Spectrophotometric Titrations with Ethylenediaminetetraacetic Acid Determination of Iron, Copper, and Nickel PHILIP B. SWEETSER AND CLARK E. BRICKER Department of Chemistry, Princeton University, Princeton, JV.J .
The disodium salt of ethylenediaminetetraacetic acid (Versenate) has found considerable use as a standard reagent in some volumetric procedures. This reagent would have even more varied applications if a suitable means of detecting the end points could be found. This paper gives procedures for the use of spectrophotometric end points for the determination of iron, copper, and nickel with standard Versenate solution. The effects of ptf and foreign ions on these procedures were investigated. With the methods given, i t is possible without any separations to determine copper in many nonferroiis alloy-s and iron in ferrous alloys and ores b y a single, simple titration.
T
ment of the spectropliotometer when the regular cover was left in a vertical position. A single hole was cut in the center of this cover, so that the neck of the titration cell would just slide through it and allow the base of the cell to rest on the regular carrier aasembly. The neck of the titration cell above the cover could he clamped to hold the cell securely in the light path. A motor-driven stirrer was employed with a wing-tipped glass rod of suitable length, so that the tip could extend down to the bulge in the titration cell but not into the light' path. The stirrer could run continuously during the titration without affecting the absorbancy readings. The 10-nil. microhurct %-asthe same a8 previously reported ( 1).
HE complexing agent, disodium dihgdrogen ethylenediamine tetraacetate (Versenate, Sequestrene, or Complexone 111) has found considerable use in the past few years as a voluiiietric reagent for the determination of various cations. The nisin limitation in its use as a titrant has been in finding a suitable means of detecting the end point of the reaction?. The indicator Eriochronie Schwartz T has been widely used in the determination of magneaiuin anti calcium for the total hardness of water ( 2 ) . Thir and other indicators have been used with varying degrees of succes in the determination of nickel, coppt%r,zinc, cobalt, iron, manganese, tin: and other cations (3,10, 1 2 ) . The acid properties of the Versenate have also been used as a means of end-point detection, both by the use of acid-base indicators and by electrical pH measureinents during the course of the titration (9, IO). This method has many limitations which make it unsuitable for most work. More recently Pribil has described a potentiometric and an aniperometric procedure for the determination of several cations (6. 7 ) . S o previow reference has lieen found t o the use of a spectrophotoinc:tcr for dotermining the end point in titrations with Versenate. A procedure for the determination of iron(III), copper(II), and nickel(I1) by a spectrophotometric titration, given in this paper, offers a rapid and convenient method for the analysis of iron, copper, or nickel solutions. KOprior reduction of.the iron is required, and few alxorbancy rcwlings are needed during the course of the titration. as the plot of absorbttncy us. milliliters of added titrant is a straight line. Data on the effect o f foreign ions in the titration of copper and iron(II1) with I'ersenate are presentetl.
SOLUTIONS
Standard Copper. .4n accurately weighed sample of electrolytic copper (ea. 2.5 grams) was dissolved in 10 nil. of concentrated nitric acid and diluted t o 1 liter.
-
-3.5 cy.+
i
APPARATUS
A Beckman ;\lodc~lB spectrophotometer was used for all the titrations with the Versenate. Yo niodifications are needed for the spectrophotometer other than a titration cell and cover for the cell compartment. The titration cell, diagrammed in Figure 1, was made from borosilicate glass. This cell consisted of a T tube 3.5 cm. in diameter with the ends of the T closed so that the light path in the cell was 7 cm. -2 bulge was blown in the cell just above the base of the T, so as to facilitate the mixing of the solutions when a motor-driven stirrer was used. The outsidr of the titration cell, with the exception of the closed ends of the T, was painted with a flat black paint to keep stray light from the photocell. This type of cell, with the increased light path, not only gave greater sensitivity than the cell previously described ( I ) , but also allowed much greater efficiency in stirring the solution after each addition of the titrant. .4 Bakelite cover a-as made to fit snugly over the cell compart-
Figure 1. Titration Cell for Becknian 3Iodel B Spectrophotometer
Standard Iron(III), ca. 0.05 J I . Reagent grade feriic ammonium sulfate was dissolved in \$ ater to which a small amount of sulfuric acid was added, and the resulting solution was diluted t o 1 liter. This solution was standardized with standard ceric sulfate after reduction of the iron(II1) employing a Jones reductor. Standard Nickel. Appro.;imately 5.3 grams of nickel sulfate hexahydrate were dissolved in 400 ml. of xater. This solution was standardized by the gravimetric method n i t h dimethylglyoxime. 253
ANALYTICAL CHEMISTRY
254
.Standard Versenate, ea. 0.10 M . Approximately 74 grams of disodium dihydrogen ethylenediamine tetraacetate dihydrate were dissolved in water and diluted to 2 liters. This solution was standardized against the standard copper and checked against the standard ferric solution, using the spectrophotometric end point. Sodium Acetate-Hydrochloric Acid Buffer. Hydrochloric acid solution, 1 X , was added to 250 ml. of 1 N sodium acetate until the pH of the mixture was 2.2. Sodium Acetate-Acetic Acid Buffer. 9 solution was prepared which was 0.2 144 in sodium acetate and 0.8 144 in acetic acid. Final pH was 4.0. Salicylic Acid, 67, solution in pure acetone. All other solutions were prepared from reagent grade chemicals. SPECTROPHOTOMETRIC
