A Continuous Electrolytic Analyzer for Acidic or Basic Corriponents of Process Gas Streams R.
L. BURNETT and R. F. KLAVER
California Research Ccrp., Richmond, Calif.
b An apparatus for continuously determining trace concentrations of acidic or basic components of a gas stream is described. The analysis is carried out by scrubbing the gas stream with an aqueous salt solution and titrating the dissolved species witn electrolytically generated hydronium or hydroxide ions. A constant pH is maintained within the titration vessel, the current required for this being a measure of the acid or base content of the gas stream. The apparatus has been used successfully in monitoring levels of HCI content from 0.1 to over 300 p.p.m. by volume.
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nlxessary, particularly in the case of petroleum refinery processes, to measL re the concentration of acidic or basic constituents present in trace amounts in gas streams. Ordinarily this is acconiplished by batch techniques, whereby the gas sample is scrubbed of the trace component which is subsequently determined by one of a variety of neutralize,tion, redox, or gravimetric methods. The advantages of a continuous method for carrying out this type of analysis i r e obvious; and the electrolytic methoc. herein described is simple, sensitive, a n j rapidly responsive to changes. Instruments and methods similar to the one below have b2en described by Coulson and Cavanagh (8) and Briglio, Brockman, and Shaffer (1). The apparatus employed by them were not designed for long-range, continuous use, however. A discussicm of helpful experimental technique:! in coulometric titrations in generd h a s been given by Fuchs and Quadt (3). The method emp1oy.d with the present apparatus consisk of continuously titrating the acidic or basic constituent absorbed in aqueous solution from the gas stream with electrolytically generated hydronium or hydroxide ions. A constant pH is maintained within the titrating vessel, the current required for this being a measure of the acidic or basic content of the gas etream. The concentration of these components in the gas is readily obtained with a knowledge of the total gas flow through the titrating vessel. To exemplify the reactions taking place within the electrolytic cell, w u m e T IS FREQUENTLY
the determination of an acidic component such aa HCl wherein the titration is carried out in the pH region 5 to 8. The reactions occurring a t the smooth platinum electrodes within the complete electrolytic cell are the following: 2H20
+ 28 = 20H- + HZ + l j 2 0 2 + 2Z
3H20 = 2H:O+
(Cathode) (Anode)
Mixing of the anode and cathode compartments is prevented, and absorption with subsequent neutralization of HC1 is carried out in the cathode chamber. If the neutralization reaction is sufficiently rapid and absorption of HCl S quantitative, the rate of addition of titrating agent-i.e., the magnitude of the titrating current-is proportional to the concentration of HC1 present in the gas stream. As proved by comparison with batch methods, these conditions are met with the present apparatus. The titrating current generated by the apparatus is related to the quantity of HCl titrated per unit time by Faraday’s law: e = 3.727 X 10-21
where e is equivalents of HCl titrated (or OH- generated) per hour. I is the generating current in amperes. Like considerations also apply to the titration of basic components where the titrating vessel is now the anode compartment of the electrolytic cell. Mixtures of acids or of bases, whether all strong or some strong and some weak, may be determined by proper choice of control pH in the titrating vessel.
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Titration of a strong acid component only in the presence of a weak one is possible in this manner. This is analogous to the titration of mixed acids by conventional batch methods. When dealing with mixtures of acids and bases, the net acidity in aqueous solution would be titrated. The present apparatus features an automatic reversal of polarity for conversion from acid-type to basetype titrations. This finds use in situations where the gas stream may contain both acidic and basic components which may alternate in being dominant. EXPERIMENTAL
Apparatus. The titration procedure is carried out by means of the arrangement shown schematically in Figure 1. The gas stream is scrubbed of HCl (in our sample case) by passing through the cathode chamber of the electrolytic cell. A Beckman No. 39183 combination electrode assembly within the cathode compsrtment allows monitoring of the pH. The pH signal is fed to a Beckman Model J pH Analyzer which serves as amplifier for the coulometer controlling the hydroxide generating current. These basic ions are produced a t the smooth platinum electrode within the cathode chamber. A recorder may be used for following the level of HCl being determined. Titration Vessel. Figure 2 is a detailed drawing of one model of the glass electrolytic titration vessel. To provide mixing of the anode compartment (in the case of acidic titration), and thereby prevent polarization of
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Figure 1.
