10 ANALYTICAL PROCEEDINGS, JANUARY 1993, VOL 30 Analytical Viewpoint ~~~~~ The following is a member of a continuing series of articles providing either a personal view of part of one discipline in analytical chemistry (its present state, where it may be leading, etc.), or a philosophical look at a topic of relevance t o chemists in general or analytical chemists in particular. These contributions need not have been the subject of papers at Analytical Division Meetings. Persons wishing to provide an article for publication in this series are invited t o contact the editor of Analytical Proceedings, who will be pleased to receive manuscripts or to discuss outline ideas with prospective authors. Overhead Projector Flow Injection Analysis Kate Grudpan Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai, 50002, Thailand Thanboosak Thanasarn Theong Witta yakom School, Chiang Rai, Thailand Flow injection analysis (FIA) is based on injecting a sample into a continuously moving non-segmented carrier stream yielding a change which can be continuously monitored. The observed change is related to the concentration of analyte in the sample. Three processes are involved: sample injection residence time; and controlled dispersion.Measurements are made without equilibrium being reached. This allows a high sampling frequency with economic sample and reagent consumption. Residence time, the period for the sample to travel from the injection point to the detector, can be optimized for any particular system. During this period the sample plug is physically dispersed so that diffusion and possible reaction occur between sample and carrier.Dispersion coefficient, the ratio of concentration of analyte in the injected solution to concentration of analyte at the point of measurement, depends upon flow rate, tube length and sample size. FIA is now an important area of modern analytical chemistry4" and laboratory exercises for students have been de~cribed.~-" Some of these can be easily set up using existing equipment. FIA systems using low-cost instrumentation, which can be adapted for student exercises, have also been devel- oped. 1s-19 The principles of FIA have been demonstrated by means of a capillary tube" or a microconduit slide2* in a 35 mm projector, allowing the shape and size of the sample zone, dispersion processes, stopped-flow FIA and the effects of intermittent pumping to be observed.The operation of a multi-line manifold has been demonstrated by using a microconduit and an overhead projector.21 Microconduit FIA systems are not readily available in developing countries: we have therefore developed a system using low-cost materials to demonstrate the principles and applications of FIA by means of an overhead projector. The system can also be used with a white background instead of a projector for student laboratory exercises. Experimental FIA System The components of the system were placed on a 30 X 30 x 0.2 cm piece of clear acrylic sheet (Fig. 1). A plastic bottle (100 ml) was used as a reservoir for the reagent. Clear polyethylene tubing (1.3 mm i.d., 2.5 mm 0.d.) was used for all reagent lines.An injection port of the Betteridge typeI5 (Fig. 2) was made of acrylic pieces. Injections were made with a plastic syringe (1 ml) without a needle. A transparent plastic tube 1 cm in diameter and 30 cm long (the protecting sleeve for a thermometer) was used as a former, around which 200 cm of the polyethylene tubing were coiled at spaced intervals of 5 mm (approximately 40 turns in 22 cm). Both ends of the tubing were fixed to the former with transparent adhesive tape. The spacing allows clear viewing of the diffusion process and the sample zone can easily be observed when a coloured solution passes through the coil. A mixing chamber (Fig. 3) was assembled from acrylic sheet, microscope slide covers, epoxy glue and opaque tape.A 0.8 cm diameter hole was drilled in the centre of a 3.5 X 3.5 x 0.2 cm sheet. A 0.4 cm wide slot was cut from the hole to one edge of the sheet in order to insert the inlet and outlet tubing and the sheet was then sandwiched with glue between