The Determination Patrick G. McCormick Marquette University Milwaukee, Wisconsin 55233
by the Winkler Method A student
In
continuing effort to keep our interesting to students, as well undergraduate as instructive, we have found, as have many others, that experiments relevant to current interests, and/or close to home in terms of the sample, can be very useful. In our course in quantitative analysis for nonchemistry majors, we often have the entire class learn a particular technique in one period by performing a reasonably uncomplicated experiment with the usual commercial unknowns. The next period, each student may choose from two or three experiments which illustrate a typical application of the technique just learned, but for which he supplies his own sample. In this way, by applying simple analytical techniques to samples ranging from aspirin tablets to his own urine, the student sees for himself the scope and importance of chemical analysis, as well as experiencing first hand some of the sample-handling problems actually encountered in the field. The current interest in ecology has made most students aware of the importance of trace analysis on samples from the environment. Unfortunately, a great many analyses performed on environmental samples require the application of sophisticated techniques or instruments not encountered in introductory laboratory courses. Invoking such techniques for such a project merely reinforces the all too common idea that soon all analyses will be performed by sophisticated instruments, and there is no real need for the fundamentals of classical analysis, nor for such methodology as titration. In light of the above, an experiment which combines many desirable features is the determination of dissolved oxygen in water. Though the determination can be, and often is, done spectrophotometrically (1), this involves preparation and use of air-sensitive color forming reagents, and presents difficulties to a beginning student. The classical method of Winkler (#), or some modification of this method, is still the most commonly used technique for dissolved oxygen studies (3). This method is based on the reaction between oxygen and a suspension of manganese(II) hydroxide in a strongly alkaline solution. Upon acidification in the presence of iodide, the oxidized manganese hydroxide is reduced back to Mn(II) with the liberation of an amount of iodine equivalent to the dissolved oxygen present. The iodine (as triiodide in the excess iodide) is titrated in the usual manner with thiosulfate. This determination, while being an excellent example of a practical oxidation-reduction titration, illustrates a determination done very frea
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courses
of Dissolved Oxygen
laboratory experiment quently in many areas of pollution control and environmental study, and allows the student wide lattitude in choice of samples, which can be collected with a minimum of difficulty. The usual procedure (/) involves a glass-stoppered bottle of known volume, filled completely with sample. The bottle is stoppered, and the overflow discarded. Reagents are added with pipets, the bottle being restoppered after each addition, and the overflow discarded. The messiness of this procedure, as well as the problem of air contact during opening and closing of the bottle, can be avoided through use of a large syringe. A plastic 60-ml syringe has been used effectively in our laboratory. This serves as the sampling vessel, as well as the measuring device and reaction vessel until after acidification, when the system is much less air-sensitive. The syringe method is simple enough to allow sample collection and treatment at a remote location, and then transportation back to the laboratory for titration. Experimental Apparatus Large disposable plastic syringes (60 ml, or 2 oz.) are manufactured by Sherwood Medical Industries, St. Louis, Mo. (Monoject), Becton, Dickinson and Co., Rutherford, N. J. (Plastipak), and others, but the Monoject syringe is preferable since the graduations are engraved into the syringe barrel and remain clear and easy to read after several uses. A short piece (1.5 in.) of plastic tubing (a/16 in. o.d.) is slipped over the Luer tip to facilitate reagent addition.
Reagents Manganese (II)—0.22 M, prepared from either the chloride or the sulfate salt. Basic Iodide—'0.3 MKI orNal, and 0.6 iR NaOH. Acid—0.36 N sulfuric acid. Thiosulfate—0.002 N sodium thiosulfate. A 0.1 N solution is prepared in the usual manner and standardized. When needed, the titrant is prepared by diluting 5 ml of the 0,1 A stock solution to 250 ml. The exact normality of the titrant is 1/50 that found for the stock solution.
