CHAPTER X TITRATIONS INVOLVING THE FORMATION OF PRECIPITATES The completion of the reactions of neutralization depends upon the small ionization of one of the products, water. The completion of reactions of oxidation and reduction depends upon the relative potentials of oxidizing and reducing agents. Certain other reactions are made the basis of volumetric determinations, completed because of the formation of a precipitate. In some cases an indicator is added while in others the cessation of precipitation with further addition of standard solution is the indicator. SILVER An example of titration without an added indicator is to be found in Gay-Lussac's1 method for silver. This method is one of the oldest of those analytical methods that have survived to the present day and, while it is not now extensively used because it is somewhat troublesome in the matter of execution, it is one of the most exact of all known volumetric processes. It depends upon the titration of the solution of a silver salt by a standard solution of sodium chloride. The very small solubility of silver chloride renders the reaction practically complete. The converse of this method may be used for the determination of chlorine, bromine, or iodine in soluble halides. Exercise: Preparation of Standard Solutions.-Calculate the weight of pure sodium chloride that is equivalent to 5 gm of silver, weigh this quantity, dissolve in distilled water and dilute to 1000 cc in a volumetric flask. Make a second solution by diluting 100 cc of this solution to 1000 cc. Record the silver equivalent of 1 cc of each of these solutions. Determination.-Silver may be determined in any alloy that contains no other metal forming insoluble chlorides but the approximate percent 1Instruction sur l'essai des matieres d'argent par la voie humide. Paris, 1832. of silver should be known. A silver coin may be used. United States silver coinage contains approximately 90 percent of silver. Weigh enough of the alloy to give 0.5 gm of silver, place in a 250 cc flask having a ground glass stopper and dissolve in 10 cc of a mixture of equal volumes of water and concentrated nitric acid. Both water and acid must be tested and found free from chlorine. Boil to expel oxides of nitrogen, assisting this action by drawing air through the flask by means of a filter pump. Add to the solution in the flask exactly 99 cc of the more concentrated standard salt solution, stopper and shake until the precipitated silver chloride flocculates and settles readily. Add from a second burette the more dilute standard solution, 0.5 cc at a time, allowing the solution to run down the sides of the flask and observing whether turbidity is produced. Shake the flask if more silver chloride is formed and continue the addition of the dilute standard solution until the last 0.5 cc fails to produce a visible precipitate in the clear, supernatant liquid. Do not use the last 0.5 cc in the calculation. It may sometimes happen that the percent of silver in the alloy is not known with sufficient accuracy and either too much or too little of the more concentrated solution is used. In the first case the first addition of the dilute solution fails to produce a precipitate while in the second case an unduly large quantity of the dilute solution is required to reach the end point. In either case the determination should be begun again, the proper alteration being made in either the weight of sample taken or the volume of concentrated standard solution. From the results of the titration calculate the percent of silver in the alloy. In the determination of silver by the method of Volhard1 an inorganic indicator is added to the solution. The silver should be in the form of nitrate, a solution of a ferric salt, acidified to suppress hydrolysis, is added and the silver is titrated by a standard solution of potassium thiocyanate or ammonium thiocyanate. Silver is precipitated as silver thiocyanate: AgNO3+KCNS-AgCNS+KNO3. When all of the silver is removed from the solution an additional drop of the standard solution of thiocyanate produces the red color of soluble ferric thiocyanate: Fe(NO3)3+3KCNS-Fe(CNS) 3+3KNO3. Mercury thiocyanate is insoluble in dilute nitric acid and mercury must therefore be absent. The color of salts of copper, 1 J. prakt. Chem., [2] 9, 217 (1874). nickel and cobalt obscures the end point and these metals should be absent although as much as 60 percent of copper may be present. The converse of this method may be used for the determination of the thiocyanate radical. Exercise: Preparation of Solutions. Make a solution of silver nitrate, 1 cc of which contains 0.005 gm of silver. Standardize gravimetrically by precipitating and weighing silver