lead, will require, for 1 liter, the amount of thiosulfate indicated by the following proportion: Pb: 3Na2S2O3 5H2O::x:5, or 17.985 grams per liter. The thiosulfate solution used for the copper determination will do. To standardize, dissolve about 0.2 gram of pure lead in 15 c.c. of 1:2 nitric acid, add 5 c.c. of sulfuric acid and evaporate to strong H2SO4 fumes and treat as above directed for an ore. Notes on the Process.-As a fungus growth forms in the acetate wash, only enough to last a day should be made, unless it is heated frequently. If the end point be passed in titrating, a few drops of a standard dichromate solution may be added and the titration finished. The iron value of the dichromate multiplied by 1.2363 gives the lead value. The constitution of the lead chromate depends upon the temperature, acidity and concentration of the solution and the precipitation of the lead from the ore must be made exactly as it is made in the standardization of the thiosulfate. Bismuth in small amount does not interfere, but if there is much in the sample, some may remain as sulfate with the lead. In such a case, just before filtering off the lead chromate, add 2 grams of citric acid dissolved in a little hot water. This will dissolve any bismuth chromate present. If the ore contains barium, it is difficult to extract all the lead sulfate with the acetate mixture. In such a case, after the acetate has been used, drop the filter in a flask, add 10 c.c. of strong HCl and boil almost to dryness, add 25 c.c. of the acetate solution, boil vigorously and filter and wash; the combined filtrates are then treated as usual. Wilders' Modification.-Treat 0.50 gram of ore as is usual for lead ores up to the point where the washed sulfate is obtained on the filter paper. Remove the paper from the funnel and fold lengthwise in such a manner that it can be introduced into a 200c.c. graduated flask. Place a few grams of sodium acetate and 1 c.c. of acetic acid in the flask and add about 50 c.c. of water. Then place the flask on the hot plate until the lead sulfate is all in solution. This may be hastened by shaking to break up the filter paper. Now run in an excess of standard dichromate solution from a burette, under the conditions given above and, after mixing by shaking, make the contents up to the mark by adding water. Invert the flask 10 times with the hand over its mouth to mix thoroughly. Take three No. 589 S. & S. filters, fold as one and place in a dry funnel with a dry 100-c.c. flask beneath. Filter 100 c.c. and transfer to an Erlenmeyer flask. Add a few cubic centimeters of sulfuric acid, enough potassium iodide to react with the excess of chromic acid and titrate with sodium thiosulfate in the usual manner, using starch indicator. The standard dichromate contains 3.558 grams of dichromate per liter. The thiosulfate theoretically contains 8.8177 grams per liter, but 9.150 will be more apt to give the desired strength. Making the solutions this way, 1 c.c. of dichromate should equal 1 per cent of lead and 2 c.c. of thiosulfate. Therefore, subtracting the cubic centimeters of thiosulfate used from the cubic centimeters of dichromate used will give the per cent of lead direct, without any further calculation. Notes on the Method. The chromate solution and the thiosulfate solution are both stable, and once standardized will not require further attention for a month or more. They may be titrated against each other occasionally and restandardized if any variation is found. The lead chromate precipitate is fine unless boiled, and, therefore, three filter papers are used. The solution filters quickly, however. In methods where the solution is boiled, the lead chromate is of variable composition, depending upon the conditions of precipitation, basic chromates being formed. This method avoids any washing of the chromate and thus saves time and avoids errors due to washing. REFERENCES: Low, "Technical Methods of Ore Analysis." WILDER, Eng. Mining J., 92, 390. CHAPTER XXX THE DETERMINATION OF TIN IN ORES Tin occurs in ores generally as SnO2 (cassiterite), sometimes as SnS2 (stannite). Cassiterite is insoluble in all acids, and to get the tin in solution it is necessary either to fuse the ore with an alkali, forming a stannate which is soluble in acids, or to reduce the SnO2 at an elevated temperature with either a reducing gas as illuminating gas or with finely divided metal as powdered zinc or aluminium. When the amount of tin is very low and a large sample is used, the reduction of the SnO2 with a powdered metal to metallic tin followed by solution of the tin in hydrochloric acid is best, because if a large sample is fused with an alkali and the fusion dissolved in acid, the liberated silicic acid causes trouble in filtration. A solution of stannic chloride in hydrochloric acid should not be evaporated, as stannic chloride boils at 114°C. Tin may be determined gravimetrically by weighing as SnO2 after precipitation as metastannic acid, or by weighing as the metal after electrolytic precipitation, or, volumetrically, by titrating the tin in stannous state by a standard oxidizing solution, preferably iodine. Stannous salts take up oxygen readily from the air, consequently the solution, which is to be titrated, must be protected from the air by a neutral gas, as carbon dioxide. The following process is the well-known one of Pierce and Low, with the reduction