Laboratory Teaching Practices in Quantitative Chemical Analysis VIVIAN RICHARD DAMERELL and HAROLD SIMMONS BOOTH
Western Reserve University, Cleveland, Ohio N THE early days a course in quautitative analysis
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have realized these difficulties for a number of years, was hkely to consist of the analysis of whatever and the following suggestions are offered for overcoming pieces of iron ore, limestone, etc., the instructor had them, a t least partially. available, with a minimum of classroom meetings, laboratory supervision, or textbook assistance. While DECREASING THE TIME SPENT IN THE BALANCE ROOM Excessive time is spent in the balance room in mauy this was perhaps an effective way to develop a student's initiative, i t was also an inefficient and time- courses in quantitative analysis. The authors have consuming way to learn the subject, particularly the known instances in which students have done as many experimental part. Although the quantitative labora- as 100 weighings in a semester. This is probably four tory in the modem college is run in a much more a-or five times the amount necessary to make the student cient manner, the authors feel that even today there is proficient, and the long periods in the balance room still plenty of room for improvement in the teaching tend to make the course seem dull and uninteresting. of experimental quantitative analysis. The results of The authors diminish this time in two ways: (a) by a a number of years of thought and experimentation on considerable use of the single-deflection method of the subject are summarized here, with the hope that weighing, and ( b ) by giving solutions for unknowns iu other teachers will read, criticize, and possibly benefit some determinations, so that samples may he quickly obtained with a pipet. thereby. Besides the learning of the theory required for an The single-deflection method of weighing is a great understanding of the laboratory procedures two ob- timesaver, and is sufficiently accurate for all elemenjectives seem clear: ( a ) From the student's stand- tary quantitative work. However, the authors believe point a chief aim of the work is the acquiring of skill that the student should also be given thorough inin the fundamental analytical techniques, which in- struction in the method of swings and the adjustment clude sampling, weighing, the handling of solutions method, as part of a comprehensive quantitative course. and precipitates, use of volumetric apparatus, use of It is probably well to give instruction in the method of instruments, etc.; (b) from the instructor's standpoint swings a t the beginning of the course, because i t is the analyses must be chosen and described so that the somewhat time consuming, and a student may slight student can get accurate results by doing accurate and it if first taught the adjustment or single-deflection intelligent work. Then results can be fairly graded, method. The use of solutions for unknbwns is also an effective and the student can he correctly classified as to his laboratory s k i . This is important information to way to avoid excessive time in the balance room. At have in the case of chemistry majors and many pre- first glance this seems to involve a great deal of work professional students, such as potential physicians, on the instructor's part, analyzing solutions and measuring out samples for students. Actually not a great dentists, and pharmacists. In the authors' opinion, quantitative analysis is deal of labor need he involved. For a small quantitaoften not taught in the best way to accomplish these tive class one concentrated stock solution is sufficient aims. Objective (a) can be most readily reached by for a given analysis. This can be added from a buret allowing sufficienttime for the perfection of each type to a 200-ml. volumetric flask and diluted to the mark, of laboratory manipulation. But in most courses at thus providing any number of unknowns. present the amount of time spent in weighing is greatly For a larger class, of say, 25 or more students, a in excess of that needed, while in other operations, like series of solutions can be made up in 2l/%-literacid botthat of transferring a precipitate to a filtering crucible, tles. Approximately 200-ml. samples of these can he or using a spot plate or a colorimeter, many students giveu out without further dilution, and without careful could profit by further experience. Objective (b) can measurement of the volume. The students can then be properly reached only if accurate work on the stu- accurately pipet out 25-ml. portions of this for samples. dent's part leads to accurate results. To the casual The difference between adjacent members of this solureader this seems obvious, hut all experienced quantita- tion series should be small, so that students will not tive instructors are familiar with the many errors en- know which sample they have. Then several samples tirely or partially beyond control of the student that of the same solution can be given out to the same may make a result inaccurate in spite of closest ad- class. Students will need the value of the density to calherence to procedure and technique. The authors
culate the weight of sample from the volume taken. The authors generally have them assume a density of one in such calculations, but the true density can be arrived a t also by furnishing a table of densities and percentages. Members of the class are told first to calculate their percentages assuming unit density. Then from the table they find the density corresponding to this percentage. The latter density is then used in the calculation of a second, more accurate result. Thus, in the determination of silver in a silver nitrate solntion by the Volhard method, the following table is furnished.' DBN~ITY OR S n v m NXTRATB SOLUTIONS AT 20'C.
