also contain hydrated silica and frequently titanium carbide and free sulfur. The silica holds water tenaciously and cannot be thoroughly dehydrated below a red-heat. Titanium carbide is decomposed by HNO3, but not by HCl; while sulfur is separated by dilute HNO3, but not by HCl. Graphite is not at all oxidized by HNO3 (sp. gr. 1.2). The use of nitric acid is preferable with gray irons low in combined carbon, while HCl is better for white iron high in combined carbon and for ferrosilicons, as these only dissolve with difficulty in HNO3. The carbon in in the residue can only be accurately determined by combustion. REFERENCES: SHIMER, J. Am. Chem. Soc., 17, 873. DROWN, Trans. Am. Inst. Min. Eng., 3, 41. Process of Analysis.-Treat 2 grams of drillings in a beaker with 50 c.c. of HCl (sp. gr. 1.12). Cover and boil briskly for 30 minutes. Dilute, filter on an asbestos filter and wash with hot water until all iron salts are removed. Then pour on a little HCl and wash again with water. Now wash the residue with a 30 per cent solution of caustic soda, then with water, then with alcohol, then with ether, and finally with cold water and then hot water till every trace of ether is extracted. Now transfer to the carbon apparatus and determine the carbon with chromic acid and sulfuric acid, or by combustion in oxygen. This complicated washing is required to remove the solid and liquid hydrocarbons which are likely to form and are insoluble in water alone. The ether must be followed by cold water; if hot water were added at once, it would make the ether boil and might throw the carbon out of the filter tube. Nitric acid of sp. gr. 1.135 can be substituted for HCl with such irons as are readily dissolved by it. If a little HF is added to the solution after the metal is dissolved, it will frequently greatly facilitate the filtration by preventing the separation of silica in a gelatinous form. By using a sufficient quantity of acid of the right specific gravity, most of the silica will usually remain in solution. DETERMINATION OF THE GRAPHITE BY DIRECT WEIGHING This method gives satisfactory results on many irons. It is quite generally used as a rapid method. It should be checked by the combustion method when applied to kinds of iron not previously tested. The residue is dried at 100°C. and then burned and the loss of weight assumed to be carbon. Any sulfur or water which the residue contains will, of course, be rated as carbon. Process. Weigh out 2 grams of the drillings into a 250-c.c. beaker. Add 100 c.c. of HCl (sp. gr. 1.1), or of HNO3 (sp. gr. 1.135). Boil gently till all action ceases. Keep the beaker covered to prevent evaporation and concentration of the solution which may cause the separation of silica. Finally add 3 or 4 drops of HF and boil again. Prepare a Gooch perforated crucible as follows: First, heat, cool and weigh it; second, fit into the bottom of the crucible a disc of ashless filter paper and dry the paper and crucible at 100° for 20 minutes and weigh again. Filter the solution through this crucible in the ordinary way, transferring the residue with cold water. Then wash the residue with hot dilute HCl, then with hot water and then with a 5 per cent solution of NH4OH. When the filtrate runs through colorless, finally wash with a mixture of equal parts of alcohol and ether. Now dry the crucible and contents at 100°C. to constant weight, which will take from 10 to 20 minutes. Now set the crucible over a Bunsen burner flame and burn off the residue. When all the carbon has burned away, weigh the crucible again. By subtracting the weight of the crucible with the filter paper from the weight of the crucible plus the filter paper plus the residue, the weight of the residue is obtained. Subtracting the weight of the empty crucible from the final weight of the crucible plus the incombustible portion of the residue gives the silica and other mineral matter with the carbon. Subtracting this remainder from the total weight of the carbonaceous residue gives the weight of the graphite. A still better way is to use an asbestos mat in the Gooch crucible. Instead of using a Gooch crucible, two small tared filters can be taken and the residue then burned out in an ordinary crucible, but this is not nearly so convenient. The above method is essentially that given by A. B. Harrison in "Methods of Iron Analysis Used around Pittsburgh," 2nd ed., p. 85. REFERENCES: For variations of the method and discussion of the results consult: CROBAUGH, J. Am. Chem. Soc. (1894), 104. AUCHY, J. Am. Chem. Soc. (1900), 47. CHAPTER IX THE DETERMINATION OF NICKEL AND COBALT IN STEEL The method for nickel here given is that of Moore, modified by Johnson. It is very rapid and accurate and no elements interfere except copper and cobalt, which are usually present in steel in very small amounts. The copper reacts like nickel and, if present in more than traces must be separated as directed on pages 154, 126. The method depends upon the following reaction: Ni(NH3) 6SO4 + 4KCN= K2Ni(CN), + K2SO4 + 6NH3. This reaction takes place in a solution slightly alkaline (with ammonia) and a very large amount of iron may be present without interfering, if, before making alkaline, a large amount of citric acid is added. The citric acid combines with the iron to form un-ionized iron citrate which does not allow the iron to precipitate when the ammonia is added. The end point of the reaction between the nickel