CHAPTER XVII PREPARATION OF STANDARD POTASSIUM PERMANGANATE SOLUTION. DETERMINATION OF IRON OXIDIMETRICALLY ANALYSIS OF FERROUS SULPHATE AND IRON ORE FOR IRON 262. After studying the first part of this chapter, the student should prepare a liter of standard approximately 0.02 molar potassium permanganate solution and a Jones reductor in accordance with the directions given, because both of these will be required in the determination of iron. The permanganate solution should be standardized by means of sodium oxalate. 263. Permanganate ion is without doubt the best oxidizing agent which the analytical chemist has at his disposal for the carrying out of oxidation-reduction reactions. The theory underlying the behavior of permanganate ion has already been presented at quite some length in §§ 250-255; it is presumed that the student is familiar with the theory, as it has an important bearing in actual practice. The great practical advantages of permanganate ion are its high potential and its characteristic purple color: the high potential insures a very small difference between the amount of reducing substance oxidized and the total amount of reducing substance present; the characteristic purple color is so pronounced that only the slightest excess of permanganate ion is necessary to establish an unmistakable end point, one drop (0.05 c.c.) of 0.02 molar potassium permanganate in a volume of 500 c.c. being plainly visible. The only disadvantage - and it is easily overcome is that the permanganate solution must be kept free at all times of even the merest traces of manganese dioxide, because as shown by Morse and co-workers1 if manganese dioxide is present, it acts as a catalyst in promoting the formation of more manganese dioxide at the expense of the permanganate ion, thereby bringing about a continuous and increasing lowering in the value of the solution. 1 Am Chem. Jour. 18, 411 (1896); 23, 313 (1900) Furthermore, when the stock bottle of permanganate solution is shaken preparatory to withdrawal of some of the solution, the manganese dioxide will become more or less unevenly suspended, so that successive fillings of pipettes or burettes will have varying amounts of manganese dioxide in them. As it takes only a few specks of manganese dioxide to effect the same amount of oxidation as several tenths of a c.c. of 0.02 molar potassium permanganate solution (the value usually employed), it means that it is impossible to get concordant or consistent results when manganese dioxide is present. The manganese dioxide may come from three causes: it may be on the surface of the permanganate salt which is used as the source of permanganate ion even though the salt be an "analyzed" or "C.P." grade; it may come from the reducing action of small amounts of organic matter and ammonia which distilled water usually contains; 2 it may come from the reducing action of the dirt and organic matter on the inside walls of the stock bottle into which the permanganate solution is to be put. This third cause, however, need not be encountered at all, as any stock bottle, burette or pipette used for containing a permanganate solution can be and should always be perfectly cleaned beforehand by treatment with dichromate cleaning mixture. To get rid of the manganese dioxide from the first and second causes, dissolve the required amount of potassium permanganate in the necessary volume of distilled water contained in a large beaker, cover the beaker with a watch-glass and heat the solution to 90°-95° for two to three hours, and then set it aside at room temperature for several days so as to insure the complete oxidation of all organic matter. After this, filter the permanganate solution directly into a clean stock bottle using a Gooch crucible with an asbestos mat and employing suction. It is important that the asbestos mat be formed from asbestos which has first been digested with strong nitro-hydrochloric acid and then thoroughly 2 See 27 for impurities found in distilled water. 3 3 Under no circumstances use filter paper to remove the manganese dioxide. The filter paper will, it is true, remove the manganese dioxide already present, but as the permanganate solution trickles down the under side of the filter paper, the reducing action of the latter is sufficient to cause the formation of a lot of new manganese dioxide which will go along with the solution. washed with water. It is also advisable, after the mat has been formed in the crucible, to ignite same over the Bunsen burner for several minutes, and then to pass through it a few c.c. of distilled water in order to wash out any small threads that might have become detached. If a permanganate solution is made up in accordance with the foregoing directions and kept out of direct sunlight, it will keep its value for months. It is a wise precaution, however, to check this value every now and then by titration against sodium oxalate. If a change of more than 2 parts per 1000 has taken place, the permanganate solution should be refiltered. With respect to the use of permanganate solutions, our general caution in regard to the use of standard solutions cannot be too strongly emphasized: portions of the solution withdrawn from the stock bottle should never be returned to it. Permanganate solutions are seldom made up to any prescribed exact value because of the reason that in the attendant dilutions, transference of volumes, etc., the likelihood of conditions favorable to the formation of manganese dioxide would be greatly multiplied. It is usual to make up the solutions to an approximate value and then determine this value exactly by titration. For most purposes a solution which is approximately 0.02 molar will be found very convenient. To make up such a solution it is preferable to weigh out an amount of potassium permanganate slightly in excess of that demanded by theory, namely, slightly greater than 3.16 g. KMnO4 for each liter of solution required, say 3.20 g. KMnO4. 