Continuous EDTA Titrations with a Dropping Mercury Electrode Automated Titrations Based on Nonsymmetrical Curves W. J. BLAEDEL and
R. H. LAESSIG
Department o f Chemistry, Universify o f Wisconsin, Madison, Wis.
b A previously reported continuous automated potentiometric titrator with direct readout has been adapted to the study of EDTA titrations at pH 10 and pH 5. The instrument, which uses a dropping mercury electrode potentiometrically, has been modified so that direct readout may be obtained for systems which have nonsymmetrical titration curves. The end point error i s constant and i s automatically corrected for. Sample concentrations are presented digitally, thereby eliminating the need for any computation or arithmetical corrections. In potentiometric EDTA titrations of metal ions, the indicator electrode potential i s established by the presence of low concentratiors of Hg(l1) in the titration mixture. A relative standard deviation of 0.570 i s obtained for 13 different metal ions in the 0.003-0.006 molar concentration range.
A
continuous potentiometric buretless titrator with direct readout has been described ( 1 ) and has been adapted to continuous EDT.1 titrations with a mercuryEDTA%electrode ( 2 ) . d continuous analytical process is defined to mean that kind of analysis in which the sample is processed and measured as a flowing stream (3). I n the titrator, shown in block form in Figure I , separate streams of sample, reagent, and buffer are pumped together a t a mixer and then passed through indicator and reference electrodes. h servo system utilizes the potential of the indicator electrode to control the reagent pump speed, to keep the resultant stream a t a preselected equivalence point potential. Once the servo has adjusted the reagent pump speed t o attain equivalence, the rate of rotation of the pump shaft is presented nunierically as a measure of the concentration of the sample. 13y adjusting the interval over which the pump rotation is measured, the titrator can be calibrated with a standard sample to obtain direct readout. Previously ( 2 ) , the titration of metal N AuTomTED
53706 CHEMICAL SYSTEM
READOUT
A NCIRCUITS D RVIDOVT
METERS
SPEED CONTROLER
iI Figure 1.
Block diagram o f continuous titrator
ions with EDTA was reported using a mercury-coated tubular platinum indicator electrode. Around p H 10, the coating of mercury on the platinum showed good stability and reproducible behavior. Titration curves were fairly symmetrical and end points could be rather uncritically selected at the mid point of the steepest region. However around p H 5, where many metals are titratable, the electrode was not stable, and a decrease in magnitude of the end point break was noted as the electrode aged. The response could be renewed by passing fresh mercury through the electrode, indicating that improvement might be obtained with a n electrode having a renewable mercury surface. In this study, a flow-through dropping mercury electrode was used to provide a renewable surface. With rapid drop times (1 second), long range inactivation and/or contamination effects were eliminated and a stable electrode permitting titrations a t p H 5 and 10 was obtained. At the dropping mercury electrode, titration curves-i.e., plots of electrode potential us. pump rate, the latter being analogous t o milliliters of titrant added in the conventional batchwise titration-exhibited characteristic shapes similar to those reported by Reilley, Schmid, and Lamson
(11, 12). A typical curve is shown in Figure 2, the asymmetry being due to the chemical characteristics of the electrode as will be explained in the following section. With curves of this type, the equivalence point occurs at the break rather than at the mid point, and results obtained with auto-titra-
a -50
I
-‘ERROR
0.8
I.o 1.2 IA PUMP SPEED (RELATIVE UNITS)
Figure 2. Typical curve for EDTA titration of a metal ion, using a dropping mercury electrode Conditions similar to those of Figure 8
VOL. 37, NO. 10, SEPTEMBER 1965
1255
tors that operate a t the mid point or a t the inflection point of the titration curve are unsuitable unless corrections are applied. Continuous methodology is particulaily amenable to the application of such corrections, so an automatic compensator was constructed that permitted completely automated continuous titrations nith direct readout on systems exhibiting nonsymmetrical titration curves.
Hg+'
+
Here, [HgY] is the concentration of the mercury-chelate complex, KMythe formation constant of the metal-chelate complex, and K,,, the formation constant of the mercury-chelate complex. In the operation of the titrator (2) the term [HgY]/[XY] is constant at the equivalence point for all concentrations of metal ion, so the potential a t equivalence is seen to be nearly constant, though shoil ing a slight dependence on total sample concentration. Several factors affect the potential of the electrode, causing it to deviate from the ideal value calculated by Equation 1. Two of these, p H and ionic strength, are kept constant in the titrator. The limiting potential-Le. most positive potential which may be assumed-is determined by the formation of mercuric oxide. Hg(!,
+ 20H-
$
HgO
