A Ten-week Course in Quantitative Analytical Chemistry LAURENCE D. FRIZZELL Northwestern University, Euanston, Illinois
N E of the temptations the quarter system offers is the course in instruments and a t h i ~ dquarter of quanto allow only one quarter for thenecessary courses. titative analysis taken before the course in instruments. The work was divided into lectures, laboratory, Many schools of medicine and engineering require or strongly recommend a course in quantitative analytical problems, and examinations; two hours of lectures, six chemistry and usually expect the course to produce a hours of laboratory, in two three-hour periods, and 10 student skilled in the art and informed in the science. problems a week, two how-examinations, and a final So much in so little time makes one wish i t could be examination of two hours. Lectures were carefully prepared lessons on theory, practice, and problems. It done by inoculation. The acquisition of the skill needed to conduct quan- was considered that a few general principles and methtitative analytical work is training rather than teaching, ods well learned were better than many poorly assimiand students must submit to discipline as they would lated. Each topic was considered in the lecture and when learning to play a musical instrument. The problems before the laboratory exercise. The work knowledge of chemistry needed to understand the lab- covered in lectures, problems, and laboratory was given oratory work must be active knowledge that can be in examinations with no required additional study in used when needed, not knowledge to which one refers textbooks. A textbook was recommended but students in notes or books. Such training is more satisfactory were requested not to use the text until the concise when done slowly over a long period of time, such as a presentation of a subject had been completed in the school year. An efficient, intensive course is needed lectures. Laboratory facilities provided one balance for two when it must be done in 10 weeks. The course described here endeavors to meet these students. Two groups of 25 each worked on a carefully requirements. It was also necessary to arrange the organized schedule which allowed ample time for those course so that two other groups of students could con- who worked diligently. No make-up periods were used. tinue the study of quantitative analysis. One group It was convenient to have an assistant whose special continued with a quarter devoted to the use of instm- duty was the supervision and correcting of laboratory ments in quantitative analysis, and another group had work and problems for one group of 25 students.
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in a ball mill. The unknown was ground and exposed to the air by the student as an example of the importance of preparation of the sample for analysis. Water, barium, and chlorine were determined. Two determinations of each were required. At the third period Group I1 studied the analytical balance. The groups alternated through the gravimetric work. All students were doing the same work by period 17 and finished together. The determination of water in the unknown included the determination of the weight of the sample by two methods. Thus the student was able to judge immediately the quality of his work. The sample was weighed by difference from a weighing bottle and the same sample was weighed directly in the crucible. The use of a weighing spoon as described by W. C. Pierce and E. L. Haenisch,' saved much time. A spoon that Preparation of a reagent chemical, oralic acid A study of the analytical balance held about one-half gram of the above unknown was Determination of aater in the unknown termination of barium in the unknown Q lo satisfactory. ~ d ~ ~ ~ i nof~chlorine t i o oin the unknown 10 11 The schedule was arranged so the barium sulfate and preparation and comparison of 0.1 N sodium hydroxide and 0.1 N hydroeh~oricheid solutions 11 12 silver chloride precipitates stood overnight. This prostandardization of 0.1 N sodium hydroxide solution 12 13 duced precipitates that filtered satisfactorily. The bar~ ~ t ~ ~ ~01 ioxdic ~ ~ acid t iinothe n reagent prepared in the firsf exercise ium sulfate from a l-g. sample was ignited 90 minutes, Determination of oraiic acid in the unknown then 30 minutes in a covered 15-ml. crucible with an standardization of 0.1 N hydroeh~oricacid solution with sodium carbonate 15 16 ordinary Bunsen burner. The silver chloride from a 17 17 Determination of sodium carbonate in thesample of soda ash meparation and comparison of 0.1 N potassium diehramate 0.5-g. sample Gas collected on a 21-mm. filter paper in a 18 18 and 0.1 N fermvs ammonium sulfate Gooch crucible and dried a t 130°C. 18 18 ~ ~ t ~ ~ ~ iofniron ~ tini iron o nore A solution of carbon-dioxide-free sodium hydroxide All the work except the first two exercises was graded was prepared such that 10 ml. of the solution diluted to by considering an accuracy and precision of 3 parts per 1 liter made a 0.1 N solution, and each student prepared his own solution from this concentrated solution. 