and titration carried out as directed by Patrick and Wilsnack. It depends upon reducing the tin in a strong hydrochloric acid solution in absence of air by means of the combined action of metallic iron and antimony. The tin is reduced to SnCl2, which is then titrated with iodine solution, thus, SnCl2 + 21 + 2HCl = SnCl4 + 2HI. Process of Analysis. Place in an iron crucible about 20 grams of sodium hydroxide and heat over a Bunsen burner until the charge is melted. Cool and add 2.0 grams of the finely ground Cover the crucible and heat cautiously to prevent spattering and, finally, heat with the full flame of the Bunsen until the fusion is quiescent and keep the mass thus fused for about a half hour to insure complete solution of the cassiterite. Cool and ore. dissolve the cake in about 50 c.c. of water and enough HCl to neutralize all the NaOH, and add 50 c.c. additional. When all is in solution, transfer the solution to a 500-c.c. Erlenmeyer flask and add 5 c.c. of sulfuric acid in which is dissolved 0.1 gram of antimony. Close the flask with a three-hole rubber stopper. Through one hole put a piece of glass tubing reaching to the bottom of the flask, through another place an iron rod, the end of which is coiled into a spiral. The rod should fit loosely enough to slip up and down in the hole in the stopper, and be long enough to have the spiral reach to the bottom of the flask, while the other end sticks out of the stopper several inches. Push the iron spiral down in the solution and pass CO2 or natural gas (freed from H2S by passing through NaOH) through the glass tube until the air is driven out of the flask, and then boil the solution for 25 minutes while continuing the current of gas. Cool the flask while a rapid current of gas passes through to prevent air entering. When the solution is cold, raise the iron coil out of the liquid, wash it by squirting a jet of recently boiled cold water through the third hole in the stopper, put in 4 c.c. of starch solution and then titrate the tin with standard iodine solution by introducing the tip of the burette through the hole in the rubber stopper and running in the iodine solution until a drop turns the starch blue. The iron coil must be so placed that the iodine solution does not drop on it. Notes on the Process.-When near the end point, which is told by the slow disappearance of the blue color of starch iodide, the titration should proceed slowly to prevent overrunning the end point. Large amounts of copper, lead, nickel, and perhaps other metals, interfere with the accuracy of the titration if they are present and ores containing them should be first treated with aqua regia, evaporated to dryness, digested with hydrochloric acid and filtered. The residue on the paper is then treated for tin as above directed. Arsenic and antimony when in trivalent condition consume iodine, if the solution is weakly acid, but in the strong acid solution, as used in this process, they are without effect. The reduction of the tin is not complete if only iron is used or if only antimony is used, but is perfect if both are used. Some chemists find it satisfactory to use 1 gram powdered Sb and no iron. Iodine Solution. To make the iodine solution, put 20 grams of KI and 50 c.c. of water in a liter flask and then add 12.7 grams of pure iodine, stopper the flask and shake the solution until the iodine is all dissolved. Then dilute to a liter and mix well. Standardize against c.p. tin, by dissolving 0.2 gram in a flask in 50 c.c. of water and 50 c.c. of strong HCl. When dissolved, add 5 c.c. of sulfuric acid containing 0.15 gram of antimony and reduce and titrate as directed for ore. Iodine solution of the above strength is tenth-normal and should equal 0.00595 gram of tin per cubic centimeter. The results obtained are accurate to 0.2 per cent when as much as 0.5 gram of tin is present and much more accurate with the small amounts of tin found in ores. The disturbing effects of the presence of arsenic, antimony and copper may be avoided by the following modification of the above process: Proceed exactly as directed above until the HCl solution is obtained after the fusion. Heat the solution to 60°C. and add 4 grams of iron filings to precipitate arsenic, antimony and copper, and to reduce the stannic to stannous chloride. This operation requires about half an hour. Filter into a 500-c.c. Erlenmeyer flask and wash the residue well with hot water. Heat the filtrate to 95°C. and add 10 grams of pure zinc to precipitate the tin. Drops of the solution should be tested from time to time on a porcelain plate with hydrogen sulfide water. A brown precipitate indicates that tin is still in solution. When the tin is all precipitated, decant the solution through a funnel containing a plug of glass wool, leaving most of the tin and the zinc in the flask. Put the glass wool with the tin, which was carried over, into the flask. Fit the flask with a twohole rubber stopper carrying glass tubes for the passage of CO2. Pass CO2 through the flask several minutes. Remove the stopper and pour into the flask 30 c.c. of HCl. Replace the stopper and warm the flask gently until the tin and zinc are completely dissolved. Close the exit tube and cool the flask. When cold, disconnect the CO2 generator, remove the stopper and wash off the tubes with water from which the air has been expelled by CO2 (1 liter of water+3 grams NaHCO3+HCl), |