James F. Corwin
Antioch College Yellow Springs, Ohio 45387
Beginning Chemistry Can Be Relevant
The continuing criticism offered by students of the sciences that the present day courses are not relevant t,o the problems can be answered in part by a laboratory program based on environmental problems. The theories usually illustrated by laboratory work can be applied just as well to supermarket products as to prepared samples that are commonly used in general and analytical Chemistry. A program of water and air pollution analysis can illustrate the usual techniques of such as acid-base, redox, precipitimetry, and colorimetry. Problems of qualitative analysis can he directed toward the often found statement on cans and bottles labeled "Inert Ingredients." Solid materials collected in plastic buckets placed a t strategic spots on campus can offera real problem of the application of qualitative analysis to the composition of the solid materials deposited from the air. A high volume air filter offers equal opportunity and a more difficult qualitative analysis. An example of the effectiveness of such an approach occurred in an analytical chemistry class when they were allowed to select "Rolaids" as their problem in acid-base analysis. Due to the advertising that one tablet would neutralize 47 times its weight in excess stomach acid, 10 out of 18 students in the class selected to do this analysis. It became apparent to them that a direct titration could not be used because of the buffering material, so an indirect titration was called for, which they performed. They found the neutralizing ability of the tablets was quite small but when the wording of the advertising was recalled "47 times its weight in excess stomach acid," they found the tablets to be as advertised. Although not assigned, a bottle of "Tnms" and a bottle of "Bisodol," tablets were available in the laboratory. Nine out of the ten students did both of these in addition in order to satiify their own curiosity as to which was the best. The same sort of curiosity was exhibited by students confronted with Vanish, Drano, Plunge, and other highly advertised materials. One girl found that the "New" cleaner containing blue crystals were colored crystals of sodium chloride. These ideas can be carried throughout the program by introducing "Fersol" or "Geritol" for an iron titration in redox reactions. The latter requires considerable preparation of sample to remove the organic A portion of this paper was presented at the Ohio Academy of Science Meeting, Wittenberg University, Springfield, Ohio, April 17, 1970, and at the Second Central Regional Meeting, American Chemical Society, Columbus, Ohio, June 4, 1970. 1 Americsn Public Health Administration, 11 Ed., 1960. American Society far Testing Materisls, S.T.P., 148-1 (1967).
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material present. The determination of the available chlorine in "Halozone" tablets offers an excellent iodometric titration. A program for determining water and air pollution involves all of the theory and operations of laboratory work that is usually contained in both the general and analytical courses. At the lower level, the use of water pollution kits such as those furnished by La Motte and other chemical companies which depend entirely on color comparison equipment can he used. At the upper level, the work can be refined to include American Public Health Administration1 Methods, and American Society for Testing Materials2Methods. The theory involved in carbonate-bicarbonate-hydroxide equilibrium can be well illustrated by the alkalinity test. This requires a two-indicator titration, (1) with phenolphthalein and (2) using a mixed indicator, of bromocresol green and methyl red. A comparison of the first and second titrations gives the carbonate, bicarbonate, and hydroxide content as follows. The acid concentration used is 0.02 N sulfuric acid. If the titration (1) is equal to one-half of titration (2) the alkalinity is caused by carbonate content. If titration (1) is greater than one-half of titration (2) then the diierence is hydroxide alkalinity. Of course, when the indicator for titration (1) does not show pink, then titration (2) will give the bicarbonate content. The calcium and magnesium hardness which also requires two titrations and the use of ethylenediamine tetraacetic acid as a titration agent demonstrates the use of a sequestering agent as a titrating medium. The direct titration using Eriochrome Black T as an indicator will give total hardness. A second titration in the presence of sodium hydroxide gives only the calcium hardness and of course the magnesium hardness is the difference. The determination of the Chemical Oxygen Demand (COD) illustrates the redox reaction verv well. The process consists of refluxing a measured amount of the water with a measured quantity of a standard potassium dichromate solution containing sulfuric acid for 2 hr using a small amount of silver nitrate so that the chlorides will precipitate. After refluxing, the excess potassium dichromate is titrated with a standard solution of ferrous ammonium sulfate and the difference can be calculated as organic carbon content of the water. The Winkler Method for the determination of dissolved oxygen involves the unusually careful procedure of collecting a sample of water in such a manner that minimum contact with the air is achieved and the immediate treatment of the sample so that a change in tempemture when returned to the laboratory will not cause a residual gas to form in the bottle. This is
