Chelating Agents in Nonaqueous Titrimetry B. D. BRUMMET and R. M. HOLLWEG' Edison Laboratory, Thomas A. Edison, Inc., West Oraiige, N. J.
Dithizone. 1-Nitroso-2-naphthol. Leeds & Northrup p H meter with a glass-calomel electrode system.
A nonaqueous titrimetric procedure using chelating, agents has been developed for determining metal ions. The procedure has been applied to nickel, coball, and copper, with accuracy and precision comparable tal those of a strong acid-strong base titration. The USE^ of nonaqueous solvents for chelate titrimetry broadens the field in the same niaiiner as i t has broadened the scope of titration of other acids and bases. IL opens a new phase of chelate tilriiiietry which mag increase ita, specificity. By proper choice of chelating agent and solvent i t is conceivable that titrations specific for nianj nietal ions or groups of ions may be developed. Analysis by this procedure is rapid and does not requirc expensive equipment.
PROCEDURE
The aqueous sample containing approximately 2 minoles of a metal chloride was evaporated on a steam bath. When the sample mas dry, 5 ml. of methanol was added and the beaker swirled until the sample had completely dissolved. An excess of the appropriate chelating agent was added followed by 20 ml. of benzene. The solution was then titrated potentiometrically on a Leeds & Korthrup pH meter, with the etandard 0.1N sodium hydroxide in methanol-benzene. EXPERIMENTAL
C
ONSIDERABLE work has bceri done in the past few years on titrimetric niethods using chelating agents. Schwarzenbach ( 2 ) has reported the use of chelating agents to titrate many metal ions. Martell and Chaberek ( 1 ) have also published in this field. In many eases, however, the chelating agent does not combine with the metal ion strongly enough to produce titration curves suitable for accurate analyses. Also, some chelating agents form intermediate eonipounds with metal ions which prevent formation of good titration curves. These difficulties have somewhat limited the applications of this method. The formation constant for metal chelates depends upon the metal, the chelating agent, and the solvent system. Nonaqueous solutions offer some advantages. First, there is a much wider choice of chelating agents, because many more are soluble in nonaqueous than in aqueous solutions. This choice may permit specificity for certain ions or groups of ions. Second, the equilibrium for chelate formation may be changed by varying the solvent system so that some metals which may not have formation constants large enough for titration in aqueous solutions may be titrated in nonaqueous solutions. The nietliod shows possibilities of being made specific for certain metals or groups of metals, provided the proper choice of chelating agent and solvent is made. Metal chelates are formed by the displacenient of one or more acidic protons of the chelating agent by a metal ion. If the moles of hydrogen ion per mole of chelated metal are determined, then the metal ion concentration can be measured by titrating with a strong base. To demonstrate the applicability of the method to accurate metal analyses, nickel, cobalt, and copper were each titrated using four different chelating agents: dimethylglyoxime, dithizone, S-quinolinol, and 1-nitroso-2-naphthol. The solvent combination used mas 1 volume of methanol to 4 volumes of benzene. The titrant was standard 0.1N sodium hydroxide in 1 part of methanol and 4 parts of benzene.
Nickel. This procedure mas applied to 0.1877M nickel chloride solution which had been standardized by a dimethylglyoxime gravimetric analysis. Dimetliylglyoxime vas also chosen as the chelating agent for the titrimetric nickel analysis. Aliquots of the standard nickel solution mere evaporated to dryness and analyzed as described above. A typical titration curve is shown in Figure 1, curve 1. The inflection a t the equivalence point is very sharp and occurs a t the point in which 2 hydrogen ions per iiiole of nickel have been titrated.
REAGENTS AND APPARATUS
Figure 1. Potentiometric titration of nickel
Standard sodium hydroxide, O.lAr, was prepared by adding the proper amount of 50% aqueous sodium hydroxide to 1 part of methanol, then adding 4 parts of benzene. The solution \vas protected from the air with a carbon dioxide absorbent. Nickel(I1) chloride, reagent grade. Co per(I1) chloride, reagent grade. Cogalt(I1) chlqride, reagent grade. Dimethylglyoxime. 8-Quinolinol.
