SIMPLE COLORIMETRIC ANALYSIS for LABORATORY INSTRUCTION LANDON A. SARVER University of Minnesota, Minneapolis, Minnesota
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ONSIDERING the great importance and numerons advantages of photometric methods of quantitative chemical analysis, it is a great pity that comparatively few chemists receive any instruction whatever along this line in their regular college courses. The reason for this undoubtedly lies in the fact that colorimeters are thought to be too delicate and costly for use by large groups of inexperienced students, while even the matched Eggertz tubes required for the dilution method would represent a considerable outlay of money for a single special experiment. The author has used for several years a standard series method for the determination of ammonia, which is sufficiently accurate for instructional purposes, but which requires only ordinary rectangular medicine bottles for making the color comparisons. Inequalities in the dimensions of the bottles are not great enough to affect the results seriously, and about twelve hundred students have performed the experiment with a remarkable degree of success. Princifile of the Method. When a solution containing a small amount of an ammonium salt is treated with an alkaline solution of potassium mercuric iodide (Nessler's solution), a yellow color is produced, the intensity of which is dependent upon the concentration of ammonium ion; if this be compared with equal thicknesses of a series of standards, the quantity of ammonia present may be easily estimated. Nessler's Solution. The concentrated stock solution is prepared by dissolving 150 g. of potassium iodide in 100 ml. of distilled water, adding 110 g. of iodine, and then 140-150 g. of free mercury. The flask is shaken continuously and vigorously for 10-15 minutes, or until nearly all of the dissolved iodine has disappeared, during which time the solution becomes quite hot; when the reddish iodine solution has begun to become visibly pale, though still red, it is cooled in running water and the shaking continued until the red has been replaced by the greenish color of the double iodide, after which the solntion is decanted off from the surplus mercury; the latter is washed with liberal quantities of distilled water, and the solution diluted to two liters. The finished reagent is made as needed by diluting 150 ml. of the stock solution to one liter with 5% sodium hydroxide solution; this reagent is made less alkaline than the conventional one in order to permit the comparison of higher concentrations of ammonium salts, without the formation of a precipitate.
Standard Ammonium Chloride Solution. About 0.0628 g. of pure dry ammonium chloride is weighed as accurately as possible, and diluted to one liter in a volumetric flask; if exactly this amount be taken, the concentration will be 0.02 mg. of NH1 per milliliter. Samfdes. A suitable stock solution is obtained by dissolving 6.28 g. of pure dry ammonium chloride, and making up to exactly ten liters with distilled water. Individual samples are dispensed in medicine bottles by measuring out 8.0 to 20.0 ml. of this solution, in steps of 0.5 ml., and adding some water a t random. Water. In most cases, distilled water freshly drawn from the regular laboratory supply will be found sufficiently pure; if desired, i t can be redistilled, discarding the k s t and last portions. Procedure. All apparatus must be washed with fresh distilled water. The unknown sample is transferred quantitatively to a 250 ml. volumetric flask, diluted to the mark, and mixed well. One buret is filled with distilled water, the other with standard ammonium chloride, after rinsing with several portions of the solution. Rectangular medicine bottles of suitable size should be labeled on their bottoms X, and 1 to 9, respectively. An approximate determination is first made by measuring 5, 10, 15, 20, and 25 ml. of the standard ammonium chloride, and one 25-1111. pipetful of the uuknown sample solution, into a series of bottles, diluting to exactly 45 ml. with distilled water in each case, mixing by gentle rotation, adding 5 ml. of the Nessler's reagent to each, mixing again, and comparing against a blue sky or other uniformly illuminated background. The approximate concentration of the unknown may thus be easily determined. For the final comparison, another unknown and nine standards of suitable concentration varying by steps of 0.5 ml. of the ammonium chloride solution, are prepared and compared in the same way. The decision is much more difficult this time, but it is rare for a person of average color perception to be in error by more than one 0.5 ml. step. The number of milligrams of NHa in the sample may be calculated by multiplying the number of milliliters of ammonium chloride solution in the matchmg standard by ten times its concentration. Summary. A colorimetric analysis problem which is suitable for laboratory instruction and which illustrates an important conventional method, but which requires no delicate or expensive apparatus, has been given.
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