Table 111.
Titration as Arsenic(ll1) in Milligram Range
As
Taken 9.73 7.29 4.87 3.64 2.43
As
Error,
Found 9.74 7.28 4.88 3.66 2.44
$0.11 -0.14 $0.20 +O. 33 $0.50
%
tions were carried out by three different operators and reasonably good results were obtained in all cases. This fact indicates that the end point is so sharp that even an inexperienced operator can recognize it without difficulties.
B. TITRATIONIN MICROGRAM RANGE. The generating medium was prepared as described in A. A current of 1.8 pa. was passed through the indicating electrode system for 5 minutes prior to the beginning of the titration. A small quantity of bromine was generated and the corresponding indicator electrode voltage recorded. For generating currents of about 1 ma., 0.20 minute was used. A proportional pregeneration time mas used for other generating currents. I n any case, the preset potential should never be more than 0.2volt from the initial potential. The sample was then introduced before any appreciable amount of bromine was lost. To obtain accurate results, the volume of the sample added should not be larger than 10 ml. Bromine generation was initiated until the preset voltage was reached. The amount of arsenic in the sample was calculated, using Equation 1. The stirring rate should be held relatively constant during the titration. Air bubbles picked up by
Table IV.
Titration of Arsenic(ll1) in Microgram Range
As Taken, y 243.30
84. 26
60.82
AS
Found, y 243.18 243.46 244
84.36 84.19 84.30 84.19 84.21 60.81 60.95 61.01 60.64 60.68
Error, m /o
-0.09 $0.06 +0.32 -0.64 +o. 02 $0. 02 -0.04 -0.02 $0.12 -0.08 $0.05 -0.08 -0.06 -0.17 $0,22 $ 0 . 32 -0.30 -0.23 $0.68 $0.06 +o .72 +0.45 $0 .39
the indicating electrode during titration will give rise to erroneous results. The results of typical titrations are tabulated in Table IV. DISCUSSION
The present end point detection method offers the following advantages: A very small amount of hydrogen is generated a t the indicating electrode before the titration. This should reduce traces of oxidizing substances (oxide) that might be on the indicating electrode. The pregeneration of bro-
mine will eliminate reducing substances in the titration cell before the addition of the sample. The above practice also pretreats the indicating electrode according to the method prescribed by Mj7er and Swift before each titration ( 7 ) . The nonreproducibility of the indicating electrode from titration to titration does not affect the results. While the present work deals only with the titration of arsenic(II1) with bromine, it is believed that the present technique can be applied to other low level coulometric titrations. ACKNOWLEDGMENT
The invaluable assistance of Jean dnderson in the preparation of the manuscript is gratefully acknowledged. LITERATURE CITED
Adams, R. N., ANAL. CHEM.26, 1933 (1954). Cooke, W. D., Furman, K. H., Zbid., 22, 869 (1950). Cooke, W. D., Reilley, C. Ti,, Furman, N. H., Zbid., 23, 1662 (1951). Delahay, P., ‘ Y e w Instrumental Methods in Electrochemistry,” pp. 251-4. Interscience, Kew York. 1954. Gauguin, R., Charlot, G., Bertin, C., Bandoz, J.. Anal. Chim. Acta 7. 360 (1952$.’ Gauguin, R., Charlot, G., Coursier, J., Zbid., 7, 172 (1952). Myer, R. J., Swift, E. H., J. Bm. Chem. SOC.70, 1047 (1948). Willard. H. H.. Furman. N. H.. “Elementary Quantitative Analysis,” p. 273, 3rd Ed., Van Nostrand, New York, 1940. RECEIVED for review October 10, 1955. Accepted August 24, 1957.
CompIexometric Titration of Copper and Other Metals in Mixture 1-(2-Pyridylazo)-2-naphthol (Dye) as Indicator K. L. CHENG Ufica Metals Division, Kelsey-Hayes Co., Ufica 4,
b The total amount of copper and zinc, cadmium, or nickel in a solution can be titrated by (ethylenedinitril0)tetraacetic acid (ethylenediarninetetraacetic acid, EDTA) with 142pyridylazo)-2-naphthol as the indicator at pH 2.5 to 10. In the absence of copper, zinc or cadmium can be titrated at pH above 5 by EDTA with 1-(2pyridylazo)-2-naphthoI as the indicator. Zinc, cadmium, or nickel can be titrated directly b y EDTA in the presence of copper which is masked by the
N. Y.
addition of a slight excess of thiosulfate. The amount of copper in the mixture can be calculated by difference. The difficulty of obtaining a sharp end point in the complexometric titration of copper with 1 -(2-pyridylazo)-2-naphthol as the indicator may be eliminated by addition of alcohol, dioxane, or other organic solvent.
