MICROCHEMICAL REACTIONS OF THE COMMON ACIDS 381 



SULPHITES, THIOSULPHATES. 



a. To a drop of a solution of potassium iodate add a little 

 potato starch and a small drop of dilute sulphuric acid. Ex- 

 amine to see that no iodine has been set free. Add a fragment 

 of the unknown. The starch is colored blue. 



b. To a moderately concentrated drop of copper sulphate 

 apply a drop of a solution of the unknown by Method /// A, 

 page 254. Warm gently sulphites, if pure and undecom- 

 posed, yield at the most only a faint cloudiness thiosul- 

 phates give a brown precipitate of copper sulphide and around 

 the circumference of the drop lemon-yellow crystals of copper 

 thiosulphate. 



SULPHIDES. 



a. The Silver Nitrate Precipitate was Black. 



b. Place a drop of solution or fragment of solid in the distilling 

 apparatus, cover with a slide holding a tiny drop of silver nitrate 

 and one of lead acetate side by side. Raise the cover and care- 

 fully run in a drop or two of dilute hydrochloric acid. Cover 

 quickly and allow to stand. Both drops turn black. 



c. Proceed exactly as in b but invert over the crucible a slide 

 carrying a drop of sodium nitroprusside. A beautiful purple 

 color results. 



SULPHOCYANATES. 



a. Give a Blood-red Color with dilute Ferric Chloride. 



b. Add Mercuric Chloride and Zinc Sulphate. There will be 

 obtained the double sulphocyanate of mercury and zinc. See 

 Zinc, page 308; Copper, page 340. Add a trace of copper and 

 increase the delicacy of the reaction. 



TARTRATES. 



Note. Before testing for tartrates always neutralize any free 

 mineral acid present. 



a. By means of Calcium Acetate. 



The solution may be neutral or acidified with acetic acid. 



