ference between the maximum signal, E,, with sample in detector
E , = E,'
- 0.0591 log(aNo3- + k )
(1)
and the base-line signal, Et,, E , = E,'
- 0.0591 log(k)
(2 )
where k = Zi Kiai is the sum of all potentiometric interferences in the eluent. Inclusion of a negative sign is appropriate for an anion-selective electrode, and the peak height, E m , becomes E,,, =
- (E, -
Eb) = 0.0591 log
[
%02;
+
"1
(3)
Assuming that the total quantity of sample is proportional to the maximum activity of sample present in the detector, Equation 3 rearranges to give
an empirical basis. Plots of peak area us. moles of sample are linear over the range of 0-10 nmoles for nitrate and 0-3 nmoles for nitrite (Figure 5 ) . Short term reproducibility of the detector is good. Over a four-hour period of operation with sample sizes of 1-300 nanomoles, peak areas are reproducible to within f3% for NO3- and f 5 % for Nos-. The liquid-membrane electrode detector should be useful in a number of ion-exchange chromatographic separations, particularly those in which ultraviolet absorption or electrochemical activity of the sample is poor or absent. Versatility is an important feature of the detector. By changing the composition of the liquid ion-exchange solution, it is possible to obtain response to many organic and inorganic cations and anions and to enhance the selectivity for a selected group of ions. Recent reviews of the ion-selective electrode literature (9, 12-14) can be consulted for membrane composition, selectivity coefficients, and potential applications. A more thorough characterization of the quantitative response of the detector and its application in the determination of ionic species is in progress.
ACKNOWLEDGMENT Logarithmic calibration curves based on Equation 4 are shown in Figure 4.Response is linear from 1 X to 3 X mole for mole for nitrate and from 3 X 10-10 to nitrite. The relative standard deviation in measuring the quantity log ( 10Em/0.0591 - 1) is f3% or better for both ions over the indicated sample range. Minimum detectable quantities at a S/N ratio of 2 are approximately 0.1 and 0.3 nanomoles of NO3- and Nos-, respectively. Davenport and Johnson report an equivalent detection limit for the liquid chromatographic determination of NO3- and NO2- using amperometric detection with a tubular Cd electrode (6). Peak areas also can be correlated to sample quantity on
The assistance of Ed McKnight in design and construction of the flow-through electrode cap is gratefully acknowledged. RECEIVEDfor review June 24, 1974. Accepted August 9, 1974. This work was supported by the Division of Sponsored Research, Florida Atlantic University. (12) R. P. Buck, Anal. Chem., 48, 28R (1974). (13) G. J. Moody and J. D. R. Thomas, "Selective ion-Sensitive Electrodes," Merrow Publ. Co., Watford, England, 1971. (14) J. Koryta, Anal. Chim. Acta, 61, 329 (1972).
Water-cooled Condenser for Unattended Operations David J. Stanonis U.S. Department of Agriculture, Southern Regional Research Center, New Orleans, l a . 70 179
Two major problems must be considered during the unattended operations of a conventional water-cooled condenser. These are decreases and increases in water pressure. Decreases lead to inadequate water-flow to the cooling condenser. Increases may cause the tubing to slip off the condenser connections and cause a flood. Robertson and Jacobs (1 ) state that the tubing should be wired onto the condenser and the water tap; however, Conlon ( 2 ) has pointed out that too high a pressure can cause the tubing connections to the condenser to expand and burst, thus causing a flood and no cooling in the condenser. Conlon suggests a four-point approach to the problem. Install a pressure regulator and a filter in the water main ahead of the valve that is used to adjust the flow to the condenser. Monitor the flow of water from the condenser to the drain and install a solenoid valve to the monitor that can cut off the source of heat after a water failure. The commercially available 12R Water-flow Monitor which takes care of the (1) G. R. Robertson and T. L. Jacobs, "Laboratory Practice of Organic
Chemistry," 4th ed., Macmillan, New York, N.Y., 1962, p 28. (2) D.R. Conlon. J. Chem. Educ., 43, A589 (1966).