T
stability constants (log K ) for complexes of (ethylenedinitrilo) tetraacetic acid (ethylenediamineHE
tetraacetic acid, EDTA) with copper(I1) and zinc(I1) are reported to be 18.3 and 16.1, respectively (6). Copper(I1) forms a more stable complex than zinc with 1-(2-pyridylazo)-2-napthol (a dye); a t p H 2.0 copper(I1) forms a complex with the dye but zinc does not. However, when a mixture of copper and zinc was titrated by EDTA with the dye as the indicator, both copper and zinc were titrated instead of copper alone (1). Zinc can be titrated with EDTA by VOL. 30, NO. 2, FEBRUARY 1958
243
Table
I.
dye is not very sensitive to zinc in relatively acid medium. At p H above 7 , thiosulfate releases copper to react x i t h the dye. The possibility of utilizing the masking action of thiosulfate in the case described is based on the fact that the thiosulfate reduces cupric ion and forms a stronger complex lvith cuprous ion than EDTA and the dye in slightly acid medium, but it does not prevent zinc from forming complexes with EDTA and the dye. The indicator solution must be added after copper has been complexed by the thiosulfate; otherwise, thiosulfate is unable to reduce and remove copper from the dye. A large excess of thiosulfate should be avoided, because it also reacts with zinc, although its zinc complex is much weaker than its copper complex. Thus the presence of large excess of thiosulfate causes a rather gradual color change at the end point for the titration of zinc. Four to 5 nil. of 10% sodium thiosulfate pentahydrate masked 0.25 mmole of copper, If the amount of copper present is unknown, thiosulfate is gradually added until the solution becomes just completely colorless. Year the end point, concentrations of free copper ions and EDTA are both very small compared to the concentration of the dye; therefore, EDTA would break u p the copper-dye precipitate slon-ly. Such competition can he overcome by addition of excess EDTA. A sharp end point is easily obtained by the back-titration technique as developed by Flaschka and Abdine (3, 4). However, copper could be directly titrated n-ith EDTA using the dye as indicator when some organic solvent 11-as added to the aqueous solution to prevent formation of copper-dye precipitate. The soluble form of copperdye complex in the solution containing 25 to 50% organic solvent makes it easier for EDTA to sequester copper. When 0.2500 mmole of copper was titrated in the presence of different
Titration of Copper and Other Metals in Mixture
Found, Millimole Total 0,5436 0.9936 0.3487 0 1750
0.1000 0.0750 0.0750 0.1750 0,1750 0.1000 0.0750 0,0750
Taken, Millimole Zinc
0.4936 0.4936 0.0987 Cadmium 0 1250 0.0500 0.0500 0.0250 0.0500 Sickel 0.1250 0.0500 0,0250 0.0250
Copper
Copper 0.0500 0.5000 0.2600
Total 0.5443 0.9937 0.3482
0 0.500
0.0500 0.0250 0,0500 0.1250
0.. ~. 1746 -. 0.0998 0.0744 0,0762 0.1754
0.0500 0.0500 0.0500 0,0500
0.1736 0.0996 0.0758 0,0754
