An Easy and Low-Cost Technique for the QualitativeAnalysis of Silicates Daniel Vilale Facultad de Quimica, General Flores 2124. Montevideo, Uruguay Silicate characterization in solid samples has traditionally been regarded by our students asone ofthe stumbling hlocks in the laboratory course of Qualitative Analytical Chemistry. The classical method' is based on the conversion of silicates into silicon tetrafluoride by means of the combined action of CaFz and H2SOa, placed in a platinum crucible. SiF4is collected in a piece of filter paper rinsed with a nitric solution of ammonium molybdate. Benzidine is then added, and a blue spot is developed with ammonia vapors. This technique has two disadvantages. I t requires expensive materials and reagents and, if cairied out hy beginners, is not always satisfactory owing to the complex manipulations needed and the relativelv low nensitivitv it Dresents. It is essential, from the a n a k i c a l point of Giew; to have an accurate method for the characterization of a given substance. However, in the case of silicates this is a crucial problem as the presence or lack of silicates will determine how the subsequent analysis of the sample is to be performed. In our laboratory we have replaced the technique described above by an easier method to analyze silicates. In this method a small amount of the substance to be analyzed (approximately 200 mg) is placed together with the same amount of CaF7 in a oolvethvlene centrifuee tube; four or five drops of co&entr&ed H ? S O ~are then added. Aelass rod withalootxasshown in the Fiaure I , isinserted intoihe cork stopper at the top of the centrifuge tube. A drop of distilled water is suspended a t the loop of the rod. The tube is then heated in a water bath (Fig. 2) a t approximately 80 "C for 1 min, and the suspended drop is observed. A turbidity or white precipitate will confirm the presence of silicates. This method is based in the following sequence of reactions:
+
HaO, CaF, SiO, + 4HF
--
Flgure 1. (len)Diagram showing the glass rod with a loop used for sillcate characterization. A = ca* stopper. B = glass rod with a loop, C = drop of water, D = polyethylene csnnifuge tube, E = sample + CaF2+ H2SO+
2HF + CaSO, SiF, + 2H,O
Due to its volatile nature, SiFa given off by the reaction mixture hydrolyzes in the suspended drop, thus producing a SiO, nHpO p r e ~ i p i t a t e ~ , ~ .
.
+
-
SiF, (n + 2)HZ0 SiO, .nH,O
+ 2H&3iF6
The method described above has proved to be easier for our students to perform. Analytical results have been a t the same time considerably improved, thereby diminishing the number of misleading analyses because of the erroneous a .n.~ r a i s aof l the oresence of silicates. Furthermore, this technique has morcsensitivity than the previous one, as it allows amounm of silicates smaller than 1% to be detected in any kind of sample.
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Feigl, F. Spot Test In Inorganic Analysis, 5th ed.; Elsevier: London. 1958; p 499. Vogel, A. TextbwkofMacmandSemlmicroQualitativeInorganic Analysis, 5th ed.; Longman: New York, 1979; p 352. Burriel. F.: Anivas. S.: Lucena. C.: Hernandez. J. Quimica AnalF
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Journal of Chemical Education
Figure 2. (right) Apparatus used for silicate characterization
