from 1 m L phosphoric acid when diluted with 20 m L of 6 N HC1. Our d a t a indicate that the sensitivity and precision of this procedure are quite adequate for t h e ACS test requirement.
(2) Private Communications, Members of the ACS Committee on Analytical Reagents. (3) K . C. Thompson and D. R. Thomerson, Analyst(London),99, 595 (1974). (4) J A. Fiorino, J. W. Jones, and S. G. Capar, Anal. Chem., 48, 120 (1976). (5) D. D. Siemer and P. Koteel. Anal. Chem., 49, 1096 (1977).
LITERATURE CITED (1) "Reagent chemicals", ~~~~i~~ chemical %ciety committee on ~ ~ ~ Reagents, 5th ed., American Chemical Society, Washington, D.C., 1974.
w
RECEIVED for review J u n e 13, 1977. Accepted September 14, i ~ l 1977.
Calibration Procedure to Minimize Errors in Continuous Analysis by Ion Selective Electrodes Lorenzo Liberti" and Antonio Pinto Istituto di Ricerca Sulle Acque-Consiglio Nazionale delle Ricerche, via De Blasio 5. Zona Industriale, 70 722 Bari, Italy brought in contact with 0.8 L of a Na2S04solution in a thermostated vessel. Stirring speed and NarSOl concentration ranged N respectively. The between 2000-4000 rpm and 1 X 10-3-1.8 chloride release was followed by a C1-Select-ion and a Lazaran reference electrode, using an Expandomatic SS-2 potentiometer (all from Beckman Instruments Inc.), and the mV variations were recorded by a Philips PM 8010 double channel recorder. The 100% of exchange was usually reached in a matter of minutes. For each resin, a calibration curve vias prepared according to the following procedure. By the aid of an automatic buret (Autoburette ABU 12 from Radiometer, Copenhagen), known amounts of a KaC1 solution were continuously added to an Na2S04 solution, using the same experimental conditions (stirring speed, temperature, etc.) as in the related exchange kinetic experiments, and the potential variations recorded. NaCl concentration and feed rate were so adjusted that an over-all amount of C1- corresponding to 1 0 0 7 ~exchange with that resin was added in real time (i.e.. the time required for the exchange reaction to be over). By comparison with such dynamic calibration curves. the potential vs. time curves in kinetic runs were converted into concentration vs. time plots.
T h e response time. Le., the time lag required for ion selective electrodes t o assume their new equilibrium potential when subjected to a rapid change in activity of the primary ion, in some cases represents t h e most critical limitation t o application of ion potentiometry t o continuous analysis(1). This report describes a simple procedure t o minimize errors related t o t h e use of ion selective electrodes in continuous monitoring. T h e electrode response after a selected time is compared with a calibration curve obtained contacting the electrodes with standard solutions for the same time. One 1s t h u s able t o continuously monitor quickly changing concentrations, provided the appropriate calibration curve is used. T h i s "dynamic" calibration procedure has been satisfactorily used t o study t h e kinetics of t h e exchange reaction between N a 2 S 0 4solutions and anion resins in chloride form.
EXPERIMENTAL In the rate measurements about 10 mg of chloride resin were
1.0
0
U
0
00
O A
?
