Harold F. Walton University of Colorado
Boulder
Experiments in Inorganic Paper Chromatography
These experiments are typical of those used in geochemical work for the detection and estimation of trace metals (3, 6). They are sensitive, fast, and simple enough to he done in a field laboratory. The solutions are spotted onto slotted filter paper (Whatman CRL/1 chromatographic paper, 11 X 21 cm) and then dried to about 52% relative humidity in a constant humidity desiccator (moist Ca(NO& or Mg(NO& crystals as desiccant). This drying is important for good results. After elution with the proper solvent, the spots are developed by spraying1 the paper with a complexing agent. Comparison with standard papers gives an estimation of the concentration of the unknown. Uranium
This method depends upon the solubility of uranyl nitrate in ethyl acetate, which is aided by "salting out" from the water phase (1-5). A 1000-ppm (one gram of uranium per liter) solution is made by dissolving 0.295 g UaOs (or equivalent amount of the nitrate) in 65 ml of cone HNO, and 80 ml of water. This solution is saturated with aluminum nitrate (about 150 g Al(N03)3.9H20)and made up to 250.0 ml with water. Solutions of 100 and 500 ppm are prepared from the 1000-ppm solution by diluting the solution with dilute HN08 (1:3 HNOBto water by volume) which has been saturated with aluminum nitrate (about 2 g/ml dil. HN03). The eluting solution is made up in a 600-ml beaker just before use by mixing 6 ml conc HNO, with 1 rnl of water and then adding 30 ml of ethyl acetate (maximum life 2-3 hr). A reference paper is made by spotting a strip of paper with 0.005,0.010, and 0.020 ml of the 100-ppm standard (0.5, 1, and 2 pg U) and 0.010 and 0.020 ml of the 500ppm standard (5 and 10 pg U). Two or three spots, with 0.005 ml of 10yo Fe(NO& added, are also placed on the paper for comparison (0.025 nd maximum volume for 1spot). The paper is air dried and placed in the constant humidity desiccator for a minimum of hr. The spotted paper is then crimped on one end strip to prevent too rapid flow of solvent, rolled into a cylinder, fastened with a paper clip, and placed in the ethyl acetate solution wit,h the spots at the bottom. The beaker is covered and t,he solvent allowed to rise to within 1 cm of the top of the strip (about 20 min). The paper is
An all glass spray bottle is best, but a perfume atomizer may be used if the metal nozzle is not allowed to rust.
drained, air dried, and then sprayed with 5% potassium ferrocyanide. Uranium appears as chocolate brown stains of uranyl ferrocyanide at the extreme limit of the area reached by the ethyl acetate. Iron will also appear as B. blue spot in the original position since it does not migrate. Samples of soil, rocks, and ores may be prepared for analysis by digesting the sample in HN03-A1(N03)asolution for 1 hr on a boiling water bath (1). Satisfactory experiments can be run on a rock or soil sample (not ores) of about 1 g digested with 2 ml of HNOa-A1(NO,), and taking 0.010-0.020 ml to spot the paper. Nitric acid oxidizes all accessible uranium to the 6+ state. Standard papers for comparison of intensity should be run simultaneously since the hydroferrocvanic acid decomuoses in a few hours to Prussian blue. Copper, Cobalt, and Nickel
The chlorides of these metals (3, 6) are extracted and migrate up the paper with methyl ethyl ketone. A standard solution (100 ppm) of a mixture of copper, cobalt, and nickel is made by dissolving 0.24 g CoC12.RHI0, 0.24 g NiC12.6H,0, and 0.17 g CuClz.2H 2 0 in a little water, adding 500 ml. cone HCl and 50 ml HN03 and making the solution up to 1000 ml with water. Solutions of nickel, copper, and cobalt (200 ppm) are prepared in the same manner. Methyl ethyl ketone is prepared for eluting by mixing 5 ml conc HC1, 3 ml water, and 25 rnl methyl ethyl ketone in a 600-ml beaker just before use. The reference paper is spotted with varying concentrations of solutions of the mixture and the individual cations. After air drying, the paper is placed in the desiccator for hr or more. It is then crimped, rolled, and placed in the methyl ethyl ketone solution. After the solvent has risen to within 1-2 cm of the top of the strip ( I / % hr or more) it is removed, drained, and air dried for 2-3 min. The paper is then suspended above 10 ml of concentrated ammonia solution in a 600-ml beaker. A thin glass rod through the slots holds the paper over the solution until fuming has stopped and the free acid is neutralized. Rubeanic acid (dithiooxamide, 0.1% in 60% ethyl alcohol) is then sprayed onto the paper. Three colored hands appear. Copper moves the farthest and gives a greenish band; it is closely followed by a yellow cobalt band. Nickel gives a blue hand close to the original spot. Unknown samples of about 1g may be fused with 1 g of potassium pyrosulfate and then chromatographed (3). After heating the mixture for 1 min in a Pyrex test tube the tube is rotated as the flux cools to distribute a thin layer of material over the entire surface. Volume 42, Number 9, September 1965
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Two ml conc HC1-HN03 s o h (50 ml HCI to 5 ml HNO3) are added, plus water to make looml. This solution is heated over boiling water until all the K&0, has dissolved (10 min minimum). The sohtion is then spotted onto the paper and compared with reference spots developed a t thesame time. The ratio of the distance traveled by the metal ion to the distance traveled by the solvent is called the R, factor. ~t will be of interest measure these R~ fa+ tors and see how they vary with, for example, the HCl: water ratio in the solvent. with a higher proportion of water the separation between copper and cobalt is sharply decreased.
478 / Journal of Chemical Educafion
Literature Cited
(1) "Uranium in Rocks," US. Geological Survey Bulletin 1036-L, U S . Geological Survw, Washington. (2) "Uranium in Waters," U.S. Geological Survey Bulletin 1036-J, U.S. Geological Survey, Washington. (3) "Analytical Methods Used in Geochemical Exploration by the U.S. Geological Survey," U S . Geological Survey Bulletin 1152, U S . Geological Survey, Washington, (4) BLOOM,H., AND WALTON, H. F., Sci. Am., 197, 41 (1957). (5) WALTON, H. F., Cha.Eng. News, 38, June 20 (1960), p. 121. (6) HUNT,E. C.,NORTE,A. A., AND WELLS,R. A., Analyst, 80, 172 (1955). This article gives details of many sirnilw tests. Most of them have been refined or modified by later workers, particularly a t the U.S. Geological Survey.
