Potentiometric Investigation of Glycinate and Tyrosinate Complexes of Copper(l1) with a Dropping Amalgam Electrode of Simple Design Charles G . Birch' a n d Stanley E. M a n a h a n Departmen1 of Chemistry, The Unicersity of Missouri, Columbia, Mo. 6.5201 DROPPING COPPER AMALGAM ELECTRODES have some advantages over the conventional D M E in the polarographic determination of formation constants of copper complexes (I). We have found that the dropping amalgam electrode (DAE) is also useful in the potentiometric investigation of copper complexes in solution. I t has the advantage of providing a fresh electrode surface at regular intervals. In addition, observation of the composite anodic-cathodic waves obtained polarographically at the DAE in the system under investigation yields information pertaining to the reversibility of the electrode reaction and provides, therefore, a convenient check on the validity of the potentiometric data. The use of a DAE is complicated by the ready decomposition of the amalgam when exposed t o the atmosphere. I t is necessary, therefore, to prepare and store the amalgam in a n inert atmosphere and several designs for electrode assemblies which accomplish this have been published (2-4). Most of these electrode assemblies are complicated and require large quantities of mercury for the amalgam. We have designed a n electrode which not only enables the preparation and storage of the amalgam in a n inert atmosphere, but is also simple and compact in design and can be used with as little as 10 grams of amalgam. This note reports the use of the DAE in the potentiometric determination of the formation constants of the glycinate and tyrosinate complexes of copper(I1) in aqueous solution,
I
c
EXPERIMENTAL
The electrode assembly consists of two parts (Figure 1). Pressure is applied to the amalgam through a manometer which has two gas lecture bottle valves for nitrogen inlet and outlet sealed into it with epoxy resin. The amalgam compartment is attached t o the manometer with a rubber hose. Two pieces of platinum wire are sealed into the amalgam compartment, one for contact with the amalgam when measurements are being taken, the other for contact while the amalgam is being prepared by electrolysis. The capillary tubing is attached t o the amalgam compartment with a '/(-inch nylon Swagelok fitting, enabling facile removal of the capillary during preparation of the amalgam. The amalgam is prepared by the electrolytic reduction of copper from a solution containing the appropriate amount of cupric nitrate and 0.1M NHaN03. Mercury and the solution (about 5 cc of each) are placed in the amalgam compartment through the tube from which the capillary has been removed. Present address, Department of Chemistry, University of Kansas, Lawrance, Kan. 66045 (1) S. E. Manahan, Ph.D. thesis, The University of Kansas, 1965. (2) J. E. B. Randles, Discussions Faraday Soc., 1, 11 (1947). (3) N. H. Furman and W. C. Cooper, J. Am. Clrem. Soc., 72, 5667
(1950). (4) Y.Okinaka, I. M. Kolthoff, and T. Murayama, Zbid., 87, 423 (1965). 1 182
ANALYTtCAL CHEMISTRY
Figure 1. Dropping amalgam electrode showing ( A ) amalgam compartment in position for analysis, ( B ) amalgam compartment in position for preparation of amalgam, (C) manometer compartment Components are (a) capillary tube, (6) l/l-inch nylon Swagelok fitting, (c) amalgam, ( d ) platinum wire for contact with amalgam during analysis, (e) platinum wire for contact with amalgam during preparation, (f) platinum wire for contact with solution during preparation of amalgam, (g) rubber hose, (h) mercury, ( i ) gas lecture bottle valve (one on either side of manometer) During the electrolysis, nitrogen is passed from the manometer through the amalgam compartment and the latter is held a t such a n angle that the nitrogen stirs both the mercury and aqueous phase and removes oxygen from the latter. A dc potential of 6 volts is applied between the mercury (negative electrode) and a platinum wire inserted into the aqueous phase. The electrolysis is allowed t o proceed for hour. After completion of the electrolysis the aqueous solution is decanted and the amalgam compartment is rinsed four times with water and four times with acetone. Throughout the rinsing steps nitrogen is passed through the amalgam compartment at a brisk rate and the acetone residue is evaporated in the nitrogen stream after the final rinse. After the capillary is attached to the amalgam compartment while a low nitrogen pressure is maintained, the DAE is ready for use. An amalgam properly prepared by the above procedure and kept under
4
2 6 0 ~ 240
Figure 2. Plot of E, - E,, mv, us. pH for solutions 1.00 X 10-*F in copper(I1) and 1.00 X 10-*F in amino acid
220 200 I80
> 160
2 140
9
‘
I
I20 100 80 60
40 20
0
3
4
5
6
7
~~
