trends in aplytical
chemistry, vol. 1, no. 5,198.!?
Automatic coulometric titrators. part II. continuous titrators Continuous titrators were first developed durin World War II for the analysis of mustard gas. They currentBy come in a variety of forms and are used in a number of industrial applications John T. Stock University of Connecticut, U.S.A. Terms such as ‘continuous analyzer’ or ‘continuous titrator’ have been applied to devices that fall into one of two classes. Devices of one class rely on periodic sampling and analysis, so that a succession of individual results is obtained. Although such analyzers may be both sophisticated and rapid, the basic concept is more akin to that of a ‘batch’ or single-sample device’. Instruments in which the readout continuously indicates the state of a streaming sample belong to the other class. Although phenomena such as reaction kinetics, indicator response time and controller lag may impose obvious limitations, an ideal analyzer of the truly continuous type would provide instantaneous indication of any change in the concentration of the analyte. The 1966 review by Blaedel and Laessigz, themselves notable contributors to the advance of continuous methods of analysis, includes valuable tables that list specific determinations, methods and sensors. Kiess has extensively surveyed the literature of continuous analysis based on coulometry. His critical account includes listings of reaction precursors and of typical determinations. Some early continuous titrators are described in the monograph by Phillips’. The present account deals with selected examples of periodic-sampling devices and truly continuous coulometric titrators.
A series of papers by Takahashi et al. begins with studies of the fundamental principles of truly continuous coulometric titrators* and of working conditionss. Fig. 1 is a schematic diagram of the titrator developed by these workers. The solution carrying the electrogenerated titrant runs into the cell through which the analyte solution flows continuously. The flow rates of both streams are kept constant. A platinum-SCE potentiometric indicator system senses the prevailing titrant concentration and keeps this at a low level. This is done by controlling the amplifier output, which is the current that flows through the chart recorder and generating system. If the analyte concentration in the sample stream rises, the titrant generation rate and hence the current must rise accordingly. The recorder
Early developments Sometimes a long delay occurs between the introduction of a technique and its development. This was not the case with continuous titrators. Manual coulometric titration was first described in 19385. A continuous coulometric titrator for mustard gas was developed during World War II and was described in the conventional literature in 19486. This device was eventually developed into an industrial instrument to determine such things as the concentration of mercaptan odorants in city gas supplies. The development of continuous coulometric titrators was particularly active during the early 1960s. At this time electromechanical integration, with final chart recorder readout, was used in a completely automated periodic-sampling titrator developed for the determination of sulfuric acid in a viscose spin bath’. 0 165.9936/82/OOCWXOOlSO2.75
Fig. 1. System for continuous titration with externally generated titrant. A, generator anodc; B, generator catho&; C, indicator electrode; D, SCE; E, set end point; F, amplifier; G, recorder; H, electrolyte stream; I, sample stream 0
1982 Elscvicr Scientific
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trends in analytical chemistry, vol. 1, tto.5,1982
interference by NO2 is minimized and the reaction is more rapid than in acidic media. These workers devised a circulating-type titration cell with potentiometric indication that was intended for the continuous determination of UDMH at ppm levels in air. The controlled bromine generation system makes use of a four-electrode potentiostat. The continuous coulometric acid-base titration of P-306 ppm of HCl in refinery gas streams has been described by Burnett and Klaveris.
