50 c.c. of ether (previously saturated with hydrochloric acid) are now added to the second funnel, and the shaking, etc., repeated in order to extract more ferric chloride from the combined aqueous layers. After the ether layer has been allowed to separate completely from the aqueous layer, the latter is drawn off into a beaker and freed from dissolved ether by evaporating on a steam bath (care must be taken that no live flames are near). The aqueous layer will contain about 5% of the iron originally present in the sample.21 This will be mostly in the ferric form, although a little of it may be reduced to the ferrous form through the reducing action of the ether. The aqueous layer will also contain all of the following elements if they were originally present in the sample: copper, aluminum, chromium, titanium, vanadium, manganese, nickel, cobalt, calcium, and magnesium; also about 70% of the phosphorus. If molybdenum or tungsten were present, they will be found mostly, if not entirely, with the ether layer. After the aqueous layer has been freed of dissolved ether, a little nitric acid is added and the solution boiled to oxidize any ferrous iron to ferric, after which it is ready for any separations that may be desired. 233. Extraction with a Mixture of Absolute Alcohol and Ether in Equal Volumes. The use of this mixture allows the separation of calcium from strontium (barium); the nitrate of calcium is soluble in the mixture while the nitrate of strontium is not. The method is used in rock analysis 22 as follows. The weighed oxide (CaO+SrO) from the ammonium oxalate precipitation of the calcium is transferred to a small flask of 20 c.c. capacity, dissolved in nitric acid, and evaporated to dryness at 150° to 160°. The thoroughly dried nitrates are treated with as little (seldom over 2 c.c.) of a mixture in equal parts of absolute alcohol and ether as may be needed to dissolve the calcium salt, solution being hastened by occasional gentle agitation. After standing overnight in the corked flask the insoluble matter is collected on the smallest possible filter and washed with more of the above mixture of alcohol and ether. After drying, a few cubic centimeters 21 If it is desired to remove more of the iron, another extraction with ether can be made. 2 Quoting Hillebrand, loc. cit., § 13, pp. 142 and 144. of hot water are passed through the filter, on which may remain a few tenths of a milligram of residue, which does not usually contain any lime or other alkaline earth and whose weight is therefore to be deducted from that of the lime, unless it can be shown that it is derived from the glass of the little flask in which the nitrates of calcium and strontium were evaporated. To the solution of strontium nitrate in a small beaker a few drops of sulphuric acid and then its volume of alcohol are added, whereby the strontium is precipitated as sulphate, in which form, after twelve hours, it is weighed and then tested spectroscopically as to freedom from calcium and barium. 234. Amyl Alcohol. The use of this solvent forms the basis of Gooch's 23 method for the separation of lithium from sodium and potassium. Anhydrous lithium chloride is soluble in boiling amyl alcohol while the chlorides of the other two elements are but very sparingly soluble. Gooch thus describes his method for separating lithium: 24 "To the concentrated solution of the chlorides amyl alcohol is added and heat is applied, gently at first, to avoid danger of bumping, until the water disappears from solution and the point of ebullition rises and becomes constant for some minutes at a temperature which is approximately that at which the alcohol boils by itself, the chlorides of sodium and potassium are deposited and lithium chloride is dehydrated and taken into solution. At this stage in the operation the liquid is cooled and a drop or two of strong hydrochloric acid added to reconvert traces of lithium hydrate in the deposit, and the boiling continued until the alcohol is again free from water. If the amount of lithium chloride present is small, it will now be found in solution and the chlorides of sodium and potassium will be in the residue, excepting the traces, for which correction will be made subsequently. If, however, the weight of lithium chloride present exceeds 10 or 20 mg., it is advisable at this point, though not absolutely essential to the attainment of fairly correct results, to decant the liquid from the residue, wash the latter a little with anhydrous amyl alcohol, dissolve in a few drops of water, and repeat the separation by boiling again in amyl alcohol. For washing, amyl alcohol, previously dehydrated by boiling, is to be used, and the filtrates are to be measured apart from the washings. In filtering it is best to make use of the perforated crucible and asbestos felt, and apply gentle pressure. The crucible and residue are ready for the balance after drying for a few minutes directly over 23 Proc. Am. Acad. Arts and Sci. (1886), p. 177; Bull. U. S. Geol. Survey, No. 42 (1887), pp. 85-86; Chem. News, 55, 18, 29, 40, 56, 78 (1887); Am. Chem. Jour. 9, 33 (1887). Consult, also, W. W. Skinner and W. D. Collins, Bull. Bur. Chemistry, No. 153 (1912). 