PROBLEMS of TEACHING QUANTITATIVE ANALYSIS I. A n a l p e d Samples
M. G. MELLON AND J. P. MEHLIG* Purdue University, Lafayette, Indiana
One of the important objectives for a course in elemen- by comparing their samples with those of their neighbors. tary quantitative chemical analysis i s to teach the student The hope of eliminating this practke as far as possible to make acceptable determinations. A s a means of as- justifies the effort to secure a supfly large enough to procertaining a n individual's ability to work effectively in vide series for alternate years with a different sample for the laboratory probably nothing i s of more value to the in- each student in any one year. Occasionally also the samstructor than a n adequate supfly of dependable analyzed ples which may be purchased are not homogeneous, stable, or carefully analyzed; or the combinations desired are unsamples. Although such samples are undoubtedly necessary available. The instructor's only feasible alternative to purchasing wherever classes contain many students, the number obtainable commer~iallyis, in most cases, not suficient for prepared samples i s to build up his o?un series. T o do large groups. Certain students are always encountered this for large classes involves various problems, some of who exert much energy and ingenuity trying to analyze which are considered in the follmving discussion.
+ + + + + + CHARACTERISTICS OF GOOD SAMPLES
P
ERHAPS the first requirement of a set of samples is that they shall be of known composition. That is, the instructor should know the amount in each sample of the constituent the student is to determine. Ordinarily this value should he obtained with the method and under the conditions used by the student, preferably with a little better precision than that of the best students. Occasionally an instructor pre-
-
* N o w at Oregon State College, Department of Chemistry.
fers to judge the work of his students in terms of the average value reported by several of the best students rather than in terms of his own analysis. Perhaps of equal importance is the requirement of stability. Samples of changing composition are obviously worthless except at the time of analysis. Lundell' emphasized this difficulty in the preservation of certain samples supplied by the Bureau of Standards. Instability and uncertainty in composition eliminate from consideration many substances, familiar exam-
LUNDELL, I d . Eng. Chem., Anal. Ed., 5,221 (1933)
ples being bleaching powder, hydrogen peroxide, caustic soda, and similar commercial products. Every compound and mixture used must be carefully observed. The authors purchased two series of sulfate samples and one series of chloride samples which have proved to be quite unreliable. As another example, mixtures of carefully dried sodium carbonate and basic cupric carbonate were unstable when stored in clear glass bottles. Smith, Hardy, and Garda studied the possibility of mechanical instability from segregation by vibration in samples containing substances of different specific gravities. From the standpoint of securing satisfactory checks in determinations i t is also very important to have the sample homogeneous. Suitable homogeneity may usually be secured for ores and salts by grinding the substances to pass a lO@mesh sieve and mixing in a ball mill. In view of the labor involved in preparing mixtures i t seems desirable to select for a given combination substances which will make the sample suitable for determining more than one constituent. Thus, a mixture of ammonium chloride and calcium carbonate might be used for nitrogen, chlorine, calcium, and carbon dioxide. One should keep in mind, of course, the objective of the determination. If it is to determine copper, for example, after separation of interfering elements, then one must use something containing those elements, as an ore, rather than pure cupric oxide diluted with inert material. By proper selection of material one may accomplish other ends also. In the mixture of ammonium chloride and calcium carbonate mentioned above it is impossible for the lazy student to guess at the amount of carbon dioxide from a simple determination of the loss on ignition. In general, a series of samples should be as nearly as possible uniform in appearance to prevent determinations being made by comparison. In a collection of one hundred brasses this becomes a real problem. The differencebetween successive mixtures in making sixty to eighty dilutions of cupric oxide with flint are hardly perceptible, although the end samples differ widely. In this case the relative degree of paleness gives some indication of the amount of sample to use. In some cases one may intentionally make a series more nearly uniform, as in the use of a solution of caramelized sugar to color diluted commercial vinegars. This particular series may be extended far beyond the range of acidity of ordinary vinegars by using the caramel solution and acetic acid. Instructors do not agree about the difference in percentage that is desirable between successive members of a series. The precision ordinarily attainable by the student is an important factor to consider. Where close checks are readily obtained in duplicate determinations the samples may be more closely spaced than in the reverse situation. FrostJ suggested diierences of 0.4 to 0.5 per cent. for samples such as chlorides and %TH, HARDY, AND G m , ibid., 1,228 (1929). FROST, J. CHEM.EDUC.. 3, 562 (1926).
