ionic to electrical charge conversion. The internal reference solution must contain the ion of interest as well as a counter ion which is reversible to the internal reference electrode. In the present case, these are the potassium and chloride ions, respectively. The outer ion selective membrane, which in the present case is a PVC film containing valinomycin and dipentyl phthalate, must selectively transport the ion of interest. The main developmental problem in constructing the wire electrode was the preparation of the internal reference medium so that osmotic gradients across the outer membrane would not cause large water flow between the test solution and the reference medium. Such flow was found to burst the outer membrane. The critical factor in producing the wire electrode was the minimum dehydration of the PVA matrix prior to the application of the PVC-DPP-Valinomycin membrane. This resulted in minimum shrinking and swelling of the PVC matrix and a minimum stress on the outer ion selective PVC coating. Minimizing the dehydration of the PVA film in combination with use of a PVA formulation 0.005M with respect to KCl, apparently, produced an internal reference medium that was close to osmotic balance with respect to clinical electrolyte concentrations. This last point was confirmed by estimating the effective KC1 concentration (activity) in the hydrated PVA polymer matrix. This was done in the following way. If one, conceptually, replaces the reference electrode shown in Cell I with a Ag;AgCl electrode, and replaces the aqueous KC1 solution on the left hand side with a hydrated PVA reference medium, then the cell potential will be given by
AE
2ttRT F
C,
= -In -
CI*
where t f is the cation transport number across the ion selective membrane and CI and CII are the KC1 concentrations of the PVA reference media and external solution, respectively. (It is assumed, for purposes of estimation, that activity coefficients are unity and that the standard state EMF value is the same in the PVA matrix as it is in aqueous solution.) By assuming that the transport number of the membrane is independent of the external salt concentration, one can extrapolate the measured potential
as a function of test solution concentration to AE = 0. The extrapolation will give the effective potassium chloride concentration in the PVA matrix since the internal and external salt concentrations must be equal a t AE = 0. By converting the measured cell potentials made with the Cell I configuration to those expected with use of a Ag;AgCl electrode, one can use the data taken from the electrode test series to carry out the extrapolation. Doing so indicated that the KC1 concentration in the PVA matrix lay in the range 0.07-1.OM. The high salt value occurred when dry stored electrodes were first rehydrated. The potential then decreased over a period of about 24 hours until the estimated effective potassium chloride concentration in the PVA layer was equivalent to the potassium chloride concentration of the external solution. If the electrode environment was changed between solutions of widely varying ionic strength-e.g., 0.1 to 0.001M-the internal salt concentration, as estimated from the potential shifts, tended to reestablish osmotic equilibrium with the new test solution. However, in solutions of constant or nearly constant ionic strength, such as those encountered in clinical systems, the internal reference media maintained relatively constant composition and, thus, a stable reference potential that was unaffected by small changes in the external test solution composition could be maintained between the internal medium and the Ag;AgCl couple. The hydrated PVA reference medium differentiates this electrode system from wire electrode systems such as the wire calcium electrode described by Freiser (11).The wire calcium electrode and similarly prepared wire electrodes (12) appear to function satisfactorily in aqueous test solutions. However, the effect on the electrode EMF stability and reproducibility from accidental short circuiting or use of a low impedance measuring device which would allow current flow in the system is unclear from a thermodynamic viewpoint. T h e P V C I P t interface i n those electrodes is not a thermodynamically reuersible couple. The fact that such electrodes function at all must be due to the use of high impedance voltage measuring devices which do not polarize them severely. Received for review January 11, 1973. Accepted April 4, 1973.