Schematic diagram of continuous acid-base analyzer VOL. 35, NO. 11, OCTOBER 1963
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ranges between 25 pa. and 12.5 ma. full scale. The detailed diagram is shown in Figure 4. The original reason for using a magnetic amplifier was to provide isolation between the p H electrodes and current electrodes which was important with some earlier pH analyzers used in this fashion. With the Beckman Node1 J analyzer this isolation, however, is not required; and other d.c. amplifiers may be used. The circuit is connected to ground a t Electrode No. 1. If Electrode No. 2 were grounded, capacitively-coupled a x . ground currents from the mag-amp would be partially rectified by the cell resulting in a d.c. titrating current which would not be indicated by the meter, giving erroneous results. This has been demonstrated in actual use. Therefore, some care has to be taken also when connecting a recorder t o the circuit.
GLASS ELECTRODE
RESULTS A N D DISCUSSION
BACKED
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Figure 2.
the electrode by adhering oxygen bubbles, the gas stream scrubbed of acid in the cathode compartment is then passed through the anode chamber before venting. Fritted glass is employed for dispersal of the gas in each chamber. Anode and cathode compartments are separated by a fritted glass disk backed on the upstream side with agar. The electrolyte used in the titration vessel is approximately 0.5 to 1.0N KXO,. Salts of other strong, nonhalogen acids may be used. Since, in the titration of acids, the electrode reaction in the anode chamber produces hydronium ion, the possibility of diffusion of those ions across the agar boundary exists if the acid concentration is allowed to build up. This may be prevented by adding a few crystals of slightly soluble BaC03 to the anode chamber. The acid produced will then react with carbonate to form COe and water. Since the gas streams analyzed will generally be quite dry, there \vi11 be a continual drop in mater level in the electrode compartment on the inlet side of the cell. It is necessary to replenish the level by addition of water a t intervals of 2 t o 5 days, depending on the rate of throughput of gas. The slow change in pH due to concentration of the solution in the titrating compartment exerts a negligible effect upon the titrating current. Controller. A simplified diagram of the controller is shown in Figure 3. The output of the p H Analyzer is fed to one of the input windings of a magnetic amplifier (Airpax Preac No. 5549), the other input winding receiving a bias current from the potentiometer marked “control point.” If the two input currents are equal, the 1710
ANALYTICAL CHEMISTRY
AGAR
Titration vessel
output of the mag-amp is zero; and no current flows through the current electrodes. If the pH rises above the set point, because of constant rate addition of base, the output of the magamp will be amplified by T I and provide a current to the electrodes as indicated by the solid arrows. This in turn tends t o decrease the pH, the final result being a constant current proportional to the rate of addition of base. If acid is being titrated, the pH drops below the set point; and the current polarities now are as indicated by the dotted arrows, amplification being provided by TP. Gain control through negative feedback is provided by the potentiometer marked “feedback.” The fullscale current is adjusted by the “range” rheostat which actually consists of a number of fixed resistors selected with a multipoint switch to provide 9 fixed
Performance. Acid and base levels producing neutralization currents on the continuous analyzer from 5 pa. to approximately 20 ma. have been monitored successfully. These have represented concentrations from 0.1 to 300 p.p.m. by volume a t the rates of gas throughput employed. It is not the concentration, of course, but the rate of throughput of acidic or basic components which determines the operating limits of the equipment. The maximum gas rate desirable with the cell pictured in Figure 2 was approximately 60 liters per hour. At high levels of titrating currente.g., above 500 pa.-the sensitivity of the system is so great as t o make damping necessary. This can be accomplished either electronically or by moving farther from the equivalence point pH (where pH changes are the greatest) in conducting the titration. For example, a t low titrating currents, HC1 may be determined a t a pH near 7; but a t higher currents it is desirable to carry out the titration a t a pH of 5.5 to 6.0.
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Figure 3.
Simplified circuit diagram of acid-base analyzer
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VOL 35, NO. 1 1 , OCTOBER 1963
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Table 1.
