To the Editor:
In patch testing, the concentration of the active agent in the inert vehicle is often of critical importance. Per cent is the usual way of specifying concentration. In the case of metal salts, the use of per cent has led to a curious error. Surprisingly enough, the "5%" nickel sulfate solution used by many dermatologists contains only 3% nickel sulfate ! This error results from technicians simply weighing out 5 gm of the commercially available nickel sulfate hexahydrate to make 100 ml of solution. As nickel sulfate hexahydrate contains 40% water, the resulting solution is actually only 3% with respect to nickel sulfate.Specific inquiries to dermatologists both here and abroad revealed this type of error to be widespread. In most cases the dermatologist was unaware that the technician making the solution was simply weighing out the percentage in terms of the hydrate. Molarity is an unequivocal way
Epstein E. MOLARITY VERSUS PER CENT. Arch Dermatol. 1965;91(1):94. doi:10.1001/archderm.1965.01600070100028
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