Procedure
A small volume (about 5-10 ml) of sample is drawn into the syringe. With the tip of the syringe pointed upward, the plunger is depressed so that all air is expelled from the syringe. With the syringe held normally, the remainder of the sample is gently expelled, until the plunger is just at the bottom of the barrel. This will leave the tip filled with sample, right to the end of the plastic tubing. Gentle pressure on the plunger will cause the liquid in the plastic tubing t.o bulge slightly out of the end. This must be done just before dipping the tubing into a solution to be drawn into the syringe to prevent trapping a small bubble of air at the end of the tubing, which would enter the syringe with the solution. Volume 49, Number 12, December 1972
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Dip the plastic tubing into the water sample to be analyzed, being careful not to trap any air in the tube. Slowly withdraw the plunger, drawing solution into the syringe, until the liquid is above the 30-ml mark on the syringe. If the plunger is withdrawn too rapidly, a large pressure drop occurs, and some air may leak past the plunger seal. Wipe the plastic tubing with a tissue and carefully expel sample from the syringe until the plunger is exactly at the 30-ml mark. Using the technique described above, dip the tubing below the surface of the manganese reagent, contained in a small beaker (to avoid contamination of stock solution) being careful not to trap an air bubble. Carefully withdraw the plunger to the 35-ml mark, drawing 5 ml of manganese solution into the syringe. (Note that overdrawing and then readjustment to the mark cannot be done here or in subsequent additions of reagents due to mixing with the sample.) Mix the solutions in the syringe by slowly tipping the syringe up and down for about 1 min. Before this mixing step, the solution may be drawn a short distance into the plastic tubing to prevent loss, but must be returned to the end of the tubing again before adding the next reagent. Now, in a manner similar to the above, add 5 ml of the basic iodide solution by drawing the plunger to the 40-ml mark while the tip is dipping into the reagent. Mix these solutions in the syringe for about 2 min. The dense precipitate of manganese hydroxide is clearly visible through the syringe walls, and this makes the mixing of the solution easy to see. Finally, draw 10 ml of the sulfuric acid solution into the syringe by withdrawing the plunger to the 50-ml mark. Mix the solutions in the syringe thoroughly using the same tipping procedure. Dark particles of precipitate will be seen floating in the brownishyellow solution. Continue mixing until all these particles have dissolved, and the solution is completely free of solid. Expel the entire contents of the syringe into a 125 ml Erlenmeyer flask. Draw 5-10 ml of distilled water into the syringe to rinse its inner surfaces, and add this to the flask, making sure the syringe is completely emptied. Titrate the contents of the flask with the 0.002 Ar thiosulfate in the usual manner, using starch indicator or another suitable means of indicating the end point. The oxygen content of the reagents themselves must be determined by performing the above procedure, using, in place of the sample, 30 ml of pure water from which oxygen has been removed, preferably by purging with oxygen-free nitrogen. The titration volume obtained for this determination can be considered a blank value, and substracted from the titration values for all other samples before calculations are performed.
Discussion
This experiment, using the techniques described above, has proven quite successful in the hands of students with average laboratory ability, who mastered it easily. The micro method of Burke (4) on the other hand, was found to be difficult to master, volumes being quite critical, and the microtitration was very difficult. His reagent concentrations are also questionable. The reagent concentrations used in the present experiment correspond to the proportions recommended by Carpenter (5), and result in a pH of the final solution of about 2.4-2.5, just before titration. Carpenter points out that pH is somewhat critical; if the solution is too acid (pH less than 2) air oxidation of iodide is a problem; if too basic (pH greater than 2.7) the reaction of oxidized manganese with iodide is not complete. He states that volume measurement of the added reagents should not be more than 5% in error, to assure this pH range. The volumes added in the present experiment are directly read from graduations on the syringe, and can be easily kept within 5% of the stated value. Nonetheless, titration should be carried out as soon as possible after the solution is placed in the flask. If necessary, it can be stored in the syringe until ready for titration. The entire reaction can be performed at the sampling location, and the treated sample brought to the lab840
