixtures Containing Tin, Lead, Arsenic, and Antimony Metals Taken, XIg. Sn 302 P b 700 Bi 5 3 . 5 Sn 310 P b 717 -4s 93 Bi 3 4 . 8 Sn 255 P b 847 As 67 Sb 72 Bi 4 5 . 3 Sn 250 P b 900 As 85 Sb 102 Bi 5 0 . 7 Sn 325 P b 736 As 54 Sb 37 Bi 2 4 . 7 Sn 400 P b 614 Bi 3 0 . 2 Sn 367 P b 624 As 102 Sb 78 Bi 1 1 . 7
53.1
Error, Parts/1000 8
34.4
11
Bi Found, Mg.
49.4
50.7
24.4
12
30.3
3
11.6
8
0 0.5
1.0
1.5
9.0
2.5
ML. OF EDTA
Figure 2. Titration of Bismuth with 0.01M Ethylenediaminetetraacetic Acid 4.18 rng. Bi; 400 m p
The stability of the lead complex of ethylenediaminetetraacetir acid is approximately the same as that of the cupric complex. Thus lead can exert no interference so far as the actual titration process is concerned, but large amounts of lead can interfere with respect t o the indicator systems. K h e n thiourea is used as the indicator under the conditions given in the above procedure, the maximal permitted quantity of lead is about 1 gram. Larger quantities of lead form a precipitate with the thiourea. With 1 gram or less of lead, there is no noticeable interference. When copper is used as the indicator, still larger quantities of lead can be tolerated. Because lead and copper react with the titrant simultaneously, and because the lead complex is colorless, the effect of lead is to lower the slope of the rising portion of the titration curve past the bismuth end point. The extent t o which this is troublesome depends upon the quantity of copper indicator present. Under these conditions, 3 grams of lead affects the slope only slightly and the end point not a t all. The presence of 5 grams of lead affects the slope more markedly, and the end point is less sharp. Thus this method possesses the advantage of being relatively insensitive to large quantities of lead, especially when the indicator is cupric ion. Obviously copper will not interfere when cupric ion is serving as indicator. With thiourea indicator, however, as little as 5 mg. of copper interferes by forming a precipitate with the indicator Merely separating the precipitate does not suffice, because some bismuth is coprecipitated. Ferric ion interferes when either indicator is used, because its complex with the titrant is stable enough t o form during the titration of bismuth. I n addition, ferric ion appears to interact with thiourea. T o test the method under fairly realistic conditions, bismuth was determined in mixtures with tin, lead, arsenic, and antimony. T h e mixtures were prepared by weighing out the pure metals and hence the analysis of actual alloys was simulated. The following procedure was used t o obtain the results presented in Table 11.
Procedure. For samples containing 0.5 to 10% bismuth, 1gram portions may be taken for analysis. The size of the sample may be adjusted within wide limits depending on its bismuth content. Place the accurately weighed sample in a 250-ml. widemouthed Erlenmeyer flask. Add 10 ml. of a mixture prepared by adding 20 ml. of liquid bromine to 180 ml. of 48% hydrobromic acid. Cover the flask, and warm gently until the sample dissolves, avoiding excessive loss of bromine. Additional bromine may be added dropnise if required for complete solution and oxidation of tin. Xfter the sample has completely dissolved, add about 10 ml. of peIchloric acid (70 to i2’%)< and heat over a flame in the hood to expel tin, arsenic, and antimony bromides. JVhen copious fumes of perchloric acid appear, heat intermittently so that condensation on the upper flask mall will wash the solid bromides back into the solution. Bumping is especially troublesome a t this stage; therefore, the mixture must be kept in constant motion. After the lead and other bromides have been converted to perchlorates, the solution becomes clear and colorless. Finally, heat the solution until copious whitr fumes of perchloric acid appear. Turbidity in the solution seems to indicate that the tin has not been removed completelv. If this is found, add more bromine-hydrobromic acid mixture, and repeat the above process. Cool the solution, and dilute it to a convenient volume-e.g., 50 to 75 ml. Bevond this point, the procedure is the same as that given for pure bismuth solutions, with the choice of indicator depending upon the quantity of bismuth present and the interferences which may be expected. LITERATURE CITED (1) .Im. SOC.Testing Materials, Philadelphia, “ASTlI Methods for Chemical Analysis of Metals,” 1950. ( 2 ) Goddu, R. F., and Hume. D. K., ANAL.CHEM.,22, 1314 (1950). (3) Gronkvist. K. E., Farm. Reuy, 52, 305 (1953). (4) Lundell, G. E. F., Bright, H. .A,, and Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., Chap. 10, Xew York, John Wiley & Sons, 1953. (5) llalmstadt, H. V., and Gohrbandt. E. C., A x a L . CHEM.,26,442 (1954). (13)Pribil, I
