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
rate are essential for a good product.
A large vessel is needed for the reaction, as the nitroso-b-naphthol
separates in a finely divided condition and there is some
tendency to foam.
The final air-dried product is pure except for its moisture content,
as is shown by the fact that on drying _in vacuo_ it has a very good
melting point. A sample of Kahlbaum's nitroso-b-naphthol melted
at 101--105.