nitrogen a t all times will not decompose noticeably for several weeks. Before any potentiometric data were taken, polarograms were run a t the D A E of buffered systems containing copper(I1) and excess amino acid in the pH range 2.00 to 8.00. In the presence of glycine and tyrosine the composite anodiccathodic waves passed through zero current without inflection whereas with tryptophane above pH 3 two separate waves were observed with the anodic wave being a t a considerably more positive potential than the cathodic wave. I t was concluded that the potential values obtained with tryptophane might not represent reversible electrochemical behavior and potentiometric measurements were not taken for solutions containing this amino acid. Potentiometric measurements were taken in solutions 1.OO x 1 O - V in copper(I1) perchlorate, 1.00 x 10-2F in amino acid, and maintained a t a n ionic strength of 0.10 with NaC104. The D A E capillary and a glass electrode were immersed in the solution and the potential of each was measured us. an SCE reference electrode in contact with the solution via a n agar bridge containing sodium nitrate electrolyte, The potentials were measured with two k e d s and Northrup Model 7401 pH meters. The potential measured a t the D A E was found to fluctuate each time a drop fell. However, when a relatively long drop time of 10 seconds was used, it was found that the potential reaches a constant value after 3 to 5 seconds and potential values taken during the latter half of the drop life are reliable. The solutions were deoxygenated with nitrogen between each potential measurement and were maintained a t 25 + 1 ’. Initially the pH was adjusted to 2.00 with HC104 and the meter measuring the potential of the D A E was zeroed. At this pH none of the copper is complexed with amino acid and any shift in potential of the D A E at higher pH values is due to complexation of the copper. Sodium hydroxide, 1.00M, was added in small increments to raise the pH, and the pH and the potential of the D A E were measured after each addition of NaOH. Average values of potential as a function of p H are given in Table I. RESULTS AND DISCUSSION
Where the effect of dilution upon the total copper(I1) concentration in solution is negligible it may be shown from the Nernst equation by a derivation similar to that described by Laitinen (5)that the equation describing the potential of the the D A E us. the reference electrode is as follows:
( 5 ) H. A. Laitinen, “Chemical Analysis,” McGraw-Hill, New York, 1960, p. 287.
Table I. E, - E, as a Function of pH for Solutions 1.00 x lO-3F in Cu(1I) and 1.00 X 10-2F in Amino Acid
Glycine -(Ec - Ed),mv
PH 2.60 2.80 3.00 3.20 3.40 3.60 3.80 4.00 4.20 4.40 4.60 4.80 5.00 5.50 6.00 6.50 7.00
Tyrosine -(Ee - E.,), mv
0 1
2 3 7 11 16 21 27 34 42 51 60 90 120 149 179
0
1 2 5
9 15 22 30 39 50
61 71 83 112 142 172 201
Table 11. Formation Constants of Glycinate and Tyrosinate Complexes of Copper(I1)
log PI log Pz ,. , 8.13 0.10 15.47 f 0.05 10.13 7.60 f 0.10 15.23 f 0.05
PK~I PK~Z ~Kas
Glycinea 2.43 Tyrosineb 2.34 (1
pK
9.62 9.11
values from (6).
* pK values from (7).
where Ec
=
Es
=
[L-] PI and
=
P2 =
the potential of the DAE L‘S. SCE in the presence of complexing ion the potential of the DAE us. SCE in the absence of complexing ion the concentration of complexing ion the overall formation constants of the complex species CuL+ and CuL2, respectively.
The value of [L-] may be calculated from the expression [L-] = O!L- CL, where O!Lis the fraction of uncomplexed amino acid present as L- ion and CL is the formal concentration of uncomplexed amino acid species. The fraction, (YL-, is
(6) F. Basolo and Y. T. Chen, J . Am. Chem. SOC.,76,953 (1954). (7) R. B. Martin, J. T. Edsall, D. B. Wetlaufer, and B. R. Hollingworth, J. Biol. Chem., 233, 1429 (1958). VOL. 39, NO. 10, AUGUST 1967
1183
readily calculated from the acid dissociation constants. In calculating CL the quantity of amino acid coordinated to copper(I1) must, of course, be taken into consideration. The titration curves for glycine and tyrosine are shown in Figure 2. It is observed that from p H 5 t o p H 7 the plots are linear with a 0.059 slope. This shows that essentially all of the copper present in solution is in the form of CuL2 and Equation 1 reduces to
E, - E,
=
-
RT 2 - In nF
02 [L-1
from which p2 is readily calculated. From pH 3 to 5 the plot is curved and all three copper species, Cuf2, CuL+, and CuL2, are present in significant concentrations in this region. The constant is determined in this p H region by finding the value of it which gives the best fit of the data. The formation constants determined for the glycinate and tyrosinate complexes of copper(I1) are given in Table 11. RECEIVED for review May 15, 1967. Accepted June 5 , 1967. Work supported by a grant from NASA through the University of Missouri Space Sciences Research Center.
