13] Filtration and Washing 13 



(/) Never fill your filter with liquid, or some of the pre- 

 cipitate may escape over the edge of the paper. 



(g) Allow each portion of washing liquid to drain away 

 before adding the next. 



(h) Always evaporate the final washings to see that no 

 further impurity is being removed from the precipitate. 



A general method of doing this is as follows : Take a 

 piece of thin glass (cheap window glass will do) and cut it into 

 strips about three inches long by half an inch wide. Thoroughly 

 clean one of these strips, and collect a few drops of the filtrate 

 on its surface. Next lay it across the mouth of an Argand burner 

 which is burning with a very low flame, and allow the drop of 

 liquid to evaporate. No residue should be left on the surface. 



Of course this method does not apply in cases where the 

 precipitate has to be washed free from acid, but this is easily 

 shown by litmus paper. 



It should be remembered that although a residue of acid 

 or of ammonium salts left in a precipitate is entirely driven oft 

 in the subsequent ignition, still much harm may be done by 

 the action of these substances on the filter paper, making it 

 brittle and difficult to handle. 



Exercise VI. — Filter the liquid contained in the flask after 

 Exercise IV. through a 9 cm. filter, taking care to observe all the 

 precautions above mentioned, washing with hot water by decanta- 

 tion until the filtrate is perfectly free from solid matter. Keep the 

 precipitate for the following exercise. 



WASH BOTTLES 



13. From the student's work in qualitative analysis he will 

 be familiar with the construction and use of the ordinary wash 

 bottle. It will be found very convenient to keep two wash 

 bottles for quantitative work, one for cold and the other for 

 hot water. 



