74-77] Volumetric Estimation of Iron 49 



Afterwards check your work by taking another 25 c.c. and 

 running in the exact amount, all but a drop or two, finishing 

 up as before 



74. Calculation. — For each operation one-tenth of the 

 amount of ferrous salt weighed out has been used. Therefore, 

 the total amount of iron in the 250 c.c. of solution is obtained 

 by multiplying in 10 x "0056 by the number of c.c. of K 2 Cr 2 7 

 used. 



75. Estimation of Iron in Solutions containing 

 Ferric Salts. — The first thing necessary is to reduce the iron 

 to the ferrous state. Several methods have been devised for 

 performing this operation, and these may be found in text 

 books on general quantitative analysis. The one here given is 

 easily and rapidly worked, and gives good results. 



76. Make up a solution of stannous chloride by dissolving 

 about 10 grams of the salt in 25 c.c. of hot dilute hydrochloric 

 acid (equal parts of strong acid and distilled water), dilute to 

 100 c.c, and keep in a bottle whose stopper is very slightly 

 smeared • with vaseline. This prevents the salt from creeping 

 up and cementing the stopper into the bottle. It will not 

 vitiate the solution, because that is always taken out by a 

 pipette as shown in the sequel. A few pieces of pure tin 

 should be kept at the bottom of the liquid. 



Make up a saturated solution of mercuric chloride in 

 100 c.c. of water. 



77. The Estimation. — Measure out 10 c.c. of a dilute 

 solution of ferric chloride 1 into a 4-oz. conical flask, add 

 about 40 c.c. of water, and heat to boiling. Suck up into 

 a 5-c.c. pipette some of the stannous chloride solution and 

 add it, drop by drop, to the boiling liquid in the flask. Con- 

 tinue adding until the liquid becomes quite colourless. 



1 If it be desired to check this estimation the iron should be deter- 

 mined in another 10 c.c. by the gravimetric method given in paragraph 22. 



E 