DETERMINATION
OF
END
POINT
Salicylic acid and ferric ions form a deep colored complex with a maximum absorption a t ca. 525 mp. This complex was used as the basis for the spectrophotometric titrations of iron(II1) with standard Versenate solutions. At a pH of approximatel~ 2.4 the Versenate-iron complex is much stronger than the ironsalicylic complex. Therefore, in the titration of an iron-saliEylic acid solution with Versenate, there will be a gradual disappearance of the iron-salicyclic acid color as the end point is approached. Although this change is not distinct enough for a good visual-type end point, the spectrophotometric end point a t a wave length of 525 mp is very sharp. The visual end point can be detected t o within 0.05 to 0.10 ml. of standard Versenate; therefore, in titrations where this volume of titrant does not cause significant errors, the visual method can be used. ,
length is set a t 525 mp and the slit width of the instrument is adjusted so that the absorbancy scale of the instrument reads approximately 0.20. At this point 1 ml. of the salicylic acid solution is added to the titration cell, whereupon the absorbancy immediately increases to an extremely large value due to the formation of the ferric-salicylic acid complex. The Srersenate solution is slowly added from the microburet until the absorbancy reading approaches 1.80, which is 0.80 to 0.40 ml. before the end point when 0.10 11' Versenate is used. The absorbancy is recorded for this volume of titrant and then 0.10- or 0.20-ml. aliquots of the Versenate are added and absorbancy readings are taken after rach addition until a t least three readings are taken beyond the end point. The intersection of the two straight lines produced when absorbancy is plotted against milliliters of titrant added gives the true end point. A representative titration curve for the determination of iron is shown in Figure 2. Determination of Copper. The procedure for the spectrophotometric titration of copper with Versenate is very similar to that given forthe ferric ion. The copper solution is added to the titration cdl, followed by 20 ml. of an acetate buffer and sufficient aater to make the volume 85 to 90 ml. The resulting solution should have a pH of 2.4 to 2.8. The cell is positioned in the spectrophotometer and the wave length is set a t 745 mp. The slit width is adjusted so that the solution has zero absorbancy. The copper solution is now titrated with the standard Versenate and absorbancy readings are taken every 0.30 to 0.50 nil. until a t least three readings are taken beyond the end point. The end point is then determined from the plot of the resulting data, as is shown in Figure 3. Determination of Nickel. The procedure for the titration of nickel is the same as for the copper titration, except that a wave length of 1000 mp is used, and the pH of the nickel solution is made approximately 4.0 with a suitable acetate buffer. The titration curves obtained for the determination of nickel are similar to those for copper, one of which is shown in Figure 3. DlSCUSSION
O0'
4'2
4'4
4'6 48 5'0 5'2 ML. OF 0.1012N VERBENATE
54
5'6
5 8
'
Figure 2. Titration of Iron(II1) with Standard Versenate Wave length 525 r n w
The titration of a copper solution with Versenate may also be follov.-ed by a spectrophotometer at a wave length of 745 mp, as the copper-Versenate complex has, a t this wave length, a molar extinction considerably greater than the copper solution alone. The spectrophotometric titration of nickel solutions is likewise possible a t a wave length of 1000 mp, where the nickel-Versenate complex shows characteristic absorption. The absorption spectra of copper and nickel Versenate complexes has been reported by Plumb ef al. (6). PROCEDURE
Determination of Iron. The Beckman Model B spectrophotometer is allowed to warm up for about 20 minutes before the titrations are started. An accurately measured aliquot of the ferric solution is added to the titration cell, to which 20 ml. of a suitable acetate buffer is added, so that when the solution is diluted t o approximately 90 ml. with water, the resulting solution has a p H of 1.7 to 2.3. If the ferric solution is very acid it should be neutralized with ammonia before the buffer is added. The cell is placed in the spectrophotometer and the tip of the buret and the stirrer rod are immersed in the solution. The wave
Schwarzenbach (8, 1 2 ) has determined the stability constants of various metal complexes with ethylenediaminetetraacetic acid. Although these constants will vary with the buffer used, a study of these values provides considerable insight on the number and amount of interferences to be expected from foreign ions. Because the stability constants of iron(III), copper(II), and nickel(I1) with Versenate are rather large, under ordinary conditions cations with much smaller stability constants should cause little or no interferences in the titrations. Tables I and I1 show the effect of various ions in the titration of iron and copper with Versenate. The effect of such cations as calcium, barium, and magnesium is not included in these tables, but a t the pH used for these titrations there is little or no interference on the part of these ions, even when present in considerable excess. The pH of the iron, copper, and nickel solutions is an important
.