Effect of Interfering Substances and Prolonged Sampling on the l,Z=Di-(4=Pyridyl)EthyIeneMethod for Determination of Ozone in Air Thomas R. Hauser and Daniel W. Bradley' National Center for Air Pollution Control, U.S . Department of Health, Education, and Welfare, 1055 Laidlaw Ave., Cincinnati, Ohio 45237 IN A RECENT PUBLICATION (1) we described a new method for the sampling and determination of O3 in the atmosphere. The method involved the collection of atmospheric 0 3 in a solution of 1,2-di-(4-pyridyl)ethylene (PE) in glacial acetic acid, reaction of the 03 with the PE via the ozonolysis reaction to form pyridine-Caldehyde, and colorimetric determination of the resultant pyridine-4-aldehyde using a modification of the 3-methyl-2-benzothiazolone hydrazone method (2). This paper describes the effect of two additional analytical parameters on the PE method. These parameters, namely, the effect of possible interfering substances present in the atmosphere and the effect of prolonged sampling time on final analysis, are very important when any analytical procedure is applied to the field analysis of atmospheric contaminan ts. The results demonstrate that there is no loss of collected O3 from the absorbing solution during a 24-hour sampling period because of a n aeration effect. Therefore, the method can be extended to 24-hour sampling by simply increasing the volume of absorbing solution. When the interfering substances tested are assumed to be present in the ambient air in normally found concentration ranges, the results also show that none of the interfering substances cause either a positive or negative interference in the method with a 30-minute sampling time; only three substances tested could interfere during a 24-hour sampling period. EXPERIMENTAL
Reagents and Apparatus. In addition t o the reagents and apparatus previously described ( I ) , the following equipment was used during this study: a sequential sampler (Model 24001, Gelman Instrument Co., Ann Arbor, Mich.) was used to collect 24-hour samples in 4-hour increments; a portable O3 recorder (Model 725-6, Mast Development Co., 1 Present address, Dept. of Biochemistry and Biophysics, University of California, Davis, Calif. 95616
(1) T. R. Hauser and D. W. Bradley, ANAL.CHEM.,38, 1529
I;
LIMITING
Figure 1. Air sampling train used to determine the effect of prolonged air sampling
Davenport, Iowa) was used t o monitor 03-enriched air streams for comparison with the pyridyl ethylene method. Simultaneous Determination for Interfering Substances. A sampling train was assembled as previously described (1). Purified air was passed through the O3 generator, and the 03-enriched air stream was split in half. One half of the air stream was sampled and analyzed by the published PE procedure (1). The other half was simultaneously sampled and analyzed by the same procedure, except that microgram quantities of the interfering substances to be tested were placed in the absorbing solution prior to sampling. The air sampling rate was regulated by a glass limiting orifice (3) a t 0.3 t o 0.5 liter/minute for approximately 30 minutes. Any increase or decrease in absorbance noted in this simultaneous determination was attributed to the interfering substance. Prolonged Air Sampling. A sampling train was assembled as shown in Figure 1. Purified air passed through the 0 3 generator and into a glass manifold equipped with 12 sampling ports. From one of the sampling ports, the O3 in the air stream was monitored by use of the Mast ozone apparatus. From another sampling port, a 24-hour sample was collected in a bubbler containing 50 ml of the PE absorbing solution at a sampling rate of 0.3 t o 0.5 liter/minute. During the same 24-hour sampling period, six consecutive 4-hour samples
(1966). (2) E. Sawicki, T. R. Hauser, T , W. Stanley, and W. Elbert, Ibid., 33, 93 (1961).
1 184
ANALYTICAL CHEMISTRY
(3) D. W. Bradley, Chemist-Amlysf,55,93 (1966).