chloride, or by Gay-Lussac's volumetric method. Make 1500 cc of a solution of potassium thiocyanate or ammonium thiocyanate by weighing 2 percent more than the calculated quantity of salt required to make 1 cc equivalent to 0.005 gm of silver. Make 100 cc of a solution (saturated without heating) of ferric ammonium sulphate, adding enough nitric acid to remove turbidity and to cause the red color to give place to pale yellow. Standardize the thiocyanate solution as follows: Measure 35 cc of the silver nitrate solution into a beaker or Erlenmeyer flask, dilute to about 75 cc, add 1 cc of ferric ammonium sulphate solution and titrate with the thiocyanate solution until a permanent red tint is obtained. Determination.-Weigh not more than 0.25 gm of a silver alloy containing no mercury, nickel or cobalt and not more than 60 percent of copper and place in a 250 cc flask. Dissolve in 10 cc of a mixture of equal volumes of concentrated nitric acid and water, boiling to expel oxides of nitrogen. Cool, dilute to about 75 cc and titrate exactly as in the standardization of the thiocyanate solution. Calculate the percent of silver in the alloy. HALOGENS AND THE CYANIDE RADICAL Volhard's method also applies to the determination of the halogen hydracids and cyanogen. A measured excess of standard silver nitrate solution is added, precipitating all of the chlorine, bromine, iodine or cyanogen. The excess of silver nitrate is determined by titration by standard thiocyanate solution by the method already described. In the original method the precipitated silver halide was not removed by filtration before titration of the excess of silver. Rosanoff and Hill have shown1 that the silver chloride reacts with the red soluble ferric thiocyanate, which is produced at the end point, as follows: 3AgCl+Fe(CNS) 3→FeCl3+3AgCNS. 1 J. Am. Chem. Soc., 29, 269 (1907). This occurs to an appreciable extent, even though the solubility of silver chloride is less than that of silver thiocyanate. Rosanoff and Hill found that as much as 43 percent of ammonium thiocyanate is changed in two minutes by reaction with silver chloride. It is therefore necessary to remove the precipitate by filtration before the final titration. Determination.-Use the standard thoicyanate and silver nitrate solutions prepared for the preceding exercise. Weigh enough of a soluble chloride, bromide, iodide or cyanide to be equivalent to about 40 cc of the silver nitrate solution. Dissolve in a small amount of water, acidify with nitric acid and add 50 cc of the standard solution of silver nitrate. Filter and wash thoroughly and titrate the excess of silver nitrate by standard thiocyanate solution. Calculate the percent of halogen or cyanogen in the sample. A method for the direct titration of the halogens by standard silver nitrate solution is described on page 338 in the discussion of water analysis. CHAPTER XI ANALYSIS OF INDUSTRIAL PRODUCTS AND RAW MATERIALS In most of the exercises in the preceding portion of this book determinations have been made of single constituents of various substances and interfering substances have usually been either absent or capable of being removed with comparative ease. Standard methods have been employed and attention has been centered upon the chemical principles underlying the method and the proper manipulation. In the pages that follow the student will become acquainted with the application of these and other determinations to the testing and analysis of some materials which are of importance to our industrial life. Such materials are often quite complicated in composition and most varied procedures are necessary in a determination of their industrial value. The chemist will then find it necessary to have at his command all of the chemical principles and methods of analysis that have already been learned and to apply these to an intelligent study of the material under examination. He will also be prepared to take up other methods of testing. Some of the tests are purely physical but they are, in industrial practice, applied by the chemist and not by the physicist because the former is usually engaged in the analysis of the same or similar materials. Other analytical determinations are empirical, rather than exact, in their nature but must be made with the same degree of care and attention as the determinations involving definite elements or compounds. CARBONATE MINERALS The most important and abundant of the carbonate minerals are the calcites and the dolomites. The calcites consist essentially of calcium carbonate and the dolomites of double carbonates of calcium and magnesium but these compounds seldom or never occur in a pure state in nature. Iceland spar is one of the best |