39 



fully cleaned, and, if new, rubbed with fine sand to roughen it and re- 

 move the grease, &c. 



When the ferric is reduced to ferrous oxide, which may be known by 

 the absence of a blood-red color on testing a few drops of the solution 

 removed by a glass rod to a watch-glass with ammonium sulphocya- 

 nide, empty one of the bottles into a large beaker, add 2 or 3 c. c. of 

 concentrated sulphuric acid, and dilute to about 500 c. c. 



Titrate with the standard permanganate solution in the same manner 

 as for standardizing. The number of c. c. of potassium permanganate 

 used, multiplied by the standard, gives the weight of metallic iron in 

 the solution treated. From this calculate the per cent, of ferric oxide. 

 The two titrations should not differ more than two-tenths of a c. c. 



The weight of the ferric oxide thus obtained deducted from the weight 

 of the combined oxides will give the weight of the alumina by differ- 

 ence. 



PREPARATION OF THE STANDARD PERMANGANATE SOLUTION. 



Dissolve 3.8 grams of pure crystals of potasium permanganate in 1 

 liter of distilled water, with constant agitation until all the crystals are 

 dissolved, decant the perfectly clear solution , and keep in a glass-stop- 

 pered bottle. 



There are several methods of standardizing the solution of perman- 

 ganate. (See Sutton Volumetric Analysis, 31, and 32, 33, 59.) Of 

 these the method proposed by Marguerite, by means of iron, is to be 

 recommended. 



In determining the iron by Marguerite's method, the presence of hy- 

 drochloric acid must be avoided, especially if the solution is at all warm, 

 since the permanganate under these circumstances will react upon the 

 hydrochloric acid liberating chlorine, as shown in the following reac- 

 tion : 



K 2 Mu 2 O 8 -f 16HC1 = 2KC1 + 2MnCl 2 + 8H 2 O + 1OC1. 



Some of the chlorine may convert the ferrous salt present into the 

 ferric state, but some will usually escape, and the results obtained will 

 consequently be higher than the truth. 



For this purpose introduce into a small flask, having a Kroouig valve 

 in the stopper, 0.200 grams of cleaned piano-forte wire, containing 99.7 

 per cent, of iron; add 25 to 30 c. c. of dilute sulphuric acid and heat to 

 incipient boiling. When the iron is dissolved allow the flask to cool 

 sloicly. When the contents of the flask are cold empty them into a 

 large beaker, wash the flask out well, and add the washings to the main 

 solution, dilute with distilled water to about 500 c. c. ; then drop in the 

 solution of permanganate to be standardized from a burette, having a 

 glass stop-cock, with constant stirring until the color, which disappears 

 rapidly at first and then more gradually, finally becomes permanent, 

 and remains so for one minute. The final color should be a light pink. 

 Note carefully the number of cubic centimeters of permanganate used, 