SO2 and CO analysis
Fig. 2. Block diagram of continuous SO2 monitor. A, light source; B, titration cell; C, opticaljlter; D, photocell; E, amplifier; F, recorder; G, generating electrode system; In, sample entry; Out, to suction pump
pen thus moves up the scale. If adjustments are suitable, the recorder reading is directly proportional to the prevailing analyte concentration. The device was used for the continuous determination of submillimolar concentrations of arsenic(II1) with electrogenerated bromine10 and of KMn04, K2Cr207 and chlorine with electrogenerated iron(II)ii. Traces of iron in water were determined by passage through a Jones reductor and continuous titration of iron(I1) with electrogenerated bromine’s*. Barendrecht and Doornekampls have described an apparatus for the continuous determination of 0.002-l% of water in liquids such as methanol. The generating solution is a modified Karl Fischer reagent that is automatically kept in a constant, almost exhausted, state by sparging with nitrogen carrying the vapor of bromine or water. This solution is pumped into a specially designed cell, where it is activated by the controlled generation of iodine and reacts with the water in the sample stream. Bi-amperometric indication and a controller with PID action regulate the generation current and operate the recorder.
Biamperometric methods Bi-amperometric indication is also employed in a device for continuously monitoring HCN in the concentration range t&l X 1W g. 1-r in factory atmospheresr4. The HCN is absorbed in KBr solution which is buffered at approximately pH 8.5 and flows into a cell for titration with electrogenerated bromine. A problem encountered in the determination of unsymmetrical dimethylhydrazine (UDMH, a rocket fuel) by reaction with bromine is that the reacting ratio varies with the conditions. By coulometric titration with bromine generated in a KBr medium of pH 8, Buck and Eldridgeis found a transfer of approximately eight electrons per mole of UDMH. At this pH, * For a similar device for continuous determination of iron II in picking bath liquor, see G. de Kainlis et al. (1971) Chim. Anal., 53, 696.
Battery-operated portable instruments have been developed, by Boniface and Jenkins, for the continuous acid-base determination of SO2 and CO in the atmosphere of steelworks 17. The underlying principle in the determination of SO2 is its conversion to H2SO4 by absorption in faintly acidic H202 solution. Fig. 2 illustrates the arrangement of the monitor, which relies on photometric detection provided by the differential response of two selenium barrier-layer photoelectric cells. Bromocresol green in the absorbent solution provides the actual signal. The responses of the photoelectric cells are amplified separately and the results are subtracted to drive a single-transistor booster. This regulates the electric current and hence the reading on the chart recorder. Based on four type 741 operational amplifiers, the electronic system is both simple and inexpensive. A platinum working electrode and an isolated silver anode form the
Fig. 3. Labyrinth electrode for continuous constant-potential electrolysis. (a) electrode face; (b) section, showing flow and electrical connections; (c) enlarged vertical section ofportions of electrode A andporous disk B that closes the reference compartment; (d) puruhings on bottom of channel. The arrow indicates direction offlow of solution
trends in ayalytical chemistry, vol. 1, no. 5,1982
electrolysis system. Two ranges, &20 and &lo0 mg. m-3 of SOP, are provided. Oxidation to CO2 is a fundamental step in the functioning of the CO monitor. This has working ranges of O-50 and O-250 ppm by volume. The CO:! is absorbed in a dimethylformamide-monoethanolamine-water medium that contains thymolphthalein and the anode depolarizer, KI. Except for the sample pretreatment tubes, absorbent solution and band in which the optical filters transmit, the device is essentially the same as that used for the monitoring of sop. In the continuous monitoring of high concentrations of active material a large electrical current may be needed. Low electrical resistance then becomes an important cell parameter preventing excessive heating effects. A cell designed for the continuous coulometric titration of from 20 ppm to 3% of chlorine in bleach solutions has a capacity of 560 mlts. The potentiometrically indicated titration is with iron( which is generated at an electrode with louvre-like openings to promote circulation. Careful cell design resulted in a calculated cell resistance of only 0.13 ohm. This, in conjunction with a specially designed controller system, allows chlorine to be determined over the specified concentration range with an average error of 0.6%. A current of nearly 7A is needed to handle the highest chlorine concentration.