-
sulfates. To maintain this difference and a t the same time to provide a large number of samples requires for dilution a stock material containing the desired constituent in an amount as high as possible. From this standpoint cupric oxide is much better than the ordinary copper ore. In hematite iron ores one cannot get higher than pure ferric oxide, containing about 70 per cent. iron. It is quite difficult to find suitable compounds containing certain elements in amounts su5cient to make the desired dilutions. Except for work in gas analysis, samples ordinarily will be either solids or solutions. From most standpoints it seems preferable to use solids for preparing samples unless there is no alternative. The authors are unacquainted, for example, with any solid vinegars. PACILITES REQUIRED
For the efficient preparation of large numbers of samples one should have several pieces of special equipment. An oven is necessary in some cases for drymg the sample before it is ground. Some materials, such as rocks and coal, are so coarse they must be crushed first, as with a jaw crusher. For finer subdivision grinding is necessary, in machines such as roller mills or rotary pulverizers. Better yet, a t least for some substances, are rod and ball mills, the former being particularly effective for hard materials such as granite. As i t is generally desirable to bring salts and ores down to a size of a t least 100 mesh, sieves should be available. Where weighed mixtures are to be made, one needs a reliable balance and weights for a kilogram or more. If one ventures upon the production of alloys, there should be available a furnace suitable for producing a homogeneous system of the melted constituent elements. I n this case one needs a drill press or other satisfactory means for taking samples. MATERIALS
If the series consists of solutions one may have considerable choice of materials. The chief requirements are that the system shall be stable and contain the constituents to give the experience desired. For solids the situation varies. Samples with high percentages of aluminum are practically unavailable, while a number of substances will serve for determining carbon dioxide. In general, one uses naturally occurring materials, such as rocks and ores, commercially prepared materials, such as steel and soda ash, or the more or less pure common chemicals, such as calcium carbonate and sodium chloride. The variety of commercial materials is frequently unsatisfactory in having a spread in analysis too limited for a good series. Mixtures may be made of certain stocks having different compositions, such as limestone and dolomite; or high-grade stocks may be diluted with inert material, such as iron ore with flint finely ground for ceramic work. By persistent inquiry - . one may s e c i e brasses with a reasonably good spread in composition.
Materials such as ores and salts must he capable of being both ground and mixed. Sometimes there is too great a difference in the specific gravities of the substances. Fluffy magnesium carbonate is troublesome. Some substances will not grind unless well dried, and even then caking may be encountered when they become sufficiently fine. Putting the materials directly from the oven into the hall mill or warming the latter itself helps in some cases. Occasionally two materials that stick when mixed in certain proportions may be handled by the addition of some flint as a third substance. Workable mixtures are indicated in Table 1 for a number of determinations. Doubtless other equally
Conslilurnl Dclnnrined
00. ca, COt Cn, CI, COt
cr
M8
Mn Ni N*
Pb PO, Soda ash (CI. COtl
TABLE 1 Mlxrua~S POX SAYPLES Midwe KHCsHgO. NaCI Solutions of HSOI HCIHIOI caramel ~olution As.0. &SO, Paris green Narc03 (NaCI) BaCOa KNOa B=(NOsb NauC01 NaCa04 KSO. CaCOs Narc01 CaCOa NaCl (NlbCI) KCoOl KC1 CuO i flint IKdOd FuOl flint KC1 NnCI %at Marble or lime$tonc dolomite Limestone FezOs, CaCOa, or silica Marble FesOa, MgCOa, or silica
+
+ + + + + + + + + + + + + + + MgCOs + MnOa + KC1 (KISOd NiCOa + NanCOa (N&)tSO< + powdered sugar (and flint) Pb(NO.1. + KNOz KHaPOa + KC1 NasC08 + NaCl KC08
KBOI --. + NaCl * h new series not chccked for stability.