Table I.
Effect of Various Ions on Determination of Iron
Iron Found, Error, Gram % .,......... 0,02797 0.00 0,02801 0.14 ........... 0.02800 0.11 ........... 0.02802 0.18 0.010 Zn(I1) 0.02798 0 .04 0.020Zn(II) 0.02805 0.28 0.030Zn(II) 0,02813 0.57 0 , 0 2 0 Al(II1) 0.02816 0.68 0.040 Al(II1) 0.02801 0.14 0.005 Cr(II1) 0.02807 0,39 0.010 Cr(II1) 0.02806 0.32 0.030 Cr(II1) 0,02815 0.64 0.005 Mn(I1) 0.04470 0,13 Steel sample" 5 Contained 8.76% Cr, 0.35% Xi, 1.00% Ma, 0.49% Mn, and 0.48% other elements. Ions Present, Gram
Iron Taken, Gram 0,02797 0,02797 0.02797 0,02797 0,02797 0,02797 0.02797 0.02797 0.92797 0.02797 0,02797 0,02797 0,04476
V O L U M E 2 5 , NO. 2, F E B R U A R Y 1 9 5 3
o.6
255
t
If till is present, tartaric acid (5 ml. of 20% aqueous solution) should be added to keep the tin(1T') in solution during the titration. S o study was made on the effects of foreign ions in the titrations of nickel with Versenate, but in general, the interferences should be similar to those for iron and copper. Table I11 lists the results of the titration of nickel with a Versenate solution which had been standardized against standard copper. The normalities in this table are within 1 part per thousand of those found by the gravimetric determination of thr nickel with tlimethylglyoxime. The spectrophotometric titration of copper may also be made at a wave length of 580 mp if an ammonia-ammonium chloride Iluffer (pH 10) is used. I n this case the spectrophotometer follows the loss of the copper-ammonia color as the copperVersenate complex is formed. The slope of this titration curve is very nearly the same as that given in Figure 3, but in the opposite direction. The main advantage of such a titration is the ease of estimating the end point from the initial absorbancy reatling: however, most of the ions given in Table I1 interfere a t this p11, so that the procedure is useful only when all such ions aw known to he ahsent. J3y means of' spectrophotometric end points, a single standard ITersenatesolution can be used to analyze an iron(II1)-copper(I1) solution. Furthermore, this 1-ersenate solution has been used t o determine copper in brass, bronze, and hell metal samples without any previous separations escept the reinoval of insoluble lend sulfate when present,. I n addition, the procedure for determining copper has been shoivn to be ertreniely useful for determining acetylene in a gas stream by the copper scetylide method (,$). The iron content of solutions of stain1e.s steels and iron ores xas determined directly without separations or troublesome reductions of iron(II1). As the interferences in the procedures given are not extensive, many other useful applications should be found for these met,hods.
.
28
2 4
32
36
40
4 4
48
5 2
5 6
6 0
Y L . O f 0 . I 2 1 2 N VERSENATE
Figure 3.