Comparison of Continuous and Batch Methods on Four HCI-Containing Gas Streams
Continuous analyzer
1000 cc./min. gas flow rate at 15' C. I , Ira. HC1, peq./hr. p.p.m. 270 10.1 4.0 585 21.8 8.6 1,060 39.6 15.6 20,900 778 306
The pH chosen for the reference point of the titration of a strong acid or base does not affectthe level indicated by the generating current other than to influence the degree of cycling. As with all potentiometric titration methods, best results are obtained when temperature fluctuations are not great. Changes in temperature will cause shifts in pH and in the reference electrode POtential. These changes, if they occur rapidly and in one direction, can be troublesome. Cycling temperatures with a frequency of several minutes will cause minor fluctuations; but the average titration current read will be a true indication of the amount of acid or base
Batch samples: 283 liters of gas scrubbed by NanSOa HC1, peq. p.p.m. 46.1 107 184 3870
3.85 8.9 15.4 323
%
Deviation +4.1 -3.5 +1.3 -5.3
being determined. It was shown experimentally that application of heat to the titrating vessel a t a uniform rate of 5' F. per hour would result in a reduction in observed titrating current of 10 pa. (when determining an acid). Lining out at an increased temperature, however, would result in the *titration current's returning to its original level. These figures enable one to estimate the degree of temperature constancy required for any given level of acidity being determined. Normal ambient temperature variations have not caused dficulties with our apparatus within the over-all &5% limits of accuracy which we have experienced with the equipment. This
level of accuracy was determined by comparison with batch methods on the same gas stream during periods of constant HCl level. The batch analyses were conducted by passing 10 cubic feet of sample gas through a sodium carbonate scrubber and subsequently analyzing the solution for chloride by potentiometric titration with silver nitrate. Table I shows the results of four such comparisons. We have had one of these analyzers in continuous service on a refinery gas stream for eight months. During that time, only replenishment of evaporated water from the titration chamber of the cell has been required for continued troublefree operation. LITERATURE CITED
(1) Briglio, A,, Jr., Brackman, J. A. Jr.,
Shaffer, p. A., Jr., u. 8, ~ ~&m-~ merce, Office of Publication Board,
(
OSRD 6183 PB5940, 1945.
2 ~ ~ ~ ~ L*~*., (3) FU&, w., Qua&, we, 2. ~ ~chern. ~ 147, 184 (1955). R~~~~~~~ for review J~~~ 17, 1963. Accepted July 16,1963.
A Rapid Radiochemical Procedure for Tin ALLEN E. GREENDALE and DANIEL L. LOVE U. S. Naval Radiological Defense laboratory, San Francisco 24, Calif.
b A very rapid radiochemical procedure has been developed for the isolation of radioisotopes of tin from their fission-product isobars. An irradiated uranium solution containing tin and antimony carriers is added to a solution of sodium borohydride. The volatile stannane (SnHd) formed is decomposed in a hot quartz tube to the metal, which i s collected on a cold surface. Stibine (SbHa), which is also formed under these conditions, is removed by absorbtion on an Ascarite column. The tin chemical yield ranges between 15% for an antimony de10' to contamination factor of 2 60% for an antimony decontamination factor of lo3. The time required for separation of the tin metal from the other fission product elements is about 10 seconds. Decontamination factors of other antimony descendents are: I = 7 X lo4, and Te 2 X lo4. Arsenic is also volatilized as the hydride; however, it is not necessary to eliminate it in this work for the determination of the Sn fission yield.
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VERY RAPID radiochemical procedure for the separation of tin from mixed fission products (MFP) was required to determine the fission yields
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ANALYTICAL CHEMISTRY
and radiation characteristics of shortlived tin isotopes. A simple one-step procedure was sought which would have the required speed and would give an exact time for the tin separation. The decontamination factors for antimony, tellurium, and iodine had to be large since the fission yield of the tin nuclide is finally determined from the amount of a longer-lived radioactive iodine descendent arising from the decay of this separated tin. The Nuclear Science Series Monograph, "The Radiochemistry of Tin" (II), lists a number of radiochemical methods for separating tin from MFP. These procedures include separations by multiple sulfide precipitations, distillation of the halides, solvent extraction, and combinations of these methods. The time for separation varies from 20 minutes to 4 hours, and chemical yields vary from 50 to 95%. Decontaminsi tion factors for descendent and parent elements are not given. Since these methods are unsatisfactory for studies of short-lived tin isotopes, other methods were mvestigated. Production of tin hydride with an akali borohydride has been reported (1, 2, 6, 1.2). This paper describes an application of these methods to give a
rapid: clean radiochemical separation of tin from MFP. Slight modifications were made to this general tin procedure so that irradiated samples could be handled rapidly for determination of tin fission yields. EXPERIMENTAL
Reagents. The concentration of carriers and reagents used are: antimony(III), 10 mg. of Sb per ml. in concentrated HCl; tin(IV), 400 mg. of Sn per ml. in 3 N HC1; and sodium borohydride solution, 120 mg. of NaBHl per ml. in 0.2N NaOH. Apparatus. The apparatus for the determination of tin is shown in Figure 1. A simpler apparatus may be used if a rapid separation is not necessary. Unit A holds the solution of carriers, acid, and mixed fission products. It is fitted with a three-way stopcock t o provide vacuum for introducing the solution of MFP into the vessel and nitrogen for forcing the solution rapidly into unit B. Unit A also has a side arm with a needle valve in series to introduce nitrogen for flushing the air out of the system before starting the reaction. This unit and the method of introducing the sample to it are the same as unit A described previously in the procedure for the rapid separa-
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