264. Standardization of Permanganate Solutions. There are four principal substances in use for the standardization of permanganate solutions. Stated in order of their preference, the most desirable being put first, they are: 1. Sodium oxalate Na2C2O4 2. Ferrous ammonium sulphate hexahydrate FeSO4 (NH4)2 SO4-6 H2O (Mohr's salt) 3. Iron wire 4. Oxalic acid H2C2O4.2 H2O. In view of the fact that sodium oxalate, because of its many advantages, has practically displaced the other three substances, we will discuss its use in detail and dismiss the other three with only a few observations and references that might be helpful in case it is ever necessary to employ them. 6 265. Sodium Oxalate. This standard was first proposed and adopted by Sörensen; some time later Blum5 studied quite critically not only the conditions for its preparation as an analytical standard but also the tests that it should answer, as well as the proper conditions for its use. Shortly after the publication of Blum's work the Bureau of Standards, Washington, D.C., added this very important standard to their list of standards and published a pamphlet setting forth in detail the reasons which led them to this action. Since all this work establishes the great superiority of sodium oxalate as a primary standard in conjunction with standardization of permanganate solutions, every laboratory should be provided with an analyzed grade of sodium oxalate, and in cases demanding extreme care a certified sample obtained from the Bureau should be used." The conditions to be employed when sodium oxalate is used as a primary standard for permanganate solutions are as follows:" In a 400 c.c. beaker, dissolve 0.250-0.300 gram of sodium oxalate in 200 to 250 c.c. of hot water (80° to 90°) and add 10 c.c. of 9 M H2SO4. Titrate at once with 0.02 M KMnO4 solution, stirring the liquid vigorously and continuously. The permanganate must not be added more rapidly than 10 to 15 c.c. per minute, and the last 0.5 to 1 c.c. must be added dropwise with particular care to allow each drop to be fully decolorized before the next is introduced. The excess of permanganate used to cause the end point color must be estimated by matching the 4 Z. anal. Chem. 36, 639 (1897); 42, 352, 512 (1903); 44, 141 (1905); 45, 217 (1906). 5 J. A. C. S. 34, 123 (1912). 6 Circular of the Bureau of Standards, No. 40. Sodium Oxalate as a Standard in Volumetric Analysis. Washington, 1913. 7 The Bureau makes a small nominal charge for such samples. Quoting almost verbatim from Circular of the Bureau of Standards, No. 40, cited above, pp. 10-11. In regard to keeping this solution of sodium oxalate any length of time before titrating, the words of the Bureau on the stability of stock solutions of sodium oxalate are to the point: "It has been shown that sodium oxalate solutions do not decompose appreciably upon boiling, but that they readily attack glass. This latter effect is noticeable to a lesser degree when the solutions are preserved in glass at the ordinary temperature, giving rise to a precipitate of calcium oxalate which renders difficult the accurate measurement of an aliquot. For this reason and also because of the possible decomposition of sodium oxalate by sunlight, the use of a stock solution for standardizing is not recommended." color in another beaker containing the same bulk of acid and hot water. The solution should not be below 60° by the time the end point is reached; more rapid cooling may be prevented by allowing the beaker to stand on a small asbestos-covered hot-plate during the titration. The use of a small thermometer as stirring rod is most convenient in these titrations, as the variation of temperature is then easily observed. The precision obtained in the titration under these conditions is limited only by the apparatus employed. By the use of weight burettes agreement of duplicates to one part in two thousand can readily be obtained. The precision with volume burettes may be somewhat less than this, depending upon the accuracy of calibration and the care used in the reading of the volume and in maintaining the temperature of the solution constant. In view of the probable warming of the permanganate solution during the titration, the use of weight burettes is recommended whenever a precision greater than one part in one thousand is desired. The absolute accuracy of the results cannot be assumed to be greater than one part in a thousand until an exhaustive investigation of the whole subject of volumetric standards is made. 266. Exercise No. 25. — Prepare a liter of approx. 0.02 molar potassium permanganate solution, and standardize same by means of sodium oxalate, running the titration in triplicate. The average deviation should not exceed 1 part per 1000. 267. Ferrous Ammonium Sulphate Hexahydrate, FeSO4 (NH4)2SO4-6H2O. (Mohr's Salt).-When freshly prepared, this salt has the composition as given, but upon standing the iron in it undergoes oxidation, while some of the water of hydration is given off with the net result that one cannot be sure of the ferrous content of iron unless the salt is freshly made. Directions for its preparation and use will be found in Olsen.10 268. Iron Wire. This standard was formerly in quite general use before the advent of sodium oxalate. The objections to it are mostly on the score that it is almost impossible to obtain pure iron, the following impurities being always present to greater or less extent: Silicon Phosphorus 10 p. 300 of text cited in § 13. |