A SYSTEM of UNKNOWNS and GRADINGfor QUANTITATIVE ANALYSIS W. C. PIERCE AND E. L. HAENISCH The University of Chicago, Chicago, Illinois
A description of the peparation and dispensing of unknowns, the tolerances allowed, and a systematic grading scheme.
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N FEW subjects is there so great a deviation among various schools as in the tolerances allowed and the grading of student unknown reports in elementary quantitative analysis. The system in use a t this school is the development of years and has proved quite satisfactory.* The authors will welcome constructive criticism and suggestions. The first course is given throughout one quarter of eleven weeks and is equivalent to three and one-third semester hours. The investigation of unknowns is preceded by preliminary weighing exercises and the calibration of a buret and a volumetric flask. The unknowns are: "base," "acid," iron by permanganate, copper by iodimetry, gravimetric chloride by use of a Gooch crucible, gravimetric iron, and gravimetric sulfur. The student prepares and standardizes all necessary reagents. A definite tolerance is set for each analysis and if the student's values agree with the actual value withii this limit the report is given a grade varying from 10 to 6 depending upon the deviation. An example will illustrate this. The tolerance allowed for volumetric iron is 6 parts per 1000 parts of iron. The grading scale is shown in Table I. Similar tables are constructed for each of the tolerances used and consist merely of an equiproportional distribution of the allowed tolerance over the five grade points.
No stigma is attached to a rejection and it rarely happens that a student completes the course without a t least one. A surprisingly large number of rejections are issued because of errors in computation rather than because of mistakes in technic. In such cases the student is allowed to recalculate his data and make a new report. PREPARATION AND DISPENSING OF UNKNOWNS
1. Unknmun Base. A 0.5 N stock solution of sodium hydroxide is used. The student receives a 25- to 50-ml. sample accurately measured from a buret into a previously calibrated 250-ml. volumetric flask. He dilutes the sample and titrates aliquot portions with a 0.1 N hydrochloric acid solution. Sodium carbonate is used as the standard. The total number of milliequivalents of base received is reported. Tolerance: 15 parts per 1000. (This is unusually large as the exercise constitutes the student's first attempt a t quantitative analysis.) 2. Unhnmun Acid. A stock solution of 0.5 N acetic acid is dispensed as in No. 1 and is titrated by the student with 0.1 N sodium hydroxide. The 0.1 N hydrochloric acid or potassium acid phthalate is used as a standard. Tolerance: 10 parts per 1000. 3. Volumelric Iron. A 1.5- to 2.5-g. sample of iron wire is weighed to the nearest half-milligram, placed in the volumetric flask, and dissolved in sulfuric acid. The student dilutes the sample, measures out 40- to 50ml. aliquots, reduces the ferric ions by means of the Jones redudor, and titrates the iron with standard permanganate. Sodium oxalate serves as a standard. Tolerance: 6 parts per 1000. 4. Iodimetric Copper. A weighed 1.5- to 2.5-g. sample of copper wire is dissolved in nitric acid in the student's flask. Aliquot portions are analyzed by titration with 0.1 N sodium thiosulfate which has been standardized with pure copper or with potassium di6 6.0 chromate. Tolerance: 6 parts per 1000. nejea Greater than 6.0 5. Gawimetric Chloride. A solid sample is analyzed The amount of iron wire given to a student on one for chlorine by the Gooch crucible method. The unoccasion was 2.056 g. His average reported result was knowns are mixtures of sodium and potassium chlo2.051 g., a deviation of 5 parts per 2000 or 2.5 parts per rides which have been ground together in a ball mill. 1000. The grade was "9" and this is considered to be A set of twenty or more samples is kept a t all times to an "A" analysis. If the deviation exceeds the limit, prevent too frequent use of the same one. Unknowns the student must obtain another sample and repeat the 1 to 4 are, of course, individual and different for each determination until an acceptable analysis is made. student. Tolerance: 0.3y0 (e. g., if 59.65% is correct * The work as outlined is the result of the effortsof Professors then the value 59.35% would be barely acceptable. W. A. Noyes, Jr., T.F. Young, W. E. Vaughan, and the writers. Since this is the first gravimetric unknown the students
handle 0.1% is deducted from the calculated percentc. Mized Onides. No separation of iron and alumiage to obtain the value for which a grade of " 1 0 is num is made. Tolerance: 0.3%. (This is perhaps given). high for dolomites of low iron content but no emphasis 6. Grarimetric Iron. Difficulties have been en- is placed on the iron determination because of a precountered in preserving unknowns made of mixtures vious gravimetric determination.) of iron salts. As alternatives we have used individual d . Calcium O d e . Determined gravimetrically as samples as in the volumetric iron or a set of standard CaO. Tolerance: 0.3%. solutions. These are made up by weight to contain e. Magnesium O d e . Tolerance: 0.3%. 