24 Hillebrand, loc. cit., § 13, p. 212. a flame turned low. The weight of the insoluble chlorides actually obtained in this manner is to be corrected by the addition of 0.00041 g. for every 10 c.c. of amyl alcohol in the filtrate, exclusive of washings, if the insoluble salt is entirely sodium chloride; 0.00051 g. for every 10 c.c. if potassium chloride constitutes the residue, and if both sodium and potassium chlorides are present, 0.00092 g.; but .. the entire correction may in any case be kept within very narrow limits if due care be given to the reduction of the volume of residual alcohol before filtration. The filtrate and washings are evaporated to dryness, treated with sulphuric acid, the excess of the latter driven off, and the residue ignited to fusion and weighed. From the weight thus found the subtraction of 0.0005 g. is to be made if sodium chloride constitutes the precipitate; 0.00059 g. if potassium chloride alone is present in the residue, and 0.00109 g. if both these chlorides are present, for every 10 c.c. of filtrate, exclusive of washings. "Amyl alcohol is not costly, the manipulations of the process are easy, and the only objectionable feature - - the development of the fumes of amyl alcohol — is one which is insignificant when good ventilation is available. "The process has been used for some months frequently and successfully, by others as well as myself, for the estimation of lithium in waters and minerals." 235. CHAPTER XV OXIDATION-REDUCTION GENERAL THEORY 1 Oxidation-reduction reactions are based upon the fact that metals, non-metals, and their ions can be made to undergo a change in the amount of electric charge associated with them, and that in this change there is a simple relationship between the quantity of electricity gained or lost and the weight of the substance which is affected. This relationship is embodied in Faraday's Law, which may be reworded to read: a change of charge of one corresponds to the gain or loss of 96,500 coulombs of electricity per formula weight of the substance involved. Since in every oxidation-reduction reaction the charge which is lost by the one reacting substance must of necessity be gained by the other, it follows that there is always a transfer of electricity, and consequently the two important factors that we have to consider are: 1. The change of charge which takes place 2. The electromotive force (e.m.f.) associated with the change of charge. NOTE. The value of 96,500 coulombs is the one most recently accepted for the value of the Faraday. See H. S. Taylor, A Treatise on Physical Chemistry, D. Van Nostrand Co., New York, 1924, Vol. I, p. 506, citing: Washburn and Bates, J. A. C. S. 34, 1341 (1912); Bates, ibid., 34, 1515 (1912); Bates and Vinal, ibid., 36, 916 (1914); Vinal and Bovard, ibid., 38, 496 (1916). The previously accepted value of the Faraday was 96,540 coulombs. 236. The Change of Charge Which Takes Place. By way of introduction to this first factor, let us take up the example of the reduction of ferric chloride by means of stannous chloride. 1 The reader is especially referred to the splendid treatment of this subject by Stieglitz, loc. cit., § 13, Chapters XIV and XV. The reaction is 2 FeCl3+ SnCl2 2 FeCl2 + SnCl4 or writing it more appropriately in the ionic form, we have 2 Fe++++ Sn++ 2 Fe++ + Sn++++ The change of charge with respect to the iron is one, namely, for every 55.84 grams of iron reduced there has been lost by the iron 96,500 coulombs of electricity; 2 the change of charge with respect to the tin is two, namely, for every 118.7 grams of tin oxidized there has been gained by the tin 2 × 96,500 coulombs; the chloride ion undergoes no change of charge since it is in the same state of oxidation after the reaction as before it. NaCl That there is an actual transfer of electricity in the foregoing reaction from the ferric ion to the stannous ion, can be shown by the following experiment. A solution of ferric chloride acidulated with hydrochloric acid to increase the conductivity is placed in a beaker and a solution of stannous chloride likewise acidulated with hydrochloric acid is placed in a similar beaker, and the two solutions connected by means of a "salt-bridge,"3 containing a solution of sodium chloride, as represented in Fig. 30. A platinum electrode (one 5 cm. X 10 cm. is a very convenient size) is introduced into each of the solutions, and the two electrodes connected to a delicate voltmeter. Upon completion of the circuit the directions of the current in the external circuit will be from the ferric chloride solution to the stannous chloride FeCl3 FIG. 30 Sn Cl2 2 If it is preferred to think of oxidation-reduction in terms of the electron rather than in terms of positive electricity, then reduction corresponds to the gain of an electron, while oxidation corresponds to the loss of an electron. The term "salt-bridge" is the technical name applied to a U-tube filled with a solution of some conducting electrolyte used to connect the separate solutions of an oxidation-reduction system. The U-tube is usually stoppered at both ends with a plug of cotton to prevent mechanical flow, and the conducting electrolyte is always selected so that it will not react chemically with either of the solutions which it connects. |