METHODS OF ANALYSIS 7 



Comparing the following equations — 

 10 FeS0 4 +2 KMn0 4 +8H 2 S0 4 =5 Fe 2 (S0 4 ) 3 +K 2 S0 4 +2 HnS0 4 f8H,0, 



and 



5 (H,C 2 4 ,2H 2 0)+2 KMnO 4 +3H 2 SO 4 =10 C0 2 +2 MnS0 4 +K 2 S0 4 + 

 I8H0O, 



it will be seen that two equivalents of iron require the same amount 

 of permanganate solution for oxidation as one equivalent of oxalic 

 acid, or 2/56 Fe, is equal to 126 oxalic acid, i. e., as 8:9. Taking 

 oxalic acid, therefore, we simply multiply the weight taken by 8/9 

 to find the equivalent in iron. 



The composition of ferrous ammonium sulphate is, Fe(NH 4 ) 2 

 (S0 4 ) 2 6 H 2 0, the molecular weight is 392, of which 56, or 1/7, is Fe. 



To Standardize the Solution with Fe Wire. — Weigh out 

 approximately .200 gram of wire, cut in small pieces, and place in a 

 flask, as before; add 20 cc. cold water and 5 cc. cone. H 2 S0 4 , heat 

 slightly until dissolved (the H 2 S0 4 often furnishes enough heat of 

 itself). When completely dissolved, add 20 to 30 cc. of boiling water, 

 then add a spoonful of granulated amalgamated zinc; put on the 

 hot plate and boil a few minutes; remove, cool by dipping into 

 cold water, test with KCNS. It should show no pink coloration 

 whatever at first. If such is the case, dilute with 100 cc. cold 

 water; stir with a glass rod; allow the zinc to settle and decant, 

 carefully, into a white porcelain dish, being careful that none of 

 the zinc passes over; wash the zinc three times by decantation with 

 cold water. The solution is now ready for titration with the per- 

 manganate solution. Eun in rapidly, constantly stirring, until a 

 faint pink appears. The titration is then finished. 



To Standardize with Oxalic Acid. — Weigh out about \ gram 

 of the pure acid, which has been kept in a well-stoppered bottle. 

 Dissolve this in a No. 3 beaker in about 350 cc. of boiling water. 

 In another beaker place 50 cc. cold water, add to it 20 cc. strong 

 sulphuric acid, mix by shaking around gently, then pour this hot 

 acid into the oxalic acid and titrate at once with the volumetric 

 solution. 



To Standardize with Ferrous Ammonium Sulphate. — Coarsely 

 powder about 3 grams of the salt in a porcelain mortar, weigh out 

 about 2.1 grams, dissolve in about 250 cc. of cold water in a white 

 dish, add 20 cc. sulphuric acid (1 part acid, 5 parts water). When 



