the temperature being kept at 0'0. During the reaction crushed ice
is added from time to time to maintain the temperature at 0'0;
about 1 kg. is usually used. After all of the sulfuric acid
has been added, the solution should react acid to Congo paper.
The mixture is stirred one hour longer at the low temperature
and then the nitroso-b-naphthol, which has gradually separated out
during the reaction, is filtered with suction and washed thoroughly
with water. The product is at first light yellow in color,
but after three to four days it gradually changes to a dark brown.
The moisture content seems to have some effect on the color.
After the product has been air-dried for about four days, the yield
is about 665 g.; it melts at 97'0. A sample of this partially
dried product, on drying _in vacuo_ over sulfuric acid for twenty hours,
loses about 10 per cent of its weight and the melting point is 106'0.
By longer drying under ordinary conditions, the melting point of 106'0
is reached. The total yield of dry product is about 595 g.
(99 per cent of the theoretical amount).

This product is satisfactory for all purposes. It may be obtained
in a crystalline condition, however, by recrystallizing from hot ligroin
(sp. gr. 0.71-0.72). About 2 g. of nitroso-b-naphthol will dissolve
in 15 cc. of boiling ligroin. The product is not very soluble
in cold ligroin, so that nearly all is recovered.


2. Notes

It is very necessary to keep the temperature near 0'0 while
adding the sulfuric acid, or a tarry product will be obtained.
Vigorous stirring and the addition of the sulfuric acid at the proper