using the dye as the indicator a t p H above 5 in the presence of copper when a slight excess of thiosulfate is added. The copper. although it was complexed by thiosulfate, did not form a complex with either EDTA or the dye in the acid medium. Zinc can also be titrated with EDTA in the presence of iron(III), if enough fluoride is added (1). It is often difficult to obtain a sharp end point in the direct titration of copper with EDTA (2-6), because a very fine and stable copper-dye precipitate is formed. Addition of an organic solvent such as dioxane, alcohol, or dimethylfornianiide eliminates the difficulty. Reagents used xere similar to those previously reported ( I ) . PROCEDURE
Titration of Copper. Adjust the p H of the solution containing not more t h a n 0.5 mmole of copper at 2.5 or above and dilute t o approximately 7 5 ml. with nater. Then add 25 ml. of dioxane or methanol and 6 drops of 0.1% dj7e solution. Titrate n i t h 0.02M E D T A solution. The end point is from red to yellow., or from red to greenish yellow if a relatively large amount of copper is present. Titration of Zinc or Cadmium in Presence of Copper. Adjust the pH of t h e solution containing less than 1 mmole of zinc or cadmium t o p H 5 to 6. Add sodium thiosulfate until t h e solution is just completely colorless. Dilute the solution t o approximately 50 t o 100 ml. with water and add 6 drops of 0.1% dye solution. Titrate with 0.0231 EDTA solution. The end point is from pink to pure yellow, Titration of Nickel in Presence of Copper. The procedure is similar t o t h a t for zinc or cadmium, b u t the solution ( p H 4) should be heated t o 50" t o 70" C.; 2 to 3 ml. of t h e titrant are needed t o reach t h e end point. Analysis of Brass. Dissolve approximately 0.25 gram of the sample in a 150-ml. beaker n i t h 3 ml. of concentrated nitric acid. Heat gently 244
ANALYTICAL CHEMISTRY
Zinc 0.4933 0.4933 0 0991 Cadmium 0.. _12.54 ~ -_ 0.0502 0.0502 0.0256 0.0500 Nickel 0.1270 0.0496 0.0504 0,0248
(by differ-
ence) 0.0510 0.5004 0.2491 0 0492
0.0496 0.0242 0.0496 0.1254 0.0466 0.0506 0.0254 0.0506
and evaporate t o about 1 t o 2 ml. Cool and dilute t o approximately 50 ml. with water. Precipitate sesquioxides, tin, lead, and manganese by adjusting t h e p H t o 8 to 9 with 1 to 1 ammonium hydroxide. Filter into a 250-ml. volumetric flask, wash, and make to volume. If a small amount of precipitate is obtained, no reprecipitation is needed. The filtrate should be clear. Take a n aliquot containing not more than 0.5 mmole of copper and 1 mmole of zinc and titrate as directed. DISCUSSION A N D RESULTS
Zinc could be titrated by EDTA in the presence of copper, if the copper was reduced and masked by thiosulfate. The total amount of copper and zinc was determined by EDTA in the absence of thiosulfate and the amount of comer was calculated bv difference (Tabie' I). Addition of thiosulfate solution to the copper solution gave a brownish coloration, but it became colorless when the thiosulfate was in excess. Copper thiosulfate forms in both acid and alkaline media. The zinc solution to be titrated should be adjusted to p H 5 to 6. 4 t p H below 5, the end point x i s reached too soon, because the
1
t12
Figure 1. Effect of pH and temperature on titration of nickel
20
I
r
3 0 4 0
so
TEMPERATURE *C.
I
60
I
7
0
8
I
0
Table 11.