Titration of Copper(L1) Solution with Standard Versenate
10 mg. of zinc( 11) added to copper solution Wave length 745 m p
factor, not only i i i the iiiolar ertinction of thc various complescs. but also in the rate and selectivity of the reaction. When the pH of the solutions is much lower than that pi,escrilicd ~ I ) o v ( ~ . the T'ersenate is no longer effective as a complesing agelit. On the other hand, if the pH values are too large many foreign ions mill interfere in the titration. Interferences by foreign ions will often be apparent from the lack of a well defined titration curve arid the appearance of a slon. equilibrium in the region of the end point. The data present4 in Tahlrs I and I1 show that considernhle amounts of zinc, caciiiiiuin, nianganese(II), tin(1V) chromium(III), and sm:tller amounts of aluminum cause little or no interference in the titration of copper or iron if the solution is properly buffered. Included in Tables I and I1 are actual determinations of copper in l~rassand hell metal samples, and iron in a stainless stcd sample. The main interferences in thc iron and copper titrations are: lead(II), bismuth(III), cobalt(II), nickel(11), iron(III), and copper(I1). I n the deterininntion of copper(II), small amounts of iron(II1) (0.00 to 0.01 gram) ma>- lie complexed and/or precipitated by the addition of 0.6 gram of sodium fluoride to the titration cell. When the iron precipitated as the finely divided basic fluoride, no interference was observed in the copper(I1) detcrniinations a$ long as the precipitate was allon cttl to forin coiiipletcly before the titration was started. Larger amounts of iron(II1) caused such a heavy turbidity t o form that the absorl,aiicy rrwlings W P I T not steady or reliahlc,.
Table 11.
Effect of F7ariousIons on Determination of Copper
Ions Present, Gram 0 . 0 1 0 Zn(IIi 0.020Zn(II)
Copper Taken, Grain 0 . 0 3 159 0.03159
Copper Found, Gram 0.03161 0.03160 0.03161 0.03156 0 03168 0 03239 0 03175 0.03161 0.03171 0.03011 0.02996 0 . 0 3 151 0 03193 0 04061
Table 111. Titration of Kiclrel with Standard Versenate Xormality of Nickel By Vereenate 0.05452 0 05433 0.05452 0 05450 0.05462 0 05452 0 05450 0.05452
B y dimethylglyoxime
Error.
%
0 35 0 04 0 00 0 04
Preliminary work in progress bv the authors indicates that the number of applications oCT'ersenate as a standard reagent can he considerably extended by the use of the ultraviolet region of the Ypertra for following the course of the titrations. LITERhTURE CITED
Error,
"0 0.06 0.03 0.06 0.10 0.29 2 54 0.51 0.06 0.38 0.30 0.20 0.25 0 06 0 32
0 010CdUI) 0.03159 0.020 Cd(I1) 0.03159 0 oo5Al(III) 0 03159 0 020 AI(II1) 0 03159 0 040AI(III) 0 03159 0 . 0 0 5 hln(I1) 0.03159 0.014 hfn(I1) 0.03169 0.004Sn(IV) 0.03002 0.03002 0.020 Sn(IT') 0.014 FelIII) 0.03159 Bell metala 0.03191 Bronsesampleb 0 04074 a Contained 20.62'7 Sn and 79,38°7 Cu. b Contained 5.0l%'Sn 5.30% Pb' 16.88% Zn, ,and 72.60% Cu. moved b y sulfate precipitation before copper titration.
P b re-
(1) Riicker, C. E., and Sweetser, P. B., - 4 ~ 4 CHEsr,, ~ . 24, 409 (1952). (2) Goetz, C.A , , Loomis, T. C., and Diehl, H.. Ihid., 22, 798 (1950). (3) Harris, IT,R., and Sweet, T. R., Ihid., 24, 1062 (1952). 14) Kvdd. P. D.. senior thesis. Princeton Universitv. June 1952. ( 5 ) Piumh, R. C., Martell, A. E., and Bersworth, F: C . , J . Ph&. Colloid Chern., 54, 1208 (1950). ( 6 ) Pribil, R., Koudela, Z., and Matyska, B., Collection Czechodoa. Chem. Communs., 16,SO (1951). ( 7 ) Prihil, R.. and Matyska, B., Ihid., 16, 139 (1951). (8) Schwareenbach, G., and Ackermann, H., Helr. Cliim. Acta, 30, 1798 (1947). (9) Schwarzenbach, G., and Biederniann, W., Ihid., 31, 459 (1948). (10) Schwarzenhach, G., Biedermann, IT., and Bangerter, F., I b i d , , 29,811 (1946). i l l ) Schwarzenhach, G., and Freitag, E., Zbid., 34, 1503 (1951). (13) Schwarzenbach, G., and Gysling, H., Ibid., 32, 1314 (1949). RECEITED for review September 5 , 1952. Accepted November 10, 19.52.
