Leo Schubert
The American University Washington, D. C. 20016
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Teaching- Analytical Chemistry with ~utomaticAnallyzerr -
The use of automated analyzers in the teaching of analytical chemistry incorporates many rxccllent didactic values. From a service point of view, it provides the student with the opportunity to use the kind of equipment that he may use later on the job. The most obvious case is that of the student who wishes to become a medical technologist. Quantitative analysis is either required or strongly recommended as part of his program. Yet it is the rare student who receives training in either the theory or the practice of automated analysis until he is in his clinical lahoratory experience. Those who administer accredited clinical lahoratory programs are most anxious to receive students with prior experience. The theory of automated analysis can be frequently taught better in the integrated course in quantitative analysis than in the medical technology clinical environment. Medical technology is by no means the sole reason for including automated analysis in the chemistry curriculum. These techniques are used in pharmaceutical work; metallurgical analysis; and agricultural, cement, food, and beverage determinations. More recently environmental analyses, such as those of air and water,' have been automated. The list of applications is very large and varied.= The incorporation of automated analysis into a teaching program carries other attractive features. Not only does it teach the student to use equipment that he may be expected to employ during his professional life, but it permits the exploitation of more versatile theory and practice in the laboratory. Different kinds of materials, some more "relevent" to students' interests may be used. The present concern with the environment and consumerism may be directed as a motivational device to interest the student in analytical chemistry. It is too frequently overlooked that motivation is still a powerful need in the teaching of chemistry, particularly a t this level. The teacher of analytical chemistry must review constantly what he considers important in its teaching. Consider, for example, the values in the traditional determination of SO3 by gravimetric BaS04. This is attractive because it emphasizes theory concerned with precipitate formation. The lahoratory work is useful because of its concern with precipitate purity, quantitative transfer technique, and the weighing operation. On the negative side are the time required, its tedium, and the lack of immediate or obvious applicability. An automated turbidimetric analysis of SOa is widely used a t present. But there is no compelling reason for "Bibliography on Air and Water Quality," Technicon Corp., Tarrytown, New York, 1969. P'cTechnieonAutoAnalyzer Bibliography,'' 1957/1967, Technicon Corporation, Tarrytown, New York, 10591, 1968.
548 / Journal of Chemical Education
an SOa determination even using automated analysis in the classroom. Other automated determinations that have stronger motivational components may be employed instead. Automated analysis is useful in the teaching of quantitative analysis for other reasons. (1) It provides a convenient methodolpgy for teaching instrumental theory. Thus, spectrophotometry, fluorometry, flame photometry, atomic absorption, chromatography, dialysis, and electroanalytical procedures, among others, are automated. Teaching the theory of these techniques becomes part of the use of automated analysis. (2) It may be used as a rapid method of checking student results and standards. Each "unknown" can be rapidly checked by the instructors. Indeed, "unknowns" in the traditional sense are not required. Commercial or natural materials may be used because the results can be checked so easily. (3) Automated analysis allows greater exercise of the use of statistical analysis for a large amount of data. More samples can be run and more immediate statistical comparisons made. (4) The analyses are inexpensive because small amounts of chemicals are used and large numbers of students can use the equipment in any one laboratory period. (5) The separate steps in an automatcd analysis are readily discernible and may be studied by the students; it does not provide a "black box" experience. Automated analyses may be carried out using modules that are connected in a train (see figure). While there are basic modules used for most purposes, others may be added for special analytical uses. The unknowns are placed into the cups of the sampler, along with standards and wash water. The sampler tray rotates automatically a t some preselected speed, as high as 120 samples per hour. A simple mechanical device allows choice of unknown-to-wash (or standard-towash) ratio. The volume of the cups may run for 0.5 ml to 8.5 ml though 2.0 ml is usually used. The solution is removed from the cup automatically through the use of a probe. At a preset interval the probe rises from the sample cup and aspirates air briefly, then dips into a distilled water wash in another container. The probe is then automatically lifted from the wash and air aspiration occurs once again. This sequence not only clears the apparatus, but also prevents contamination of samples. The mechanical heart of automated analysis is the proportionating pump and manifold. The manifold sits on top of the pump. Solutions and air enter €he YY-
MIXER
Steps in wtomated anolpir,