Curve 1, dimethylglyoxime; curve 2, 8-quinolinol
1
i
CURVE I
CURVE
2
600 550
!/
500j 450
00
40
l
l
8.0 120
l
l
l
l
~
-
l
160 200 2 4 0 2 8 0 320 36.0 400 ML. N n O H 0.0755 N
l
The analysis was evaluated statistically by doing a series of nine separate determinations. The results are shon-n in Table I. The precision is comparable to that obtained in aqueous strong acid-strong base titrations. Equilibrium was obtained rapidly after each addition of base; the potentiometer was steady. The nickel chelate came out of solution but did not interfere with the titration.
Present address, Chas. Pfizer a n d Co., New York, N. Y.
448
V O L U M E 2 8 , NO. 4, A P R I L 1 9 5 6 CURVE
449 2
Table I. Titrinietric Analysis of 0.1877M Nickel Chloride Standardized Gravimetrically with Diniethylglyoxime Mean Dev.,
Chelating Agent Diniethylglyoxinie 8-Quinolinol 2nd inflection 3rd inflection
Molarity
%
0.1869
0.2
0.1SG4 0.1875
0.4 0.2
550
450
-
4000 0
4 0
8 0 I2 0 16 0 200 2 4 . 0 280 32.0 36 0 ML.
Figure 2.
NaOH
O0755N
Potentiometric titration of nickel
Curve 1, 1-nitroso-2-naphthol; curve 2, dithizone
A series of titrations was made on the standard nickel chloride solution using 8-quinolinol as the chelating agent. A very different-type curve was obtained as shown in Figure 1, curve 2. The first inflection was not very reproducible; however, the second and third inflections came a t points equivalent to 2 and 3 hgdrogeii ions per mole of nickel and the reproducibility was good. The mean Concentration and the mean deviation found by this inethod are also shown in Table I. Figure 2 shows two other titration curves for nickel. Curve 1 was obtained by using 1-nitroso-2- naphthol^ The first inflectiqn :it 6.00 ml. is approximately equivalent to 1 hydrogen ion per mole of nickel, and the second a t 18.20 nil. is equivalent to 3 hydrogen ions per mole of nickel. This inflection is sharp and ,would be suitable for an analysis.
10.45
0
5
350-
2
z Y
300250-
00
20
40 6 0 00
8 0 100 I 2 0 140 2 0 4 0 6 0 8.0 100 120 14.0 CURVE 2 ML
NaOH
0.0768 N
Figure 4. Poten tioinetric titration of copper Curve 1, dimethylglyoxime: curve 2, 8-quinolinol
Figure 2, curve 2, shows a titration curve for nickel using dithizone for the chelating agent. Again two inflections 1%-ereobtained. The first is equivalent to 2 hydrogen ions per mole of nickel and the second equivalent to approximately 4 hydrogen ions per mole of nickel. Here the first inflection mas very sharp and would be suitable for an analyeis. Cobalt. A solution of cobalt chloride was prepared and standardized; the concentration was 0.0791M. This solution was analyzed by the titrimetrie proredure using each of the four chelating agents. The titration curves obtained with dimethylglyoxime and 8-quinolinol mere not suitable for accurate analyses. However, dithizone and I-nitro~,o-2-naphthol gave titration curves with sharp inflections a t a point equivalent to 2 hydrogen ions per mole of cobalt, as shown in Figure 3. Copper. A solution of copper chloride was prepared and standardized: the concentration was 0.0800 iM. This solution %vas analyzed by the same procedure using each of the four chelating agents. The titration curves obtained 17-ith dithizone and 1nitroso-2-naphthol \+-erepoor, but those with dimethylglyosinie and 8-quinolinol n-ere suitable for accurate analyses. The inflections occurred a t points equivalent t o 2 hydrogen ions per mole of copper (Figure 4). DISCUSSION
OlO
4:O
E'O 1 2 0
00
16'0 200 24.0 28'0 CURVE I 40 8 0 12.0 160 CURVE 2 ML.
Figure 3.
NaOH
0.0768 N
Potentiometric titration of cobalt
Curve 1, dithizone: curve 2, I-nitroso-2-naphthol
The method described has been applied to the analysis of metal salts in aqueous solutions. The naniple aliquot was evaporated to dryness to remove the 1%-aterplus m y excess acid present. However, the method may be applied when a slight excess of acid remains with the metal salt. Bnalyses have been made on nickel
ANALYTICAL CHEMISTRY
450
salts containing excess acid, using 8-quinolinol for the chelating agent. The second inflection in the titration curve measures the excess acid plus 2 acid equivalents per mole of nickel. The litrant required for the third inflection then measures only nickcl. When dimethylglyoxime is used for the chelating agent, no separation of excess acid and nickel is possible. The method described was used to determine only one metill ion in a solution. It could equally well be applied t o determining the total nickel plus copper plus cobalt in a solution.