Auto mat ic G as- Me a su ring Device M. Z. Galbacs and L. J. Csanyi Department of inorganic and Analytical Chemistry, A . dozsef University, 6720 Szeged, Hungary
In general. two basic methods are employed to follow reactions accompanied by gas evolution or gas absorption. In one of these methods, the pressure of the gas in the system is measured at constant volume. The most widely known procedure, which is also suitable for micromeasurements, is that of Warburg ( I ) . In the other method, ( 1 ) 0 Warburg
1926 1784
Uber d e n Stoffwechsel der Tumoren." Springer, Berlin
the change in volume of the gas is measured at constant pressure. In the case of an appropriately graduated gas buret, the measurement can also be made on a semimicro scale. Both measurement principles can be used to measure slower processes. A restriction with faster processes is the time required to read off the pressure or volume changes. The automatic measurement of the amount of gas is an even greater problem and is to carry out.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, A U G U S T 1973
With the aim of following automatically processes accompanied by the absorption or evolution of gas a t relatively high rates, we have modified the capillary gas buret operating with a liquid plunger (2-4). A polyvinyl chloride (PVC) tube of uniform cross section was used instead of capillary, with a small (-0.01 ml) mercury drop in it as a plunger. One end of the tube was connected to the reaction space while the other end was in contact with the external atmosphere. On the evolution or the absorption of gas, the plunger moves in the appropriate direction. If the tube is nearly horizontal, the mercury drop faithfully follows the change up to the compensation of the pressure. For convenient following of the movement of the mercury drop, the tube is placed between metal electrodes connected to a high-frequency oscillating circuit. As the mercury drop arrives between the pair of electrodes, the loss factor of the oscillating circuit changes and may then be measured as a potential signal in a suitable way. If the potential signal is coupled to a recording potentiometer, the change of the “gas volume” in time can conveniently be recorded (Figure 1). The construction of the measuring element can be seen in Figure 2. A PVC tube with a diameter of 60 mm and a wall thickness of ea. 2-3 mm was used as a support for the measuring tube. A spiral channel with a pitch of 5 mm was turned into the external wall of the support tube, so as to take a PVC measuring tube 2.5 mm in external diameter, with a constant cross section. (Such a uniform tube is readily obtained commercially.) Four vertical grooves were also cut into the external wall of the support tube to sink the back electrodes of the four electrode pairs. The measuring tube was filled with mercury free of gas bubbles, laid into the turned channel, and fixed with PVC cement. The preliminary filling of the tube with mercury or some other liquid is necessary to avoid its deformation. Twenty to twenty-two turns of this tube were used, and this permitted the measurement of a volume change of about 10 ml. The four electrode pairs used for the volume measurement were prepared from brass ribbon, 2 mm wide and 0.8 mm thick. These were placed on the generator parallel with the axis of the tube a t 90”, in such a way that a n airgap did not remain between the measuring tube and the front and back electrodes. The electrodes were similarly fixed by embedding them in PVC cement. A thermostat jacket was next formed around the measuring spiral by means of a PVC tube with diameters of about 80 and 40 mm. The electrodes (the back ones above, and the front ones below) were soldered, each to a separate copper ring, and a terminal was made for these so that the gas-measuring device could be connected, as an adapter, directly to a high-frequency titrator (in our case the Radelkisz OK 302 oscillotitrator). With appropriate adjustment of the sensitivity of the oscillotitrator, a signal of about 2 mV was transferred to a recording potentiometer. To avoid the disturbance of the high-frequency signal, the thermostating was carried out with paraffin oil, and the adapter was surrounded by a screening plate. As Figure 1 shows, the PVC tube used is of satisfactorily uniform cross section, for in the case of uniform admission rate of gas, the potential peaks on the recording appear at equal time intervals. For the tube size employed, the volume of a quarter of a turn, i . e . , the distance of two potential maxima from each other, was 0.118 & 0.003 ml. ( 2 ) W. E. Hoare. J . Sci. lnstrurn.. 32, 1 (1955). (3) R. Gerischer and H. Gerischer. Z. P h y s . Chern. (Frankfurt am Main). 6, 178 (1956). ( 4 ) J. Proszt and L. Poos, Period. Polytech., Chern. E n g . . 1, 25 (1957).
1
2
3
Figure 1. Recording p r e p a r e d with the g a s - m e a s u r i n g device Chart speed, 25 crn/min; rate of gas feed: (1) 15.81 ml/min, (2) 5.24 ml/min, (3) 9.16 ml/min
Figure 2. Construction of
the m e a s u r i n g e l e m e n t
Cross-sectional view of the thermostated apparatus (upper), and plan view of the PVC measuring tube showing positions of ribbon electrodes (lower): (1) PVC measuring tube, ( 2 ) eiectrodes (four pairs), (3) connection to the oscillotitrator. Arrows show the direction of flow of the paraffin oil used to regulate the temperature
Naturally, the external air pressure must be known for the conversion of the gas volume thus measured to the standard state. In more precise measurements, it is also necessary to take into account the pressure excess for the frictional force involved in the movement of the plunger, which may result in 0.2-0.5% error in the gas volume. This size of the pressure excess (Le., that of the correc-
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
1785
ml 8-
X
*
n .a
6-
. K
.X
.*
x
t
4a #
2.