Constant potential electrolysis Another approach to continuous coulometric analysis is to maintain the working electrode at a potential at which only the analyte is electroactive. The process involves direct electron transfer, there being no titrant in the ordinary sense. In suitable cases a single electrode can act both as ‘generator’ and ‘indicator’. Constant-potential electrolysis is a well established technique, but the time for a single-sample determination may be as long as 40 min. The time factor must be drastically reduced if the technique is to be used for continuous analysis. The effective area of the working electrode should be large and the solution layer in contact with the electrode should be thin and well agitated. Eckfeld’s electrolysis cell has a working electrode that is made from a 3.125 inch diameter gold disklg. A labyrinth-like channel is milled in the front face of the disk, as shown in Fig. 3. The disk is mounted on a plastic. backing plate. Ports drilled through this into the center and outer end of the channel allow the sample solution to flow into and out of the system. When the cell is assembled, the milled face of the gold electrode presses against the porous disk closing the compartment containing the large Ag/AgCl reference electrode. The geometric active area of the working electrode, approximately 38 cm*, is further increased by punch-roughening the channel. To promote electrode efficiency even more, the solution in the channel is agitated at line frequency by a small-amplitude vibrator. Under these conditions 100% electrode
TIME-, Fig. 4. Idealized generation
response after ORCsingle-shot triangle-programmed
efftciency was achieved whilst monitoring iodide ions at a concentration of approximately 0.1 mhi.
On-line processes Following a study of the use of control-engineering principles in chemical analysis*O, Griepink et ~1.21 have used these principles in an experimental continuous coulometric titration system to suit certain on-line processes. Here there may be a need to monitor concentration fluctuations extending over a few seconds. Acid-base titration using a previously developed split-beam spectrophotometric indication technique was chosen for study. Constant-rate streams of methyl red indicator solution and of 0.04~ NaC104 solution containing the analyte are used. The streams are fed through concentric tubes so that mixing begins near the point of entry to the optical cell. The generating electrode is near to the cell and in the NaC104 stream. Suitable controller design makes possible titration times of 10 s for a few nmols of acid in an on-line system. The collaboration of two groups of Hungarian workers has resulted in a novel technique for the analysis of streaming solutions**. This triangleprogrammed reagent addition technique was first applied to the coulometric argentometric determination of chloride ion in a continuous stream by a series of ‘single shot’ titrations. A chloride ion-selective potentiometric indicator electrode is used. Chloride solution streams at constant rate and under laminar conditions in a tube. Titrant is added at a linearly increasing rate to reach a suitable overtitration point. The addition is then decreased at the same rate when, under ideal conditions, two related symmetrical titration curves such as those depicted in Fig. 4 are obtained. The distance Q is a measure of the concentration of chloride ion. ‘Tailing’ in the tube causes some distortion of the curves but an almost linear plot of Q against chloride in the concentration range l-5 mM was reported.
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In this account general and theoretical aspects, including the response with a linear or linearized detectorzs, the simultaneous determination of chloride and cyanide or iodide, as well as titrations with electrogenerated bromine have been described. The triangle-programmed technique can be extended for use in coulometric acid-base titration&“+. The most advantageous detection systems appear to be photometry (with a suitable color-change indicator) and differential potentiometry. The latter involves the response obtained from two microcapillary glass electrodes that are separated in the stream by a delay coil.