and the cyanide is shown by the disappearance of a turbidity due to the presence of silver iodide. The reaction is: AgI+2KCN = KAg(CN)2 + KI. Copper can be separated from nickel easily by the use of H2S, but cobalt is not so easily separated. Nickel can be quantatively separated from small amounts of cobalt by precipitating it with dimethylglyoxime and cobalt can be separated from nickel by precipitating the cobalt with nitroso-ẞ-napthol. Nickel and cobalt along with vanadium, copper, manganese and aluminium can be separated from iron by extracting the chlorides with ether, only a small amount of iron remaining with the nickel, etc. (See page 125.) For rapid routine determinations of nickel the cyanide titration method is recommended, but for the most accurate results the dimethylglyoxime should be used. Cyanide Titration Method. Dissolve 1 gram of steel drillings in a 150-c.c. beaker with 20 c.c. of 1:1 HCl. When action ceases add 10 c.c. of 1:1 HNO3. Reduce the volume to 15 c.c., remove the beaker from the heat and pour into it 8 c.c. of sulfuric acid diluted with 25 c.c. of water. Transfer the contents of the beaker to a 400-c.c. beaker containing 12 grams of powdered citric acid and stir until it is all dissolved. Make the solution faintly but distinctly alkaline with 1:1 NH4OH. Do not add much excess as it causes low results. Cool the solution and dilute to about 300 c.c. If it is turbid, filter it. Add to the cold solution 2 c.c. of 20 per cent solution of potassium iodide and then run in from a burette a standard solution of silver nitrate, with stirring, until a distinct turbidity due to silver iodide is produced. Then titrate with the standard cyanide. Run in the cyanide with constant stirring until the turbidity just disappears. The cyanide first reacts with the nickel then attacks the iodide. If it is thought that the end point is passed, add another measured amount of silver nitrate until a turbidity is formed and again titrate with the cyanide until the turbidity just disappears. It is best to have another beaker containing a solution to be titrated to which no silver nitrate has been added sitting beside the one being titrated, so as to have a clear solution of the same color to compare with. If the citric acid was dirty, the solutions will be cloudy and should be filtered before the silver nitrate is added. Standardization of the Solutions.-Dissolve about 5 grams of KCN and 1 gram of KOH in water and dilute to 1 liter. The KOH makes the KCN solution keep better. Also dissolve 2.900 grams of AgNO3 in water and dilute to 500 c.c. One cubic centimeter of each will be equal to about 0.001 gram of nickel. To standardize them, weigh out 1 gram of nickel-free steel and add 0.3370 gram of NiSO4(NH4)2SO46H2O, which contains 14.85 per cent of nickel, and treat just as above directed for a nickel steel up to the point of titration. Make the titration carefully until the turbidity just disappears. Now add 10 c.c. of the cyanide in excess and titrate with the silver nitrate until a turbidity just appears. This second titration gives the relation between the silver nitrate solution and the cyanide solution. For example, suppose that in the first place 0.5 c.c. of silver solution was added and 50.5 c.c. of cyanide were used. Then suppose that it took 10.8 c.c. of silver nitrate solution to produce a turbidity after the 10 c.c. of extra cyanide were added. The cyanide required to titrate the nickel would be 50.50.5 X 10 10.8 = 50.04 c.c. 50.04 = Since the amount of nickel present was 0.3370 X 0.1485, or 0.050 gram, the strength of the cyanide is 0.050 0.0009992 gram Ni per cubic centimeter. Notes on the Process.-The presence of sulfates is necessary to obtain a sharp end reaction. Silver iodide is soluble in a large excess of NH4OH, so care should be taken to have the solution only slightly alkaline with NH4OH, but it must be alkaline. If the titrated solutions are allowed to remain in the open beakers for some time a white film forms on the surface, but no account is to be taken of it. When chromium is present, proceed exactly as described above, except add 24 grams of citric acid. Instead of using so much citric acid, some chemists use citric acid and sodium pyrophosphate. The silver nitrate solution used should not be stronger than that indicated above, for when a strong silver solution is used, the silver iodide, instead of forming a turbid solution, settles out as a curdy precipitate which does not readily react with the cyanide. If a ferronickel is being analyzed, a stronger solution of KCN should be used. Such elements as vanadium, chromium, tungsten, molybdenum or manganese do not interfere, even when present in large amounts in the sample. REFERENCES: JOHNSON, "Chemical Analysis of Special Steels, Alloys and Graphites." GROSSMAN, Chem. Ztg., 36, 673. JAMIESON, J. Am. Chem. Soc., 32, 757. BOYLE, Chem. Eng., 14, 288. DAUGHERTY, Chem. News, 95–261. THE DIMETHYLGLYOXIME METHOD FOR NICKEL This is a very accurate and fairly rapid method. Copper and cobalt, if present, do not interfere and for this reason steels containing more than traces of copper or cobalt should be analyzed by this method. In cases of dispute, umpire analyses should be made in this way. From faintly ammoniacal solution, dimethylglyoxime precipitates nickel promptly and completely as a voluminous red compound, nickel. dimethylglyoxime, which is soluble in acid solutions. The precipitate |