A NEW METHOD oJ APPROACH for QUALITATIVE ANALYSIS D. J. BROWN
AND
H. ARMIN PAGEL
The University of Nebraska, Lincoln. Nebraska
I
N contrast to the usual method of procedure of "group knowns" for the first few analyses, we require the complete analysis of pure salts, acids, bases, or anhydrides. In a preliminary discussion on the constituents of pure inorganic substances, the instructor makes the following assignment: Compile a list of (a) six salts, each having a diierent acid radical with two basic radicals; (b) salts with one basic radical and two acid radicals; and ( 6 ) salts with a total of four or more radicals. The entire list is to be selected from the usual manufactured substances. Manufacturers' catalogs and textbooks are made available to the class. At the next meeting of the class this assignment is discussed and serves as the basis of the following conclusious:
(1) Salts usually contain only one acid radical. If they contain two basic radicals (the usual maximum), at least one belongs to the alkali group (including ammonium). (2) All basic radicals of the analytical groups preceding the alkaline earth group, and magnesium, may appear as constituents of the common double salts. (3) Except in the alkali group, only one of the basic radicals occurs in any one analytical group.
These facts are discussed in relation to the text. We use A. A. Noyes's "Qualitative Analy~is,"~ and all references will be to this text. Special attention is called to the fact that we may have a salt of an amphoteric hydroxide, permanganate, chromate, arsenate, arseuite, antimonate, or stannate as an acid radical, in which case a precipitate from the acid radical appears with the basic group. In such cases the tests for all other common acid radicals will be negative. Since the usual salts of the amphoteric acid radicals are those of the alkalies and less frequently those of the alkaline earths, these introduce no complicatious. Chromates and permanganates of the iron group are not issued, but the fact is stressed that if all other basic radicals are proved ahsent, the complete analysis of the iron group would be necessary. To simplify the work, no phosphates or salts which are insoluble in procedures 2 and 3 are issued. Any common salt within the limits of Noyes's text, and l i t a t i o n s given above, may be quickly identified by procedures 1-3; 11-13; 21-3; 51; 71; 81-3; 91-2; 101-5; 115-16; with the following additions. (a) Use about one-third of a ml. of the powdered samples for procedures 2 and 101 and, in the latter case, use only 10 ml. of sodium carbonate solution, MacmiUan Co.,New York City, 1923, 9th edition.
filter and dilute to 12 ml. (b) To differentiate between chloride and cyanide: To 1 ml. of the filtrate from procedure 101, add 1 ml. 6 N HNOa and a few drops of ferrous chloride solution and shake thoroughly. A blue color indicates cyanide. ( 6 ) To differentiate between chlorate and hypochlorite, repeat procedure 102, omitting the addition of sodium nitrite. If a precipitate forms, filter. See Note 9, procedure 101, and Note 1, procedure 102. (d) To differentiate between oxalate and fluoride: To the contents of the test-tube in procedure 103, add 2 ml. of 18NH2504;heat nearly to boiling and add two drops of 0.2 M KMn04. Decolorization of the permanganate with evolution of gas indicates oxalate. (e) The nature of any gas evolved in procedure 2 aids in identifying carbonate, sullide, sulfite, nitrite, and cyanide. To distinguish which member of the alkaline earths may be mesent. the carbonate precipitate in procedure 7i is dkolved in 6 N HC1 and the resulting solution examined spectroscopically. Since the student does no preliminary procedures, the following mimeographed method of procedure is given : To "run a blank" repeat the procedure or series of procedures in absence of all constituents that would give a positive result. For P 102, for example: To 1 ml. 3 N NaCOs instead of the filtrate from P 101, add 5 ml. water, etc., as directed. If no prscipiiale remains, the reagents are free from any constituent that would give a positive result in this group. If "a blank" gives any other product than a clear solution, a special statement is usually included as a part of the procedure or the nates. In this case if a prscipitale remains, it proves that at least one of the reagents used is impwe. To detect which, omit the 3 N NaCOs, and if the impurity is not proved to be in that solution, omit the NaNO1, and if necessary, omit both, in each case treating the mixture with all other reagents as directed in making an actual determination. The reagents that might intluence the results in P 102 may be checked by the method given in P 107, Note 2, but if the chemist does not know the probable effect of the variation, or if he desires to observe the chemical and physical changes, the steps of the procedure must be duplicated as nearly as possible. To "run a control" duplicate the procedure or series of prcedures with the addition of a test solution. For example, if iodide is thought t o give the positive results in P 102: To 1 ml. 3 N Narc01 add 0.1 ml. to 2 ml. of iodide test solution, the amount indicated by the permanent precipitate formed by the sample that you are analyzing, and in all other respects complete the procedure as in making the actual determination. If you have done the procedure correctly and your reagents are correct, the observed results are those indicated in the procedure and notes for iodide. Sometimes constituents which are detected in other groups have effects that are not observed with a "blank" but give peculiar reactions. For example, "run a control" for P 102 using the chromate test solution.