0.75 to 1.50% iron; they consist of iron wire dissolved NOTES ON THE DOLOMITE ANALYSIS in acid. The student weighs out 10- to 15-g. aliquots I. Determinations (b) to (e) must be reported siand analyzes them by the usual procedure including reprecipitation. The percentage of iron expressed to multaneously and considered as a whole. In case all three significant figures is reported. Tolerance: 0.04%. analyses but one are good that one may be accepted 7. Grarimetric Sulfur. The unknowns are mix- even if the results are considerably beyond the tolertures of potassium sulfate, anhydrous sodium sulfate, ances listed. and sodium chloride. These are dried, weighed, and 11. If, as often happens, the aualysis is correct exground together in a hall mill. The student may use cept for the calcium and magnesium the student is either a Gooch crucible or a filter paper during the given a synthetic mixture to analyze instead of being analysis. Tolerance: 0.3%. compelled to repeat the tedious and time-consuming The second course covers a period of time similar to complete analysis. I n no case is the calcium or the thefirst. As a preliminary exercise the student cali- magnesium report accepted if the other is rejected. brates his set of weights. The unknowns include: The synthetic mixture is a known volume of a standard .volumetric chloride, electrolytic separations, dolomite, solution of calcium carbonate in hydrochloric acid and Rjeldahl, and a problem. a solution of magnesium oxide in the same acid. The solutious are of such strength that when 40 to 50 ml. : PREPARATIONS AND DISPENSING OF UNRNOWNS of each are mixed and diluted in a 250-ml. volumetric ' 1. VolumetriG Chloride. The same samples are flask a 40- to 50-ml. aliquot will contain the same used as for the gravimetric doride. Silver nitrate is amounts of calcium oxide and magnesium oxide as a onestandardized by sodium chloride according to the gram sample of dolomite. The grams of the respecMohr, the Volhard, or the Fajans adsorption method tive oxides received are reported. Tolerances: Calwith dichlorofluorescein indicator.* The unknown is cium oxide, 8 mg.; magnesium oxide, 10 mg. analyzed by the Fajans method and one of the others. 111. A record is kept of each student analysis on Tolerance: 0.25%. every solid sample. This has proved very valuable in 2 . Electrotytic Separation of Co9er and Nickel. detecting changes in the samples and inaccurate analThe unknowns are solutions made up by weight with yses. The dolomites have been particularly troublecrystalline copper and nickel sulfates as hydrates. some. In Table I1 is given the difference between the Sulfuric acid is added to prevent hydrolysis. The manufacturer's and student analyses for a particular composition varies from 0.200 to 0.50070 copper and sample. The student results are the average of a set 0.200 to 0.500% nickel. The student uses a weighed of ten consistent determinations. 10- to 15-g. sample for analysis. An electro-deposiTABLE 2 tion apparatus with a rotating anode is furnished as FAULTY A N UY~~ 09 S A DOWYIT~ part of the laboratory equipment. This is used for Diffarmcr i n % the separate depositions of the two metals. Tolerances: D~lrrminolion Sludcnr - Mnnuf. Vdur 0.006% for each. Loss OD Ignition 1.6 Silicon Dioxide -0.11 3. Dolomite. Purchased samples of synthetic or Mired Oxides -0.37 natural dolomites are used. caieium O*tde -1.12 Magnesium Oxide -0.41 a. Loss on Ignition. This is determined in the usual way with platinum crucibles. The residue is 4. Choice is offered of the following: used for the subsequent analyses. Results are rea. Nitrogen by the Kjeldahl Method. Analyzed ferported before continuing the analysis because a loss at tilizer samples are used. Tolerance: 0.3%. this point would invalidate the future work. Tolerb. Simple Electrometric Titrations. The method of ance: 0.5%. Hildebrandt is recommended. Curves are made for b. Silicon Dioxide. Neither a second filtration nor strong and weak acids and bases. purification with hydrofluoric acid is required for 5. Problnn. For his final determination each stuminerals of low silica content (below loyo). Tolerdent is assigned a problem which serves to introduce ance: 0.3%. him to the chemical literature and to simple research : , * FAJANS AND WOLPF, Z. anorg. allgem. Chem., 137,221 (1924). methods. An effort is made to illustrate many types For an extended list of references see WILL- AND EVP.MAN, of analytical technic in the choice of problems for a "Elementary quantitative analysis," D. Van Nostrand Co., New York City. 1933, p. 117. t H~DEBRAND. J. H., I.Am. Chem. Soc., 35,847,1538 (1913). ,
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class so that the students, by observation of other prob- to develop or are allowed to assist in current dbpart* lems, become acquainted with new methods. Among mental research where analytical methods are being the more common assignments are: micro determina- used. The student must look up his own procedures in the tions such as copper, lead, phosphorus by colorimetric methods; indiiect determinations such as chloride and literature, select a method of analysis, bring it to the bromide; gas absorption methods such as carbon in instructor for approval, test it with known reagents, and steel; use of new reagents such as ceric sulfate and or- finally analyze an unknown by the method. His ganic precipitants; spot test determinations, such as written report must include a complete discussion of the Gutzeit arsenic test. Most of the problems are the method and of the special dificulties encountered. The students have indicated many times that they selected from current articles in the Analytical Edition of Indust~ial and Engineering Chemistry. Occasion- consider the problem the most interesting and valually better students are given completely new methods able part of their analytical training.