1000 or better as the maximum grade. At the first period both groups began the preparation This solution was used for all the subsequent acid-base of a reagent chemical and Group I1 continued this work work. The comparisons were made by measuring 25 during the second period. The preparation required ml. of the 0.1 N hydrochloric acid in a pipet and titratone period but many students with little manual ability ing with 0.1 N sodium hydroxide solution from a buret. needed more time. It had to be completed and ana- This taught the proper use of pipet and buret a t the lyzed in a subsequent exercise in acid-base titrstions. same time. Five or more comparisons were required This exercise confronted the student immediately with and the rules for rejecting results were applied. The the nature of his ability and enabled him to improve i t sodium hydroxide solution was standardized with pounder circumstances where losses were not irreparable. tassium acid phthalate. Three results for standardizaImpure oxalic acid was crystallized once from a 3 N tions and determinations were always required. The oxalic acid in the reagent prepared in the first hydrochloric acid solution and twice from water until chloride free. The student prepared a 25-ml. Gooch exercise was determined by the use of the standardized crucible with an asbestos mat covered by a Witt plate. 0.1 N sodium hydroxide solution. The results were A 15-g. portion of impure oxalic acid was used and the such that a maximum grade was given for work with a student had to prepare 1 g. or more of the pure oxalic precision of 3 parts per 1000 or better and a value for oxalic acid of 99.5 per cent to 100.5 per cent. The acid. Group I devoted the second period to a study of the oxalic acid unknown was a mixture of oxalic acid and analytical balance. The sensibility of the balance for a sodium chloride ground in a ball mill. The 0.1 N hydrochloric acid solution was standard20-g. load was determined and a table constructed to convert rest point differences from 0 to 1.5 pointer ized with reagent sodium carbonate, using methyl scale divisions into milligrams to apply to the weights orange as indicator and the method of aliquot parts. upon the balance pan. Rest points were determined by Blanks were also determined. Consistently satisfacthe method of swings and chainweight balances were tory results were obtained when the sodium carbonate used. Most of theobjects used in the gravimetric work reagent was heated for an hour or more shortly before it weighed about 20 g. Two 15-ml. porcelain crucibles was used. The normality of the 0.1 N hydrochloric and covers were then weighed and these weights were acid solution from this standardization was then comused in the determination of water in the unknown. (Continued on @ge 252) Each student was given an unknown prepared by "Quantitative Analysis," Second edition, John Wiley and mixing sodium chloride and barium chloride dibydrate Sons, Inc., New York, 1940, page 25.
Many teachers consider gravimetric analysis the proper subject to begin the study of quantitative analytical chemistry. The care and thought needed for the preparations and measurements teach the student the significance of quantitative work more effectively than any other method of teaching. The care and use of the analytical balance should be the first lesson the student learns. When the work begins with volumetric methods so little attention is given to weighing that a sufficientknowledge of how to weigh an object is never acquired, or the student must unlearn later and begin properly when confronted with the careful weighing needed for gravimetric analysis.
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A TEN-WEEK COURSE I N QUANTITATIVE ANALYTICAL CHEASISTRY (Catinued from Page 249)
pared with the calculated value from the standardiza- mination of iron in the iron ore. The ore was analyzed tion of the sodium hydroxide solution, and a precision by dissolving the sample in hydrochloric acid in the of 3 parts per 1000 or better was common. The un- usual way. Samples can be obtained that will give known for the determination of sodium carbonate was a complete solution of the i n n in 30 minutes or less. sample of soda ash from a purchased sample. The The course contains the three main divisions of sample was dried in an oven for the student, who placed quantitative analysis, gravimetric, acid-base titrations, it in a desiccator until i t was used. The method of and oxidation-reduction titrations. The exercises used standardizing the 0.1N hydrochloric acid solution with teach the general principles and the ordinary methods of sodium carbonate was used. measurement. The work progresses consistently and A determinate 0.1 N potassium dichromate solution gives the student a well-rounded knowledge. Students was prepared and used for comparison with the 0.1 N who complete the course satisfactorily do so with a ferrous ammonium sulfate solution and for the deter- feeling of accomplishment. 252