accomplished in many ways which depend on the books that you are reading. The method used depends on the depth of the water to be sampled but all of the methods require minimum contact with the air and immediate addition of 1.0 ml of manganese(I1) sulfate and 1.0 ml of a potassium iodide-potassium hydroxide solution. These additions are made to a completely full bottle by pipet that is immersed well below the surface and will cause overflow at the top. The stopper is then replaced allowing all the water in the neck to escape. Immediate shaking causes the dissolved oxygen to be converted into hydroxide of manganese of higher oxidation number. Upon returning to the laboratory, the addition of sulfuric acid to a 100-ml portion will allow the oxidized manganese to react with the potassium iodide to form iodine, which is then titrated with a 0.01 N solution of sodium thiosnlfate to a starch endpoint. The reactions lead to the relationship 4SnOaP- = 21, = On
The dissolved oxygen can be calculated in parts per million by considering that water has a density of 1 ml= lg. The Biological Oxygen Demand (BOD) can be achieved by the same process except that water collected at the same time is untreated and allowed to stand in bottles for five days. The dissolved oxygen is then determined and compared with the original content. Original Dissolved Oqgen - Dissolved Ozygm After 6 Days = BOD/Day
The Volhard Method for determining chloride can be demonstrated by using titration or colorimetric methods. The determination of total dissolved solids allows the use of a cation exchange resin in conjunction with two acid-base titrations. The first step requires the titration of a measured sample of water with 0.02 N H2S04to a methyl orange endpoint. This step accounts for the ions associated with the carbonate and bicarbonate contained in the water. The second step in the process is to treat a second liquid sample of water by pouring it through a cation exchange resin. This will capture all metallic ions and replace them with hydronium ions. Of course, the OH- ions present and determined in the first titration will react with hydronium ions to form water. The excess hydronium ions are then titrated with 0.02 N NaOH to a methyl orange endpoint. The totals of step one and step two are converted to CaCOa and the results reported as parts per million of CaCOa. The pH of the water can be tested calorimetrically by a mixed indicator and a calorimeter or by simple comparison tests within a comparator block. A pH meter of course gives a more exact result. This author has found Table 1 quite useful in giving a beginning student a conceptual understanding of the p H scale. A number of highly colored dyes are acidic or alkaline. When placed in solutions where the pH number is different from their own p H number, the indicator changes color. A combination of these dyes when
Table 1.
A Conceptual Representation of the Meaning - of pH .
0, 1. 2, 3
4, 5. 8
Strong add
Weak soid
Hasor,HCI
Vinegar Lemon Lime
HNO8
Table 2. Ammonia
Nitrate
Phosphate
Color Turbidity
7
8. 9. 10
Neutral
Roiled
Distilled
11. 12. 13 14, 15
Weak alkaline
Strong base
Soap Ammonia
Lye Strong detergenta
Colorimetric Procedures for Various Ions Nessler reagent SulfamalimideAPHA RromineASTM Ammonium hosphate v a n a d a t e A P H A Platinum ootalt test-ASTM Standard apparatus-ASTM
mixed in water solution will show a color which will indicate the pH number of the solution as follows. pH pH pH pH pH pH pH pH pH
3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0
Red Red-orange Orange Yellow Yellow-green Green Blue-green Blue Purple
Indicator Mixture Phenol hthalein ~rorntfyrnolBlue Thymol Blue Methyl Red a-dinltrophenol
Colorimetric procedures can be used either with a calorimeter for the advanced students or a block comparator for the beginning student. Some of these are in Table 2. Directions can be found in the American Public Health Administration (APHA) Method of Water Analysis, in the American Society for Testing Materials Methods (ASTM), or in many analytical chemistry books as indicated in Table 2. The last measurement in the table is made on an arbitrary scale; however, the apparatus can be constructed easily and is not a chemical measurement. Methods for cyanide, sulfate, sulfide, and iron are also available by colorimetric measurements. These measurements are particularly useful if the water samples are collected in industrial areas. The water analysis becomes more meaningful if the classes can be presented with a problem. These problems can be a lake or stream on or near the campus. A chance to see and analyze the results of their measurements gives a direction to the program that cannot be achieved by having samples collected by others and then presented to the students. An interdisciplinary approach with the Biology Department (bioassay of the spots at which the water is collected and bacterial counts), the hydrology of the area by the Geology, Geography, or Engineering Departments make the measurements made in the Chemistry Department have much more meaning. A complete program of examination of the environment from a chemical standpoint requires the addition of much more sophisticated equipment, such as a very sensitive gas chromatograph equipped with both flame ionization, and electron capture equipment for the examination of water for pesticide and identification of the organic residues. For trace metals such as mercury, an atomic absorption spectrometer becomes necessary.
Volume 48, Number 8, August 1971
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