Sumher 1
2
Determination of Copper and Zinc in Brass
Present, % Sample Copper Zinc SInthetic" 60 05 36 54 Studentstandardb 59 49 38 65
3
Found, % Copper Zinc 60 18 36 54 59 62 37 90 59 24 38 78 59 58 38 52
Difference, c& Copper Zinc $0 13 0 00 $0 13 -0 7 5 -0 25 $0 13 + O 09 -0 13
Silicon bronze, -0 03 90 83 2 05 90 86 2 07 SBS 158 4 dluminum biass, -0 56 63 20 21 83 63 76 21 89 XBS 16-1 a Similar t o 2. b Thorn-Smith student sample contained A1 l . 1 5 ~ cPb , O.lO~c, and traces of Sn and Xi. organic, solvclnts, tht. folloning results werr obtaincd : Solvent Methanol Ethyl alcohol Isopropyl alcohol 1,4-Dioxane S,S-Dimethvlforniamide
Copper Found, Mmole 0 2504 0 2502 0 2498 0 2500 0 2499
These solvents were equally effective in improving the rnd point for the direct titration of copper. The direct titration is simpler. Hon-ever, the back-titration technique is cssrntial for titrating cobalt, lead, or bismuth with the dye as indicator. It is not necessary to add the organic
-0 03 -0 06
solvrnt for titrating zinc directly in the absence of copper or if the copper is complexed by thiosulfate. The masking of copper has bern similarly applied to the titration of the mixtures of cadmium and copper, and nickel and copper. The difficulty of obtaining a sharp end point for the direct titration of nickel with EDTA with the dye as indicator was overcome by heating the solution. The optimum pH range and trniperature for the titration of nickel were investigated (Figure 1). Below 50" C.. higher results a t p H 3 or4indicate that much rxcrss EDTA was needed to wqurster nickel from thr stable nickeldye complex a t l o r e r temperatures. Above 60" C., low results a t pH 3
indicate that the cnd point \vas rraclicd too soon, because the stability of nickeldye complex n as decreased by both lo^ acidity and high temperature. To denionstrate the proposed procedure, an attempt w s made to determine copper and zinc in brass. Satisfactory results n e r r obtained (Tahle 11). Silver and uranium formed strongw complexes n-ith the dye than v i t h EDTA in the alkaline medium. They did not interfere in slightly acid nicdliim. The use of t h r dye a' a ,ensitiic ant1 selective method for uranium IT ill 1)c covered in a scqiarate report. LITERATURE CITED
(1) Cheng, I-, R. H., A s . 4 1 . . CHEX 27, 782 (1955). (2) Cheng, K. I,., Killiama, T. R., Jr., Cheniist-dnal?jst 44, 96 (1965).
(3) Flaschka, H., .4bdine, H., Ibid., 45, 2 (1956). 14) Ibid..D. 58. (5) FlascGka, H., Abdine, H., J ~ i k r o c h i u ~ . Acta 1956, 770. (6) Schn-arzenbach, G., Freitag, 1: , Helv. Chinz. Acta 34, 1503 (1951). RECEIVED for revien March 12, 1957. Accepted September 26, 1967. Presented in part before the Pittsburgh Conference on -4nalytical Chemiqtri- anti Applied Spectroscopv, March 1957. Preliminary ivork carried out a t Chemical Tahoratory, Westinghouse Electric Corp., East Pittsbnrgh, Pa.
Determination of Alkyl pyridines by Infrared Spectroscopy Rapid Methods of Analysis ROBERT L. BOHON' and RAYMOND ISAAC The Anderson Physical laboratory,
609 South Sixth St., Champaign, 111.
HENRY HOFTIEZER2 and ROBERT J. ZELLNER The Ansul Chemical Co., Marinetfe, Wis.
b Analyses are described for a number of synthetically produced alkylpyridines, utilizing both the sodium chloride and potassium bromide regions o f the infrared spectrum. These rapid, flexible techniques are useful for analyzing a limited number of samples of widely varying composition. High-concentration components are successfully determined without the customary solvent-dilution methods in several cases b y using thin cells, appropriate standards, and the base-line method o f measuring 10. Differential spectra are used for impurity detection and identification, as well as for trace analyses in highly purified pyridines.
I
spectroscopy lend3 itself to the analysis of many complex mixtures, and the pyridines are no exception. Wet chemical methods are time-consuming and nonspecific in most cases (1, 17). The infrared method is rapid, specific, and accurate, and enables the analyst to check for the presence of unsuspected impurities. This last property is invaluable when TT orking with samples from the laboratory or pilot plant where evperimental conditions, and hence product composition, vary greatly and unpredictably. Processes for producing alkylpyridines (2, 3, 19, 20) involve reaction a t elevated temperatures of saturated XFRARED
aldehydes or ketones with ammonia-in the presence of a suitable catalyst. Variation of reactants and experimental conditions permits product'ion of mixtures rich in specific nlkylpyridines, such as t8he picoline^. lutidines. or collidines, and fractional distillation is generally sufficient for isolation of t'lie desired components in a state of high purity. This obviat'es costlj- crystallization (14) or clirornatographic (18, 21) separations. Previous infrared determinations for
PE
Present address, Llinnesota Mining
Mfg. Go., St. Pan1 6, SIinn.
* Present address, IIarathon C o r p , Rot,hschild, \Vis. VOL. 30, NO. 2, FEBRUARY 1958
245
