Carl Mahr University
of
Marburg Germany
Preparative Separations Replace Typical Qualitative Analysis
Until a few decades ago, only inorganic reagents were used in inorganic qualitative analysis. It was carried out by systematic separations which were essentially a continuous repetition of certain operations, such as precipitation, decantation, filtration, washing, and dissolving. The occupation of the student with qualitative analytical problems was the recommended way of getting a freshman acquainted witb the laboratory techniques and with inorganic chemistry. The Changed Situation
Meanwhile, methods in applied qualitative analysis have changed completely. Sowadays, selective organic reagents and novel methods are widely used, e.g., spot tests, paper chromatography, and ion exchange techniques. This causes some pedagogic difficulties: One can teach qualitative analysis in the form which is used today, hut then it is necessary to postpone it to a later training period. The alternative is to keep the classical form, but to neglect bringing the student up to date in this branch of chemistry. As a third possibility, the analytical introduction to chemistry can be replaced by inorganic preparation techniques or physicocbemical measurements. But in doing so, one loses the chauce of putting before each student a different problem. By varying the composition of the samples the instructor independent charges the student in a certaiu sense 1%-ith research work.
chemicals used are not necessarily the most selective and sensitive reagents for the detection of the particular metal ion. Instead, the tests reveal the reactions of the element with simple inorganic precipitating agents, the chemistry of the complex compounds i t forms, and the inorganic reactions in whicb the element diiers from those of the same group of the periodic system or from others similar in chemical hehavior. It is an old experience of the chemistry teacher that the important reactions of the elements impress the student only if they are performed repeatedly. Also, skill in the most important practical chemical methods like precipitation, decantation, washing, dissolving, etc., is obtained only by repetition. What was formerly the pedagogic task of qualitative analysis is now taken over by the preparative separations mentioned here. These preparative separations are essentially a reinstatement of the classical qualitative analytical course of separation. The usual classification of analytical groups is therefore retained. The Separations
The student obtains 150-200 mg of a soluble mixture of metal salts or 5 ml of a solution containing 15-20 mg each of different metal ions. He is supposed to separate the metals in form of precipitates that are sparingly soluble. First the HCI group is precipitated witb dilute hydrochloric acid and separated by centrifugation. The precipitate is washed and the lead chloride is extracted witb hot dilute hydrochloric acid in the usual way and then treated with chlorine or A New Possibility bromine water. Mercury present as Hg2Cladissolves as However, there is still a possibility of exploiting the HgC12 while silver chloride remains undissolved aud is pedagogic advantages of the old inorganic analytical separated by centrifugation and washed. The centriftraining without giving the student a completely wrong ugate is boiled briefly to remove the halogen, and picture of today's standard of analytical chemistry. A sodium hydroxide is added. HgO precipitates which few years ago, the author1 suggested replacing qualitais separated and washed. tive analysis of cations by so-called preparative separaThe H2S group is precipitated from the centrifugate tions in the beginner's laboratory course. The students of the HC1 group and separabed into t,he arsenic and get a mixture of metal salts, but they do not have to copper groups in the usual manner. The next steps also identify the cations whicb are present. Instead, they follow closely the classical analytical procedures. are required to deposit the metals as inorganic precipiHowever, it is not sufficient to recognize copper on the tates in as pure a form as possible. Thus, they are basis of the blue color of the ammonia complex. It bas trained in a method which bas become most important to be precipitated as CuSCN and separated by centriftoday for pre-separations in trace-analysis, for separaugation. Tin, after reduction by iron or nickel, is not tion of radioactive elements, and for preparative work. simply identified by its reducing properties but is preIn the present time, these techniques are more significipitated as SnS and separated. cant than the qualitative analysis which, carried out in In the (NH,),S group more pronounced deviations of a solely chemical manner, has become less i m p ~ r t a n t . ~ the separations suggested here from the classical proIn the general chemistry laboratory at the University cedures are necessary. After precipitation as hyof Marburg the beginners study the reactions of the droxides, iron and titanium are separated by extracting metals in test tubes in the usual lvay. However, the the HCl solution with methyl isobut,yl ketone. Nickel is precipitated as [Ni(NHs)6](C104)2 and the cobalt, dissolved in t,he filtrate as pentammine aquo complex, is ' MAHE,C., Angm. Chem.,67,712 (1955). STRONG,FREDER~C E., J. CIIEIL.EDL-c.,34, 400 (1057). precipitated as the sulfide. The separation in the Volume 38, Number 8, August 1961
/
407
ammonium carbonate group finally is based on the usual precipitations as barium chromate, strontium sulfate, and calcium oxalate. In general, a separation of the alkali metals is not required. I t can be carried out relatively easily by extracting lithium as chloride with ethanol and by precipitating potassium as perchlorate. After performing the separations the student hands in the centrifuge tubes which contain each of the precipitates of the metal ious present in the original mixture. The instructor judges the student's performance on the basis of purity, color, form, and quantity of the precipitates. This is done more easily and correctly than is possible by reading the usual written lab reports.
has been recommended recently for qualitative analysis.3 For several semesters, this method has proved to be quite successful in the department of chemistry a t the University of Marburg. The separations are carried out according to the textbook of the author4and, with the usual amount of training, they yield good results in independent work and critical thinking. Also they effectively prepare the students for qualitative analysis which can now be taught in a completely modem form. In the book mentioned above4 the description of the separations is therefore followed by a discussion of the new techniques of qualitative analysis, the semimicro method with special reagents and the analysis by paper chromatography.
Ofher Advanfagas
Acknowledgment
In these separations, the student is asked to carry out only inorganic reactions. He cannot apply in advance color tests with organic reagents, the nature of vhich he cannot yet understand. The separation problem has the same wide scope in varying the components as the former qualitative analysis. The student has much choice in selecting his 0 ~ ~ methods 1 1 for separation, as
The author wishes to thank Professor B.W. Starke of the University of Marburg for help received during the translation of this article.
408
/
Journal of Chemical Education
a
STUBBE,MORRISF., J. CAEM.EDUC.,36,387 (1959).
MARE, C., "Anorganisehes Grundpraktikum," 2nd ed.. Verlag Chemie, WeinheimfBergstr., 1961. 4