SO. --.
,
good combinations are possible. Data regarding their practical value tend to accumulate slowly since one is likely to use a satisfactory combination without further search. In making any series one would choose, if it is equally good, a compound of sodium rather than of potassium on account of the cost. Likewise, of two substances, the one that is stable under the greatest range of conditions would be selected. Thus, anhydrous salts are preferable to hydrates, although reasonably good results have been obtained with mixtures containing ferrous ammonium sulfate, the samples being used without drying. Potassium sulfate is preferable to ammonium sulfate, but certain samples containing the latter salt have been used for several years. Stable rocks and ores may generally be diiuted with some non-interfering or inert substance, such as potassium sulfate, sodium chloride, calcium carbonate, or flint. PROCEDURE
The materials for preparing mixtures of solids having been decided upon, the separate substances, previously ground to 100 mesh, are weighed and mixed in a hall mill, the number of balls found to give the best results being used. If caking is encountered, only a few halls
should be used. Sometimes the use of glass jars (large screw-cap bottles serve) and marbles is of advantage both in enabling one to see how the mixing proceeds and in avoiding caking, presumably because the surface is smoother. Usually twenty minutes' mixing at 60 to 70 R.P.M.is sufficient. With the mixing proceeding satisfactorily, a triple ball mill with a different mixture in each jar keeps one individual reasonably busy screening, weighing, bottling, and operating the mill. The amount of materials to use depends chiefly upon the nature of the material and the amount of sample desired. A mixture of 400 to 800 grams is suitable for a ball mill ten inches high and nine inches in outside diameter, if the material is not too light. This provides a stock sample sufficient to last some time. Frequently, one has the alternative of mixing and analyzing each product or of analyzing the stock material and making careful dilutions of it. In the former case one may weigh less carefully, unless he later wishes to reproduce the mixture; but every mixture has to be analyzed. In the latter case careful handling of material is necessary. Although this method has been found reliable in a number of cases, in a new series several samples should be checked to determine the correctness of the calculated percentage in the diluted sample. The screw-cap bottles recently adopted by several chemical manufacturers are particularly suitable as containers for finely pound solids. Different sizes may be selected to suit one's requirements. If solutions are used, glass-stoppered bottles are probably most useful. For preparing, storing, and handling samples an adequate sample room should be available, containing sufficient shelving, a work-table with sink and the necessary service lines, and a truck or small table on wheels. Special shelving is desirable for stock pieces of metals. DISPENSING SAMPLES
Where several thousand individual samples have to he given out to students each year, one must consider how best to do it. Experience indicates the preferable method is to put into suitable containers during the summer enough samples to provide for some excess over the estimated requirements for the coming year. For solids such as limestone 16-ml. homeopathic vials with cork stoppers serve well. One company supplies such individual, bottled samples serially numbered. The dri11.ings of certain metals and alloys are better handled in small envelopes, a practice followed in commercial laboratories. Solutions are probably most safely handled in test-tubes drawn out and sealed off, thus avoiding difficulties with cork or glass stoppers. I t is desirable to have at least a semester's supply of the individual samples in a case or cabinet conveniently located for distribution, with each drawer or compartment clearly labeled. While the stock samples kept in a locked room may
be labeled to suit the instructor's inclinations, the individual samples must be designated in such a manner as not to reveal anything to students, some of whom show their greatest originality when trying to work out the instructor's system of samples. Although it involves more work than some "key" systems, in the authors' experience the only fwl-proof practice is to
give each sample a serial number. At the time this is done there is entered at the corresponding number in a record book the stock number of the sample. In checking an analysis one must 6rst consult the book of serial numbers to determine the stock number. The latter, together with the analyses, are kept in a separate book.