Dmsuy
Silon, Per CIW
Dcnsily
Siluar. P n Ccnl
Denrily
Silusr. Per Ccnl
more than one or two per cent apart in composition, so that students will not know which sample they have. The instructor is then able to give out several portions of the same sample to a class and, if he desires, the analysis can be established or checked by a class average. Coin envelopes have been found by the authors to be the most convenient containers for giving out solid quantitative samples. These can be thrown away after use, thereby eliminating the problem of what to do with returned sample bottles of dubious cleanliness. The code marks identifying the sample can also be written on such envelopes more readily than upon sample bottles. Samples standing around in these containers will, of course, pick up a variable amount of water, but since they must be thoroughly dried by the student before weighing this does not introduce any error. CALIBRATION OF WEIGHTS AND VOLUMETRIC APPARATUS
The authors believe that calibration of weights and of volumetric apparatus should be part of a comprehensive course in quantitative analysis, not only beIf i t can be arranged so that a t least six or seven cause of the trainimg involved, but also because in most samples of a solution are given out for analysis, and if institutions the weights, burets, pipets, etc., are in such the laboratory directions are clearly stated and the condition that calibration is a necessary prelude to the method is capable of good results the authors have obtaining of accurate - results. Furthermore, it is found that properly computed class averages are in thought that the explanation of the calibration procegeneral as reliable as the instructors' own analysis. dure is within the grasp of quantitative students, if This is a most important point, since few instructors properly presented. The accuracy of student calibrations is checked by have the time or help available today to analyze their the authors by the use of unknowns. The weight caliown solutions. Even in a small class, where a single stock solution is diluted, the class results can be used brations are checked by giving out pairs of weights.' to give the strength of the stock solution by taking one 50-g. and one 2-g., each of which has been iiled so that the mass is a little less than the face value. into account the volume used in each sample. The 50-g. weight then requires the use of most of the student's brass weights and part of his fractionals, and PREPARATION OF SOLID SAMPLES the 2-g. weight requires the use of his 1-g. weight and Experienced instructors will recall the all-too-fremost of the fractionals. The required result is the quent cases in which solid samples have given erratic value obtained by dividing the mass of the 50-g. weight results because of indlicieut grinding or mixing, or by the mass of the 2-g. weight. The use of this ratio both. The authors have largely overcome this d a eliminates arm-length error and the error caused by culty by making two-component samples, each comeach student's calibrating his set in terms of one of the ponent of which contains the radical to be determined. Thus, a sulfate sample series is made up by mixing weights of that set. The calibratiou of volumetric apparatus is checked powdered ammonium sulfate (60.59 per cent SO%)and by giving out a dilute sulfuric acid solution and having powdered potassium sulfate (54.93 per cent SOa). the student determine an effective density by dividing Errors due to improper mixing are much smaller with the weight of solution delivered by the pipets or buret this type of mixture than with the more common types, such as potassium sulfate and potassium chloride. As by the observed volume. The results must be coran example, five per cent too much potassium sulfate rected for temperature, and the authors furnish the in a supposedly 50-50 mixture of this with ammonium equation, Dss = DT + (0.00025)(T- 25) r sulfate will chance " the SO* content from 53.26 ~ e cent to 53.99 per cent, while a similar error in a 50-50 mix- where the corrected density at 250C,, and DT ture of potassium sulfate and potassium chloride will is the density at centigrade temperature T,quantichange the SO3 content from 30.30 per cent to 33.28 (Continued mz pagc 206) per cent.
The members of a series not be BOOTH DM~BUELL, "Quantitative Analysis." 2nd ed., 1
AND
McGraw-Hill Book Company, Ine., New York, 1944, p. 247.
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Six of each weight, properly marked with notches, make 36 &fierent combinations, enough for most quantitative sections. a BOOTH AND DAMERELL, "Quantitative Analysis." 2nd ed., McGraw-Hill Book Company, 1nc , New York, 1944, p. 156.
LABORATORY TEACHING PRACTICES I N QUANTITATIVE CHEMICAL ANALYSIS
(Continued from page 179) tative students do better work on the calibrations, the authors find, when they know that unknowns are going to be given out to check them. RECORDING OF DATA
The authors require quantitative students to use a duplicate copy data book, pages 8.25' X 10.75", and when a page has been completed the student dates it, signs it, and turns in the duplicate carbon copy. The duplicates are filed and the folder marked with the student's name. This procedure has several advantages. A copy of the student's data book is always available for examination. The altering of figures is discouraged, and if a student loses his notebook the data are still available in carbon copy form. This type of notebook is manufactured for us by the National Blank Book Company, Holyoke, Massachusetts. REPORTING AND ~RADINGOP RESULTS
Results should be reported in a way that teaches the students responsibility. The authors give out samples in coin envelopes with code numbers written on them. The student is given the responsibility of correctly including this in his report, with the understanding that an incorrectly reported code number or none a t all will result in a failure. For reporting their results the students use 3" X 5" cards which have on them space for important data (weights of samples and precipitates, and the volumes and normalities of standard solutions used). Data can then be checked against the carbon copy notebook pages, so that the altering of results is discouraged. Students are also warned a t the beginning of the course that results once handed in cannot be changed, even if mathematical errors or other errors are later
found. This rule was adopted because students are under no real time pressure in calculating results, as they would be in an examination, and i t is excellent training for them to have to do it correctly the first time. The authors grade results by use of a number representing the error in parts per 1000 (usually). The number is referred to by students as their "golf score,'' since the lower it is the better. The basis for computing the error is occasionally varied, as in the weight calibration unknown, where it is in parts per 10,000, and in the gravimetric determination of silica, where i t is in parts per 500, because of the greater di?3iculty of this analysis. A maximum of 33 parts err& is given for any single determination, regardless of how poor the result. This prevents one bad result from ruining a laboratory grade. At the end of the semester the errors for the determination are totaled and the sum is converted into a number similar to a per cent by the formula:
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Laboratory grade 100 - (3) (total error)/(total number of determinations)
Thus a student with a total error of, say, 20 parts for 10 determinations will have a laboratory grade of 94, while one with the maximum total error of 330 parts will have a laboratory grade of practically zero. Twofifths of the course grade is determined by the laboratory work, one-fifth by the weekly problem quizzes, and two-fifths by the examination average. The turning in of analytical results in duplicate is encouraged but is not made an absolute requirement, since in this case a student who is behind in his work with one result that he is sure of, but no check, is apt to arrive a t a check result very quickly by the graphitecellulose method, and conditions leading to this practice should obviously not be encouraged.