It appears probable that two metal ions could be separately analyzed-for example, nickel and copper chelated with 8-quinolinol (Figure 1, curve 2, and Figure 4, curve 2) give titration curves which show this possibility. LITERATURE CITED
(1) Martell, A. E., Chaberek, S., AXAL.CHEW26, 1692 (1954). ( 2 ) Schwarzenbach, G., Schweiz. Chem. Ztg. 28, 377 (1945).
RECEIVED for review August
26, 1955.
Accepted February 4, 1956.
Amperometric Titration of Two- and Three-Component Mixtures of Metal Ions with (Ethylenedinitri1o)tetraacetic Acid C H A R L E S N. REILLEY, W I L L I A M G. SCRIBNER, and C A R R O L TEMPLE D e p a r t m e n t of Chemistry, University o f N o r t h Carolina, Chapel H i l l ,
The combined use of cathodic diffusion current of metal ions and anodic diffusion current of (ethylenedinitri1o)tetraacetie acid under controlled pII conditions made possible the successful amperometrie titration of multicomponent mixture. In this way the successive titration of a bismuth-lead-calcium, ironmanganese, or copper-calcium mixture was effected. The results of an experimental survey of the stability of the 15 common nietal ions with (ethylenedinitri1o)tetraacetic acid at pH 2, 4, and 9.1 indicate the feasible titration of each under certain conditions. Cobalt(I1)(ethylenedinitri1o)tetraacetate was found to give an anodic wave with a half-wave potcntial of +0.140 volt cs. S.C.E., independent of pH from 4.5 to 10.5. The stability constant of cobalt(II1)-(ethylenedinitri1o)tetraacetate was then computed to be 1040.7 ( p = 0.1). Two new polarographic methods for determining stability constants are described and thcir limitations discussed. i E methods have been reported for the amperometric T H R C titration of metal ions with the disodium salt of (ethylenedinitri1o)tetraacetic acid (ethylenedianiinetetraacetic acid, EDTA, or Na2H2Y). The first method (8) utilizes the decrease in height of the reduction mave of the uncomplexed metal ion during the course of the titration.
11+ +
+ HnY --
+
+ 2H ;\I++ + 2e
MY --
Electrode reaction:
+
-+
pH = 4
(1)
I\fo
(2)
N. C.
The third method ( 7 ) is based upon the appearance of the anodic wave of free chelating agent (EDTA) after the end point. Electrode reaction: Hg
+ HY---
-+
HgY--
+ H + + 2e pH = 8
A conibination of these techniques permits the successive determination of multicomponent mixtures in certain favorable cases. I n this report successful analyses were obtained for the three-component system bismuth-lead-calcium, and for the twocomponent systems iron-manganese(I1) and copper(I1)-calcium. POLAROGRAPHY OF EDTA AND METAL-EDTA C O ~ I P L E X E S
Before optimum solution conditions and applied potentials cguld be selected, a study of the polarographic behavior of EDTA and metal-EDTA complexes under a wide variety of solution conditions was required. Anodic Waves. Goffart, Michel, and Duyckaerts ( 2 ) and hIatyska and Ilossler (6) have studied the anodic diffusion wave
."I
W
Because of the large stability constants of the metal-EDTA complexes, the half-wave potentials corresponding to the reduction of the free metal ion and its EDTA complex are widely separated, and the selection of the correct potential is not difficult. The second method ( 4 )utilizes the addition of an indicator ion to allow amperometric titrations in cases where the free metal ion does not yield a suitable polarographic wave. For example, in the titration of calcium, zinc ion can be added to act as an indicator because zinc yields a suitable polarographic wave but forms a weaker chelate than calcium in 1JI alkali. Reaction 3 therefore proceeds in preference to Reaction 4.
+ Y-4 Cay-2 H z 0 + ZnOz-- + Y-4 ZnY-- + 40HElectrode reaction: ZnOz-- + 2HzO Zn + 4 0 H Ca++
(3)
-+
+
-+
(4)
- 2e
(5)
(6)
3
4
5
6 ~ H 7
lo
I'
Figure 1. Potential-pH-pX diagram for mercury