References Stock, J. T. (1981) Trena!sAnal. Chem. 1, 59-62 Blaedel, W. J. and Laessig, R. H. ( 1966) Adv. Anal: Chem. Instrum., 5,69 Kies, H. L. (1975) Rev. Anal. Chem. 2, 229 Phillips, J. P. (1959) Automatic Titrators Ch. 7, Academic Press, New York Szebelledy, L. and Somogyi, Z. (1938) Z. Anal. Chem. 112, 313 Shaffer, P. A. Briglio, A. and Brockman, J. A. (1948) Anal. Chem. 20, 1008
7 Jeffcoat, K. and Akhtar, M. (1962) Arm&t 87,455 8 Takahashi, T., Niki, E. and Sakurai, H. (1962) J. Electroanal. Chem., 3, 373 9 Takahashi, T. and Sakurai, H. (1962) J. Electroanal. Chem. 3,381 10 Takahashi, T. and Sakurai, H. (1962) Talanta 9, 195 11 Takahashi, T. and Sakurai, H. (1963) Talanta 10, 971 12 Sakurai, H. (1962) Runseki Kagaku 11, 83 13 Barendrecht, E. and Doornekamp, J. G. F. (1962) Anal. Chem. Acta, 186, 176 14 Strbfelda, F., Doleial, J. and Pokorny, P. (1966) Collect. Czech. Chem. Commun. 31, 2851 15 Buck, R. P. and Eldridge, R. W. (1965) Anal. Chem. 37, 1242 16 Burnett, R. L. and Klaver, R. F. (1963) Anal. Chem. 35, 1709 17 Boniface, H. J. and Jenkins, R. H. (1978) Analyst 103, 1185 18 Eckfeldt, E. L. and Kuczynski, E. R. (1962) J. Electrochem. Sot., 109,427 19 Eckfeldt, E. L. (1959) Anal. Chem. 31, 1453 20 Eelderink, G. H. B., Verbruggen, H. B., Jutte, F. A., van Oort, W. J. and Griepink, B. (1976) Fresenius’Z. Anal. Chem. 280,273 21 Veenandaal, G. Jutte, F. A., Eelderink, G. H. B., Verbruggen, H. B., van Oort, W. J. and Griepink, B. (1977) Fresenius’ Z. Anal. Chem. 285, 337 22 Nagy, G., T&h, K. and Pungor, E. (1975) Anal. Gem. 47, 1460 23 Nagy, G., Fehtr, Z., T&h, K. and Pungor, E. (1977) Anal. Chim. Acta 91, 87 24 Nagy, G. FehCr, Z., T&h, K., Pungor, E. and Ivaska, A. (1979) Talanta 26, 1143 John T. Stock is Emeritus Professor of Analytical Chemistry at the University of Connecticut, Storrs, CT 06268, U.S.A. and is the author of some 200 publications on the subject. He has been actively engaged in the conservation of historic scientific instruments for the past 15years and was recentlyappointed a Fellow of the ScienceMuseum in recognitionof this work.
methods of urea
Consideration of the chemical principles upon which an analytical method is based is an essential step in the optimization of that method. It can also contribute to the simplification of procedures used and hence cut the costs of many routine analyses. Anthony R. Butler St. Andrews, U.K.
assumes an analysis rate of 0.217 per capita of population, this, when extrapolated to the U.K. population, is 12.07 million tests in a year.
David Walsh Dundee, U.K. Urea has many uses: the manufacture of fertilizers, ruminant feeds, plastics, drugs, paper, dentifrices, and pretzels. Thus, many industrial laboratories need to be able to identify urea and measure its concentration. However, it seems likely that the bulk of such analyses are performed on human body fluids as an adjunct to the investigation and monitoring of renal and hepatic disease, or in the crude determination of nitrogen balance in patients fed on elemental diets. Indeed, in the National Health Service (NHS) laboratories of Scotland alone, there were 1,130,720 urea estimations on blood in the financial year 1979/801. Figures are not available for the U.K. as a whole; however, if one 0165-9936/82/m-0$02.75
Choosing a method Most of the industries involved in the analysis of urea obtain samples composed of many organic components. Despite this, human body fluid remains the most complex and variable matrix. In choosing an analytical method this should be taken into account. Thus the method must be specific or involve the minimum of interference from similar compounds. Where importance is attached to low concentrations, the method must be sufliciently sensitive to measure the difference between compounds. With human fluids, clinical decisions may be based on concentrations ranging from less than 1 mmol/l, to those in excess of 165 mmol/l and whilst urea is the main nitrogen-containing excretion product, intermediates 0 1982 Elrevier Scientific Publishing Company