last two points can also be purchased with an accessory solenoid valve to turn off the water at the supply to prevent floods. Although some dangerous experiments may call for the precautions suggested by Conlon, the author has found that many condenser-failure problems 'during unattended operations can be avoided by making the condenser part of a syphon as shown in Figure 1. This is done conveniently as follows. One end of a piece of large-bore tubing is connected to the water tap, and the other end is attached through a tapered connector to the small bore tubing leading to the condenser. The connector is then placed under water in a beaker filled with water. The beaker is located in a sink or held over a drain pan (funnel) with some large bore tubing attached which goes to the drain. To provide a driving force (head) when the apparatus will be operating as a syphon, the end of the tubing coming from the condenser should be placed in the sink or drain pan in such a manner that the exit is lower than the surface of the water in the beaker. The condenser can now be operated in the usual way by turning on the water. For unattended opera-
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tion, the connector, while under water, is disconnected a t least from the large-bore tubing. The condenser will now operate as part of a syphon. The flow of water into the beaker should be adjusted to keep the beaker overflowing a t all times. The volume of water flowing through the condenser depends upon the head employed. T o reduce friction or drag, the lengths of tubing in the syphon portion should be kept to a minimum. Although air bubbles tend to form in the tubing, especially where the water leaves the top of the condenser, they will not stick if the surfaces are clean and smooth. Glass tubing connected directly to the condenser with short pieces of polyethylene tubing proved to be quite satisfactory for long-term operation. One such set-up was operated continuously for over twelve days. To go back to attended operations, the connector is replaced in the line and the water flow adjusted.
RECEIVEDfor review June 13, 1974. Accepted July 8, 1974. Apparatus to control pressure and to prevent flooding when using a water-cooled condenser Figure 1.
The mention of firm names or trade products does not that they are endorsed or recommended Over other firms or similar products not mentioned.
Improved Absorption Tube for Arsenic Determinations. Stanley C. Elliott Department of Agricultural Chemistry, Oregon State University, Corvallis, Ore. 9 733 1
Bobby R. Loper
U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station, Forestry Sciences Laboratory, Corvallis, Ore. 9733 1 5:.n
The standard method for the determination of arsenic (1-3) involves the reduction of arsenic to arsine by use of zinc in an acid solution in a Gutzeit generator. The generated arsine is then passed through a lead acetate impregnated glass wool plug and then to a silver diethyldithiocarbamate pyridine solution in which it is absorbed to produce a red colored solution. The absorbance of the solution is read a t 535 nm, and the arsenic concentration is determined by use of a standard curve. Most publications show two- and three-piece absorption tube apparatus. This laboratory found such apparatus cumbersome and difficult to clean. To solve these problems, this laboratory has improved the design of the absorption tube assembly, producing an apparatus that is easier to handle and occupies less space.
1 \ )I ' y
EXPERIMENTAL APPARATUS T h e apparatus shown in Figure I is constructed by joining a 24/ 40 standard tapered joint t o 36 cm of 3-mm i.d. capillary tubing connected t o a 15-ml centrifuge tube; t h e capillary tubing is bent as in Figure 1. T h e bulb in the capillary tubing is needed t o stop syphoning of solution when the absorption tube is removed from the generating flask. T h e glass beads are added to enhance mixing when generated gas passes through the silver diethyldithiocarbamate solution. T h e lead acetate impregnated glass wool plug is inserted a t the entrance t o t h e capillary over the standard tapered joint. (1) "Standard Methods for the Examination of Water and Waste Water," 13th ed., M. J. Taras, A. E. Greenberg, R. D. Hoak, and M. C. Rand, Ed., American Public Health Ass., Washington, D.C.. 1971, pp 62-64. (2) V. Vasak and V. Sedivec, Chem. Listy, 46, 341 (1952); Chem. Absb., 47, 67e (1953). (3) H. K. Hundley and J. C. Underwood, J. Ass. Offic. Anal. Chem., 53, 1176-78 (1970).
2256
1
2%s
5
3 \ T
1
i d Figure 1. Schematic diagram of arsine absorption apparatus DISCUSSION In use, a measured amount of silver diethyldithiocarbamate solution is added to the 15-ml centrifuge tube portion of the apparatus, usually between 5 to 10 ml, an amount less than enough to cover the glass bead bed will make it hard to remove solution for spectrometric measurement, too large an amount will cause solution to be lost when generated gas is passed through the apparatus. Enough glass beads are added to the apparatus to cause a good dispersion of bubbles; in this laboratory the conical portion of the apparatus is filled. Glass beads should be larger than 3 mm
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