In our course we require the analysis of four difierent salts, chosen with a view to emphasizing both the various group isolations and the relations previously discussed. This brings forcefully before the student the importance of accuracy in every procedure and thorough observation of all results. Since lead is the only b&ic radical which occurs in two different groups. it constitutes the only case in which a smaller F e cipitate follows a larger one, hence the student has a
check on his work of group separations. We constantly stress the fact that proof of absence of a group is as important as the proof of its presence, since the validity of all later work is based on these clear-cut group separations. The student must report the formula (including water of crystallization if present) and the name of the salt, acid, base, or anhydride. Special attention is called to the fact that the substance reported must have properties which agree with those observed in procedures 1, 2, 3, and 101, etc. If the report is incorrect the data are carefully reviewed with the student. If the error is due to incorrect manipulation the student must repeat the work under the direct supervision of the laboratory instructor to eliminate a repetition of the incorrect procedure. Preceding the first report a number of positive and negative procedure results are discussed in class. However, with all of this preliminary discussion, the poorer student frequently has trouble in orienting his observations. Therefore in case a student's report is wrong due to incorrect interpretation, he is given arbitrary examples requiring a similar type of interpretation. This method of instruction is rigidly followed during the earlier part of the course. Following the analysis of the four simple substances, the next unknown is a mixture containing a maximum of two salts (one of them present in small proportion) together with an insoluble residue. Only the acidsoluble constituents are determined. In some cases the salts may have the same basic or acid radical. In every case, however, the basic constituents are limited to the copper, arsenic, and iron groups, and the acid constituents to the arsenic and chloride groups. The student reports the actual salts present in the acidsoluble extract, and the residue is reported as a component of the mixture. Attention is called to the fact that the separation of any group is in fact only a repetition in principle of the analysis of a pure salt, but that the method for the initial treatment of the precipitate or solution is very definite, since the acid radical for the basic groups, or basic radical for acid groups, is always known. Also, since many of the substances precipitated in a group have properties more nearly alike, greater skill is necessary for their separation. The next unknown mixture consists of a maximum of three salts in addition to an insoluble residue. Again, one salt may be present in very small proportion. These radicals are usually distributed between only two basic and two acid groups, including the akaline earth and sulfate groups, in addition to those groups assigned in the preceding unknown. The analysis is made for "aqua regia-soluble" constituents. In addition to the usual report, the student includes the approximate percentages of certain radicals which are easily estimated. For this estimation we give the following mimeographed procedure: To determine the approximate amount of a constituent present
in a sample analyzed, treat two definite amounts of the apprapri-
ate test solution in parallel by a slight modification of the final
procedure, usually as outlined in the method of running a "control." Pour into a clean, dry test-tube sufficient to run duplicate "controls" and return t o your desk t o do the actual procedure. If the product of the procedure is very small use 0.1 ml. and 0.5 ml. of test solution but if exceedingly large, 5 ml. and 20 ml., and far intermediate amounts use appropriate volumes. Run the control on each, using vessels of the same shape, volume, and color as used in the actual determination. Test-tubes usually are best. Compare the intensity of color, the turbidity, or amount of precipitate produced by your sample with that produced by the test solutions and estimate the milliliters of test solution necessary t o produce the same results. The estimation can he made more accurate by m n i n g a larger number of controls within the probable range. Another method is t o dilute the more concentrated solution until i t has the same intensity of color or turbidity and then compare the volumes. The estimated volume of test solution necessary t o yield the product that duplicates the amount in the portion analyzed or represented in the test is used t'o calculate the percentage if the sample is a solid, or grams per liter if the sample is a liquid.
The preceding analysis is followed by a complete analysis of a mixture of salts within the limits of the text. This in turn is followed by various types of
analyses such as alloys, solutions, etc., as suggested in procedure 1 of the Noyes text. The constituents of these analyses are so varied as to bring before the student a maximum variation, both in procedure and quantities of constituents present. SUMMARY
This method brincs - before the student the simplest separations iirst. The idea of "blank" and "control," actually used bv chemists.. eives the student a ~racticalmethod of checking his work which we believe superior to the usual analysis of "knowns." The "estimation" gives a check on the quantitative separation of radicals, a point that should be stressed in qualitative separations. (Compare Noyes's preface.) Except for the first two mixtures, the student is dealing with complete analyses. These two mixtures, even, bring before the student special methods of approach.
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