ex X
L
5
IO
15
20 minute
Figure 3. Decomposition of hydrogen peroxide (0)Curve obtained with the proposed gas-measuring device, ( x ) curve obtained with a traditional gas buret
tion) can be determined in a separate experiment in the following way. The gas-measuring device is connected to an automatic piston buret and a sensitive manometer is also fitted into the system. The piston buret is then moved at a definite rate, and the pressure excess is read off on the manometer. Since the frictional force is proportional to the rate of movement of the plunger, it is understandable that different excess pressures develop for dif-
ferent rates of change of the gas volume. For the buret made in our case, in the range of rate of gas admission of 0.08-0.27 ml sec-1, the pressure excess varied between 46 and 66 mm of water, or 3.4 and 5.0 mm of mercury. It follows from this that the excess pressure correction varies with the rate of change of volume. Since the data obtained with this device are usually subjected to computer processing, however, the correction is not accompanied by any particular inconvenience. The sensitivity of the device can be varied by selection of the diameter of the measuring tube and the distance of the electrode pairs, while the total volume to be determined can be varied by the length of the PVC tube. The range of reaction rate which can be determined automatically depends in practice on the chart speed and the response time of the recording potentiometer. It should be noted that the recording also gives direct information on the rate of reaction. The number of peaks in unit distance (in unit time) is proportional to the rate of reaction. The suitability of the developed gas-measuring device for the study of hydrogen peroxide decomposition is shown in Figure 3. For purposes of comparison. the experimental points obtained with a traditional gas buret are also reported. Received for review February 14, 1973. Accepted March 28,1973.
/-Ephedrine in Chloroform as a Solvent for Silver Diethyldithiocarbamate in the Determination of Arsenic John F. Kopp U.S. Environmental Protection Agency, National Environmental Research Center, Analytical Quality Control Laboratory, Cincinnati, Ohio 45268
The most widely used colorimetric technique for the separation and determination of arsenic in water samples is evolution as arsine from a hydrochloric acid solution followed by a spectrophotometric measurement using silver diethyldithiocarbamate (SDDC) as the color forming reagent ( I ) . Pyridine, customarily used as the solvent for SDDC, has a very disagreeable odor and is often objectionable to the analyst and his immediate associates. Recently, it has been reported that chloroform is a satisfactory solvent for the SDDC when an organic base is present. Bode and Hachmann ( 2 ) describe the use of l-ephedrine in chloroform as particularly suitable. This solution is less expensive and without disagreeable odor.
EXPERIMENTAL Apparatus. The arsine generator and absorber tube have been described previously ( I ) . I t has been observed, however, that a commercially available generator assembly (Fisher, Cat. KO.1405) with more uniform physical dimensions will provide an increase in precision. A Perkin-Elmer, double-beam spectrophotometer with 1-cm cells was used for the absorbance measurements. Reagents. The solvent for the SDDC was prepared by dissolving 0.41 g of 2-ephedrine (Aldrich Chemical Company, Cat. No. ( 1 ) "Standard Methods for the Examination of Water and Wastewater," 13th ed, 1971, p 62. (2) H. Bode and K . Hachmann, 2. A n a / . Chem., 224,261 (1967).
1786
13,491-0) in 200 ml of chloroform. Silver diethyldithiocarbamate, 0.625 g, was then added and the volume adjusted to 250 ml with additional chloroform. The reagent was filtered and stored in a brown bottle. A 1570 KI solution, a 40% SnC12.2HzO in concentrated HCI, and a 1070 Pb (CzH302)2-3HzO solution are also required. The stock arsenic solution was prepared by dissolving 1.320 g of arsenic trioxide, As203, in 10 ml of distilled water containing 4 g of NaOH. Approximately 100 ml of distilled water was added and the solution acidified with "03. The final volume was adjusted to 1000 ml with distilled water; 1.00 ml = 1.00 mg of As. An intermediate arsenic solution was prepared by diluting the stock solution 1:100 and a working arsenic solution was prepared by diluting the intermediate solution 1:10. Procedure. Because of the possible presence of organically bound arsenic, a digestion step must be included to ensure conversion of the arsenic to a n inorganic form. This is necessary as only inorganic arsenic compounds will form the hydride. Concentration of the sample and oxidation of organic matter is accornplished by evaporation with "03 and H2S04. To a suitable aliquot of sample containing from 1 to 10 fig As, add 7 ml of 1 : l HzS04 and 5 ml of concentrated "03 and evaporate to SO3 fumes. Cool, add about 25 ml of distilled water, and again evaporate to SOa fumes t o expel oxides of nitrogen. Maintain an excess of " 0 3 until the organic matter is destroyed. Do not allow the solution t o darken while organic matter is being destroyed because arsenic is likely to be lost. Transfer the sulfuric acid-sample concentrate to the generator, add 25 ml of distilled water and cool. Prepare the scrubber and absorber tube by impregnating the glass wool in the scrubber
A N A L Y T I C A L CHEMISTRY, VOL. 45, N O . 9, AUGUST 1973