manifold on one side, flow through plastic pump tubes and glass mixing coils, and exit on the other side. Reagents and diluents are added from containers and admixed in the glass mixing coils. The solutions in the plastic pump tubes are made to move forward through the use of motor-driven rollers which squeeze forward the liquids and air contained in the plastic tubes. The inside diameter of the plastic tube establishes the volume of fluid moved forward by the rollers. Air is sucked into the manifold through an open-ended pump tube. The air bubble is of such a size that it completely fills the diameter of the tubing. It is moved along with the liquids by the action of the roller. Its function is to separate solutions and prevent contamination by removing liquid from the walls. Where a separation such as centrifugation or filtration is required, a dialyzer module is introduced into the stream. A fine pore membrane such as cellophane is used. Two symmetrical dialyzer plates fitted with spiral grooves are placed on either side of the membrane. The assembly is immersed in a constant temperature bath because the rate of dialysis depends upon temperature. The output from the dialyzer module (when it is used) or the mixer (when a dialyzer is not used) is moved onward to a heating bath. The purpose of the bath is to provide that temperature which causes the required color reaction to occur. This module consists of two 40-ft coils immersed in mineral oil. These coils may be used singly on in tandem. An adjustable thermoregulator permits temperature selection. After the reaction is completed in the heating bath, the stream is moved forward into the colorirneter module. A flowcell permits continuous regulation of the percent transmittance (or optical density). Before the fluid enters the flowcell, the air bubbles are removed by a debubbler. The electrical output from the colorimeter is sent to a recorder, the last of the modules. Materials important in the environment or consumerism make excellent subjects for analysis. Clusters of experiments may be devised around such materials. The usual theory deemed important in the teaching of an introductory course in analytical chemistry need not be subverted. The teaching of the theory will be amplified because it is possible to run many determinations in a relatively short period of time and repetition of these important techniques associated with the theory reinforces their retention. Some suggested materials and experiments are 1 ) River, lake, and waste waters Total Phosphate Total phosphate is hydrolyzed to o-phosphate. The latter is determined colorimetricallv using the molybdenum blue color formed w o n treatment -with ammoni& molvhdate and aminonaphtholsnlfonic acid reagent. Nitmte-Nitrite Nitrate is reduced to nitrite (using alkaline hydrazine sulfate and Cuzf catalyst). A diazo compound is formed with sulfmilamide and the amount of dye is measured colorimetrically.
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Methyl Orange and Thymol Blue Alkalinities The color transition range of the indicator is extended by the use of a buffer. Small changes in alkalinity result in a gradual change of the indicator color. The influence of fertilizers, detergents, river flow, eutrophication, sewage wastes, etc. can be discussed with the above. Ammonia in Water The indophenol color results from the addition of sodium phenate to ammonia treated with hypochlorous acid. Hardness in Water EDTA (disodium magnesium form) reacts with Cat+ to free MgC+. The released MgC+ then reacts with buffered "cslmagite" to form the red-violet complex. Protein in Milk by Kjeldahl Total Nitrogen The digestion can be done separately with micro equipment and after the completion, the solution can be transferred to the AutoAnalyzer. Alternatively, the Technicon Continuous Digestor may be used. The blue indophenol color is formed. This determination is recommended because of its consumer interest. Hydrogen Sulfide in Air The sulfide is absorbed in alkaline cadmium hvdroxide solution. The methvlene blue color is develooed uoon treatment with p-rtminodimethylaniline and femc chloride. An additional or optional determination is that of nitrogen dioxide in air. Pharmaceuticals Pyridozine (Vitamin Bs) The indophenol blue color is formed after reaction with diethyl-p-phenylenediamine and scid.8 F w e Salicylic A i d in Aspirin-Containing Products A violet-colored inner complex of salicylic acid and Fe8+ forms in acid solution.' Ascorbic acid (vitamin C) in Multivitamin Preparations The sample is dialyzed into a metaphosphoric acid solution (for n H control) and the diffusttte is reacted with indo~henol dve ' and meakred colorimetricallv. Consumer iiterest commends these pharmaceutical determinations. Urea Nitrogen in Urine This uses a modified Marsh method in which urea is reacted with diacetylmonoxime in the presence of thiosemicarbazide under acid conditions. This determination is ~articularlv useful because of the oersonal involvement it makes nossibie and the opportunity it'offers for the analysis of data. ' Blood Glucose The yellow ferricyanide is reduced to the colorless ferrocyanide calcium Interference from Mg" is eliminated with 6quinolinol. The serum samnle is treated with HC1 to release motein-bound calcium. This is dialyzed into an acid solution containing 8quinolinol and diethylamine. A colored complex between calcium and dye is formed.
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Conclusions
Automated analytical procedures provide rapid, inexpensive alternatives to the more traditional techniques. They may be used in place of or in addition to some of the more traditional standard procedures. They introduce motivational values because they allow the analyses of consumer, environmental, and healthrelated materials. Many procedures are readily available." 'TAUJI, K., Ann. N. Y.Acad. Sci., 153, 446 (1968). 4 CULLEN. L. F..PACHMAN. D. L.. AND PAPARIELLO. G. J. N. Y. ~ e a d&i., . 153, 525 (1968): 6Technicon Symposia 1965, 1966, 1967, 1968, 1969, 1970, Published by Mediad h e . , 60 E. 42 St., New York.
Volume 49, Number 8 , August 1972
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549
