274 AGRICULTURAL ANALYSIS 



is titrated by treating with potassium permanganate. 3 * The reac- 

 tion is represented by the equation : NaI-|-2KMnO 4 -|-H 2 O 

 NaIO 3 H-2KOH-}-2MnO 2 . The iodin also may be set free and 

 determined in the usual way by titration with standard sodium 

 thiosulfate solution. 



The titration of free iodin is represented by the following re- 

 action : 



2Na 2 S 2 O 8 +2l=2NaI+Na 2 S 4 O 6 . 



In this reaction thiosulfuric acid is converted into tetrathionic 

 acid and the free iodin into hydriodic acid, both of which com- 

 bine with the sodium present. The decinormal solution of sodium 

 thiosulfate may be used. Grind the crystals of the salt to a fine 

 powder, dry between blotting papers, and use 24.8 grams of the 

 dried salt per liter. The quantity of iodin found in phosphates 

 is so minute that it is hardly worth while to make a quantitative 

 determination of it. 



235. Occurrence of Chromium in Phosphates. In some phos- 

 phates a small quantity of chromium has been found. In a 

 sample of phosphate from the Island of Los Roques in the Car- 

 ibbean Sea, Gilbert found three-fourths per cent, of chromium 

 oxid (Cr,O 3 ). The phosphates containing chromium have a 

 greenish color and are characterized by great insolubility in solu- 

 tions containing organic acids. The chromium is to be deter- 

 mined by the usual methods described in mineral analysis. 



236. Estimation of Vanadium. In the complete analysis of 

 basic slags it becomes necessary to determine the presence of 

 vanadic mixture with this solution until a drop of the clear liquor, 

 the volumetric process of Lindemann. 35 It is conducted as fol- 

 lows : Dissolve four grams of the finely powdered slag in 60 cub- 

 ic centimeters of dilute sulfuric acid (i 14), boil for a few min- 

 utes, cool, make the volume up to 100 cubic centimeters, filter and 

 add decinormal potassium permanganate solution in slight excess 

 to an aliquot part of the filtrate to secure the oxidation of the vana- 

 dium to vanadium pentoxid. Add, drop by drop, a weak solution 

 of ferrous sulfate until the pink color just disappears. Prepare 



34 Sutton, Volumetric Analysis, gth Edition, 1904 : 219. 

 36 Ueber die quantitative Bestimmung des Vanadins in Eisenerzen, 1878. 

 Zeitschrift fiiranalytische Chemie, 1879, 18 : 99. 