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Journal of Chemical Education
oratory in the syringe. In such an instance, the plastic tubing should be clamped, or replaced with a plastic cap, and the syringe suitably protected. The titrant concentration was chosen as that which would require reasonable volumes in a 50-ml buret. Typical titration volumes are 20 ml for air-saturated samples, 5 ml for the blank. Admittedly, with this dilute titrant, the starch end point is not as sharp as is usually experienced, but it is easily determined to within 3 drops of 0.002 N titrant. If a microburet is available, more concentrated titrant could be used with a commensurately sharper end point. It is, of course, also possible to use amperometry (6) or spectrophotometry (7) to detect the end point, providing even more sensitivity. If desired, a 50-ml glass syringe could be used for the experiment, but plastic syringes have given us very satisfactory experience. They are more durable, and cheap enough that a student could be given two or three for sample collection outside of laboratory. A good feature of syringe collection is the ability to attach a long piece of plastic tubing to the tip for depth sampling in a natural body of water. In the presence of reasonable amounts of solids
or organic matter, a microporous filter assembly can easily be attached to the syringe tip and the sample filtered as it is being taken, without exposure to air at any time. The volume of sample taken can be determined by weighing the syringe before and after drawing a sample volume. Since the volume of the sample is the only really critical measurement made with the syringe, this is the limiting factor in the precision of the method, and calibration is advisable. Determination of the oxygen content of the reagents, the blank value, is necessary. We have found that nitrogen-flushed distilled water works extremely well, but have successfully used solid C02 (Dry Ice) with comparable results. Unfortunately, this chills the water considerably, and gives a gas concentration in the water which causes bubble formation inside the syringe. Boiling or evacuation of the water after treatment with solid C02 should reduce this problem. Sample substances which interfere specifically in alkaline solution can be accounted for by running a determination in which the reagents are added in the order basic iodide, acid, manganese (6). A variety of samples have been analyzed by this technique. Air-saturated water, obtained by drawing filtered air through distilled water for several hours, gave a value of 7.9 ppm, while water shaken vigorously in a bottle for several minutes gave a value of 7.7 ppm. Both of these values are acceptably close to the theoretical value of 8 ppm (8) for air-saturated water. Natural samples, such as lake water, have given equally reasonable values. Oxygen saturated water could easily be prepared from tank gas, and unknowns prepared by mixing various proportions of oxygen(or air-) saturated water with oxygen-free water. One sample which has not worked well is Milwaukee tap water. The values are always higher than airsaturated water by about 20%. Some of this is undoubtedly due to the presence of chlorine and hypochlorite in the water, but this would hardly account for the entire difference. Though a procedure does exist for removing these and other interfering
substances (9), it is very involved. No ready explanacan be given at this time for our difficulty with Milwaukee tap water. Finally, it is very instructive for students to be given the pertinent equations, balance them, deduce the relationship between moles of thiosulfate used and oxygen originally present (4:1) and then convert from moles of oxygen in a 30 ml sample to mg oxygen per liter (ppm). Having actually determined and calculated a value of significance by a method widely used in several areas (analytical chemistry, pollution analysis, oceanography, sewage control, etc.), the
tion
student should gain
an
appreciation for the contribu-
tions which at least today.
one
classical analysis is making
Literature Cited J. D.T and Silver, W. S., Anal. Chim. Acta, 30,49(1964). Winkler, A. W., Ber. Deut. Chem. Ges., 21,2843 (1888), Malik, A. U., Indian J. Technol., 4,32 (1966). Burke, J. D., J. Elisha Mitchell Sci. Soc., 78, 145 (1962). Carpenter, J. H., Limnol. Oceanogr., 10, 141 (1965). Potter, E. C., J. Appl. Chem. London, 7, 285 (1957). Trotti, L.t and Sacks, D., Arch., Oceanogr. Limnol. (Venice), 21, 257 (1962); C. A. 58,12295d (1963). Meites, L., “Polarographic Techniques" (2nd ed.), Interscience, New York, N.Y., 1965, p.383. Theriault, E, J., and McNamee, P. D., Ind. Eng. Chem. Anal. Ed., 4, 59(1932).
(1) St. John, P. A., Winefordner, (2) (3) (4) (5) (6) (7) (8) (9)
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