two cover slips. Finally, the mixing chamber was covered with tape cut to expose only the aperture. - T R ~ ~ ~ ~ ~ _ _ _ _ Fig. 1 Arrangement of the FIA system on an acrylic sheet (A) (30 X 30 x 0.2 cm) for placing on an overhead projector or white background: R, reagenucarrier bottle; T, polyethylene tubing (1.3 mm i.d.; 2.5 mm 0.d.); C, former made of clear plastic; I , injection port; M I , mixing coil; MZ, mixing chamber; D, detection point; P, receiverANALYTICAL PROCEEDINGS, JANUARY 1993, VOL 30 11 Side view Fcm 1.3 cm I I I Fig.2 screws; 4, syringe socket Injection port: 1 , acrylic body; 2, rubber septum; 3 , retaining A chamber of similar design to the one just described, but with a 0.3 cm diameter hole, was used as an observation point for the visual discrimination of coloured products or for timing the period of a colour change. From the detection point, the flowing solution was collected in a 100 ml plastic bottle for possible re-use. In order to start an experiment the tubing at the exit (P in Fig. 1) was connected to a syringe (10 o r 20 ml). The system was filled with the reagent solution by suction via the syringe, and allowed to siphon. The flow rate was regulated by suspending the reagent bottle at various heights.To wash the system, the tubing was removed from the reagent solution (R in Fig. 1) and suction was applied at the exit. The reagent bottle was replaced by a bottle containing water and the system flushed by siphoning until clean. / OT / l _ . Fig. 3 Mixing chamber: A, acrylicsheet (3.5 x 3.5 x 0.2 cm); H, hole (0.8 cm diameter); T, polyethylene tubing for inlet and outlet. fixed to A with epoxy glue; C, cover glass; OT, opaque tape FIA Demonstrations Dispersion and Residence Time A solution of sodium hydroxide (0.02 or 0.1 rnol 1-' in 1% bromothymol blue) is used as the carrier. Injection of a volume (0.15 ml) of 0.6 mol 1- ' hydrochloric acid into the moving blue stream yields a yellow plug which is observed as it moves along the mixing coil. The further the plug moves, the greater its elongation, illustrating the effect of coil length on dispersion.When the plug reaches the mixing chamber the yellow colour disperses further before passing to the detection point. The effect of sample size on dispersion can be demonstrated by injecting a series of volumes (0.07-0.25 ml) of 0.6 rnol I-' hydrochloric acid. The effect of flow rate on dispersion can be investigated by varying the height of the reagent bottle and injecting 0.15 ml of acid. Residence time and its reproducibility can be studied by recording the time from the moment of injection to the appearance of the first colour change at the detection point. Students using wrist watches have recorded residence times of, for example, 18 s with standard deviations of +2 s. Acid-Base Titrations When acid is injected into the carrier, the elapsed time, At, between the colour changes, blue to yellow and back to blue, at lo-' 2 3 4 5 6 lo" 2 3 10-2 2 3 4 5 6 7 8910-l 2 3 Injection volume/ml Fig.4 Elapsed time ( A t ) between colour changes as a function of log(HC1 injection volumes) (curve A) or of log(acid concentration) (curves B, C and D): height of reagent bottle, SO cm; flow rate, 4.8 ml min-I. A, 0.6 mol I-' HCI versus 0.02 mol 1-' NaOH; B, 0.10 ml HCI standard versus 0.02 mol I-' NaOH; C, 0.15 ml HCI standard versus 0.1 mol I-' NaOH; D, 0.15 ml CH,COOH standard versus 0.1 rnol I-' NaOH Acid concentration/mol I-' I I I I I I I I I I I I the detection point corresponds to the peak width at half height in the equation' where V is the volume of the mixing chamber, v the flow rate, C, is the concentration of standard injected, S, the volume injected and CNaOW the concentration of the carrier stream.Plotting At versus log C, should yield in a straight line calibration with a slope of Vlv-lnlO. Results obtained by the authors, displayed in Fig. 4, confirm the logarithmic relationships of Equation 1. At constant flow12 ANALYTICAL PROCEEDINGS, JANUARY 1993, VOL 30 Table 1 Determination of acetic acid in vinegar: a comparison of flow injection with conventional titrimetry Elapsed time/s