0.5
0
100
Figure 1. Degree of exchange for CI-/SO,'X rnequiv/L at time 0 to about 45.6 X
300 t (set)
500
exchange kinetics on anion resin Kastel A 105. (Actual variation of CI- concentration from 42.6 mequiv/L at equilibrium; 6 X N Na,SO,) (different symbols are used to identify repeated
experiments) ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
2377
RESULTS AND DISCUSSION I n ordinary exchange runs. very small amounts of resins were used, so t h a t final C1- concentrations in solution up to 5X mequiv/L were reached. With the fastest resins, the exchange reaction was completed after about 200 s: hence. an average buildup of about 2.5 X mequiv/L s-' of chloride was t o be continuously monitored. With system response times experimentally ranging from about 3 to 30 s. depending mainly on N a 2 S 0 4 concentration, conventional calibration techniques based on equilibrium potentials were not useful in this situation. Experimental potential vs. time plots were then referred to dynamic calibration curves prepared with C1- feed rate 52.5 X 10-j mequiv/L s-'. The ratio of electrode response time to C1- feed rate ranged accordingly between 1 x 10''and 1.2 x 10' L s*/mequiv. T h e results of some typical rate measurements made with this procedure are shown in Figure 1. where the degree of exchange (U= [Cl-],;[Cl-],) with time
over a quadruplicate set of runs is reported. As can be seen, good reproducibility is obtained, with a standard deviation lower than l t 6 7 ~ .Furthermore, by allowing the same limited time for potential measurements in the kinetic runs and in the related calibration curves, the precision of the method was assured (actual C1~concentrations. obtained by this technique, differed less than k770 from values calculated on the basis of weight and full exchange capacity of the resin). T h e proposed procedure is expected to work also in similar applications of continuous ion-selective potentiometry, provided that a sufficiently high ratio (110" L s'lmequiv) between electrode response time and primary ion feed rate is allowed.
LITERATURE CITED ( 1 ) 6 . Fleet. T. H Ryan, and M J D Brand, Anal Chem , 46, 12 (1974)
RECEIVED for review March 4, 1977. Accepted July 18, 1977.
Demountable Temperature Jump Cell John R. Sutter" Chemistry Department, Howard University, Washington, D.C. 20059
Richard M. Reich American Instrument Company, Silver Spring, Maryland 209 10
In genersl, Temperature J u m p Cells are designed with permanently mounted cylindrical or conical quartz rods to guide monochromatic light through the solution onto the detector. This arrangement, however, presents certain limitations for any given cell. Depending on the reaction being investigated, one might need a cell with: long path length in order to use very dilute solutions or compensate for a low absorptivity; minimum heated volume to conserve a precious reactant or to compensate for a small reaction V-l: a geometry designed to study the fastest reactions (minimize to maintain the temperature after perturbation for the longest possible time in order to study a slower reaction. We have been experimenting with the cell design shown in Figure 1. T h e center, transparent, sample compartment is the important feature of the cell. This section is milled from Lucite and the optical surfaces are polished. It is sandwiched between the two Kel-F halves forming the body of the cell and holding the electrodes. The four bolts holding the cell together are not shown in Figure 1. T h e center section shown has a 1.0-cm by 1.0-cm hole and a thickness of 0.2 cm. T h e lower electrode mounts flush whereas the upper one is recessed 0.1 cm from the Lucite plate. This recess increases the heated volume to 0.3 mL. T h e cell has a 1.0-cm light path, and a conductivity cell constant. d / A , of 0.3 cm-'. Also of importance is the improvement in light gathering ability of this cell. T h e 1.0- by 0.2-cm cross-section yields a 0.2-cm area which is to be compared to a 0.3-cm diameter circular rod functioning as the cell window. The rod offers only 0.07 cm2 of window. This represents a maximum increase of 2.8 in available light, producing a corresponding increase in SIN. T h e Temperature-Jump apparatus used with the cell has 2378 * ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
Figure 1. Temperature Jump Cell
been described previously ( I ) . The cell of Figure 1 was tested in the manner outlined there. A block diagram of the cell is also contained there. .4 heating time constant of 300 ns is easily obtained. T h e temperature of the heated solution will persist, with no noticable sag, for about 3 s after perturbation. After this time, the observed absorbance will drift slowly and exponentially (30-45 .) back to the absorbance corresponding to that at the thermostat temperature. Various sample spacers have been tried, e.g. a. 0.5- by 0 5 c m hole with 0.3-cm thickness will give twice the jump height compared to the cell just described, at a sacrifice in path length
