12 MICRO-ANALYTICAL METHODS 



The test for the presence of boric acid (also for borax) is made 

 as follows: Dip the end of a prepared thread in a lo per cent, 

 solution of hydrochloric acid and allow to dry. Lay the thread on 

 a slide, cover with cover glass and examine. It should be of a 

 reddish-brown color. To the edge of cover glass apply a droplet 

 of a lo to 13 per cent, solution of sodium carbonate, followed 

 by a droplet of the suspected solution. In the presence of boric 

 acid, the thread is colored blue, which coloration remains for a 

 longer or shorter period and then changes to gray and violet. The 

 test is a very delicate one and is not hindered by the presence of 

 sodium chloride, magnesium sulphate and aluminium sulphate. 

 Strong solutions of phosphoric acid, silicic acid^ calcium chlorite 

 and magnesium chlorite, interfere with the reaction more or less. 



4. Formaldehyde Test. — Concentrated hydrochloric acid 

 added to weak solutions of formaldehyde (i :sooo) or substances 

 containing formaldehyde, forms stellate clusters having a some- 

 what crystalline appearance. The formaldehyde can be de- 

 posited on a slide by sublimation (as for benzoic acid) and the 

 acid added. The stellate clusters appear upon evaporation 

 of the hydrochloric acid. The test requires further verification 

 to determine its value. 



5. Sulphurous Acid Test.— Moisten starch paper with a very 

 dilute solution of potassium-iodide iodine solution which colors 

 it blue. In the presence of the merest trace of sulphurous acid 

 the paper is decolorized. Do not use heat in this test. 



6. Iodine Reaction.— The color reaction of starch with N/so 

 iodine solution is of great importance in the examination of fruit 

 products, such as jams, jellies, catsups, etc., as it shows whether 

 or not ripe or green fruits and juices of unripe fruit were used 

 and whether or not starch paste may have been added as a filler 

 or thickening agent. As is known, green fruits generally contain 

 more or less starch, whereas ripe fruits are quite generally free 

 from starch. The reaction may be observed only in the fruit 

 pulp cells, indicating the presence of unripe fruit, or it may be 