Acetic acid concentration Actual Mean FIA Conventional titrimetry Nominal mol I-' (%w/v) mol I-' (./w/v> (%w/v) Standards - 0.49 mol I-' 15.4,15.5,15.5 15.5 0.58 mol I-' 16.6,16.2.16.6 16.5 0.78 mol I-' 18.4.18.3.18.3 18.3 0.98 rnol I-' 19.5,19.5.19.7 19.6 1.17 mol I-' 21.1,21.0,21.0 21.0 Samples * - 1 17.1,17.3,17.3 17.2 0.655 3.93 0.710 4.26 4 2 19.8,19.7,20.6 20.0 0.930 5.58 0.883 5.28 5 3 19.2,19.3,19.3 19.3 0.990 5.40 0.883 5.00 5 4 19.0,19.1,19.0 19.0 0.880 5.28 0.936 5.62 5 5 16.9,16.3,16.3 16.5 0.550 3.30 0.581 3.48 - * Different commercial vinegars.rate and carrier concentration, plots of t versus log injection volume (A) and t versus log analyte concentration (B) are parallel lines. The slope of the calibration is governed by the injection volume and the carrier concentration (C), and also by the dissociation constant of the acid analyte (D), the sensitivity of response from a weak acid such as acetic being greater than that from a strong acid. From the slope of a calibration, a value for the volume of the mixing chamber can be calculated.From lines A and B, a value of 0.1 ml was obtained, which corresponded closely to the true one. An estimate of detection limit under a particular set of conditions can be made by extrapolating a calibration to At = 0. Conditions for line C for example correspond to a limit of 0.1 rnol 1-' of acid analyte. The calibration line for acetic acid (D) was used to determine the acid content of commercial vinegar (Table 1). FIA for Chloride By using the manifold without the mixing chamber, 0.20 ml of 0.3,0.8 and 1.2 rnol 1-' solutions of chloride (HCI or NaCl) are injected into a carrier stream of a methanolic (10.8%) aqueous solution containing mercury(I1) thiocyanate (0.002 rnol 1 - I ) , iron(II1) nitrate (0.075 rnol I-') and nitric acid (0.075 rnol I-').Visual comparisons of the colour intensities of the red [Fe(SCN)]*' complex are made. With practice, the approximate concentration of an unknown chloride solution can be estimated by matching the intensities. Discussion The effects of tube length, sample size and flow rate on dispersion can be clearly shown using the overhead projector in a lecture room for up to 100 students. The set up can also be used to illustrate the principle of stopped-flow FIA by lowering the carrier reagent bottle to the plane of the overhead projector screen when the colour appears in the mixing chamber. An increase in colour intensity with time is then observed. This set up has been used at Chaing Mai University for training high-school students in a science camp project.Without previous experience, the students obtained results for the determination of acetic acid in vinegar which agree well with those obtained by conventional titrimetry (Table 1). In a lecture period of 1-2 hours the system reported here can provide a simple illustration of the principles of FIA and its application within a budget of less than &lo. Without the projector, students themselves can perform the experiments described within a laboratory session of 2-3 hours. This system has been found to be of value in a developing country. Rather than accepting Nernst's statement, 'Believe me my dear colleague, when I say it is so then it is so',' students of FIA can be convinced by practical demonstrations.We thank Ian McKelvie and Ian Campbell (Monash Univer- sity, Melbourne, Australia), and Colin Taylor (Liverpool John Moores University, UK) for useful discussions. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 References RSii-ka, J . and Hansen, E.. Flow Injection Analysis, Wiley, New York. 2nd edn., 1988. Valcarcel, M.. and Luque de Castro, M. D.. Flow Injection Analysis: Principles and Applications, Ellis Horwood, Chiches- ter, 1987. Stewart, K. K . , Beecher, G. R., and Hare, P. E., Anal. Biochem., 1976, 70, 167. Skoog, D. A., Principles of Instrumental Analysis, Saunders College Publishing, Tokyo, 3rd edn., 1985. Braun, R. D., Introduction to Instrumentational Analysis, McGraw-Hill Book Co., Singapore, 1987, pp. 957-967. Skoog, D. A., West, D.M., and Holler, F. 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