Each year usually brings to the writer of this article at least one student who, wishing to prepare a solution containing picric acid, is surprised and perturbed to find the stock bottle of this substance containing‘… a lot of watery liquid above the crystals’. Students are even aggrieved, thinking that some previous user has been careless, and are annoyed with the presumed prospect of having to dry out the wet mass of crystals.

Since there appears to be a steady stream of similar misconceptions, the following notes might profitably be drawn to the attention of all biologists, especially those engaged in histological work.

Recipes for preparing fixatives incorporating picric acid are very conveniently given in terms of a dozen or so ‘basal fixatives’ of which volumetric quantities are combined variously to give a great range of standard fixatives. (For details see Gray or Bolles Lee.) Consequently, a saturated solution of picric acid being one of the basal solutions, it is never necessary to contemplate weighing out dry picric acid. Where picric acid in alcoholic solution is called for, this may conveniently be prepared by filtering the water-wet crysals, preferably through a Büchner funnel under suction, washing with alcohol and taking up the crystals from the filter paper in alcohol of the appropriate strength. One of the basal fixatives is 95% alcohol saturated with picric acid; this may therefore be obtained by allowing the acid to stand covered with 95% alcohol, and the supernatant solution withdrawn as required, replacement being made each time with 95% alcohol. Picric acid is several times more soluble in ethyl alcohol than in water, maximal solubility being achieved at about 80% alcohol.

Particular attention is to be drawn to the picrates. ‘Metallic salts of picric acid are much less stable than the free acid, and should always be stored damp.’ (Mann and Saunders.) Fortunately, very few fixatives in common use include both picric acid and cations of heavy metals (which produce the most dangerous picrates), but those which do should be handled with respect in the realisation that the metal picrates are even more explosive than is the acid itself. One of the standard tests for explosiveness is that of the ‘fall-hammer’ in which a 2-kilogram hammer is allowed to drop on to the substance under test. The following figures, taken from International Critical Tables, indicate the potential dangers involved: height of fall necessary to explode picric acid — 35-95 cm.; ammonium and sodium picrates — 80 cm.; zinc picrate — 60 cm.; copper, iron, lead and page 98 silver picrates — 5 cm. These are to be compared with compressed guncotton (15% water) — 85 cm.; nitroglycerine (dry or moist) — 4 cm.

Emphasis is laid on the fact that heavy metals are common impurities of other chemicals, especially where commercial grade chemicals are being used; traces of such metals, inadvertently and unconsciously introduced into picric acid solutions are hazardous. The cautions against allowing the solution to evaporate to dryness and against the use of glass stoppers are therefore strongly reiterated. They should be applied to all stock solutions and working solutions of picric acid, picrates, fixatives and staining solutions containing any of these materials, whether in aqueous or alcoholic solution.