Safe Working With Picric Acid

Usually one of the first points made in any article about picric acid is that it can be explosive, or that it and its salts have been used as military high explosives before the introduction of TNT. That is all true. However, the dangers to histotechnologists from this are badly exaggerated, and picric acid can be used very safely in a histology laboratory. Its use should not be curtailed from a fear of an explosion, since the conditions under which that could take place are extremely unlikely to be present.

For an explosion to happen, the picric acid must be completely dry. Yet, picric acid is purchased wet with either water or ethanol, and as long as it remains in that state it will not explode. In fact, since it is nearly always used as either a saturated aqueous solution or a saturated ethanolic solution it makes sense to store in two jars, one with an excess of distilled water and the other with an excess of absolute ethanol, thereby ensuring that both saturated solutions are always available. Simply replace the amount of saturated solution removed with distilled water or ethanol, as appropriate, after removing the amount required at the time. Following this practice ensures there is no danger of an explosion.

Instructions are sometimes encountered to weigh an amount of picric acid for incorporation into a solution. Since this involves handling a dry sample technologists have become wary of doing so, but it was never the intention to completely dry picric acid for weighing, and in the past this was understood. Histological solutions are rarely required to be accurate to anything more than one decimal place, and often not even that, so extreme accuracy is not needed. The usual practice was to place some wet picric acid onto a filter paper, then place the filter paper onto some other absorbent paper (newspaper, paper towels) for a few minutes to drain off the excess moisture. While still damp the amount of picric acid required was weighed, but erring on the heavy side. This was then placed into the solvent being used and the papers rinsed and discarded. No completely dry picric acid was involved.

Even this practice is not usually necessary. Since picric acid is usually used in aqueous or ethanolic solution, it is simpler just to measure the required amount of picric acid by volume, then dilute as necessary. The volumes are based on the amount of picric acid in a saturated solution. Below are the figures (taken from the Merck Index) used on this site as the volume of a saturated solution that will contain one gram of picric acid at room temperature.

Water:1 gram in 78 mL Ethanol:1 gram in 12 mL

Simply multiply the volume that contains one gram by the number of grams needed, and use that. For other solutions, such as those that use acetone, it is very unlikely that the small volume of ethanol used as a carrier for the acid would interfere with the staining reaction.

It is advisable to check the jars of picric acid periodically to ensure that dried acid is not crusted around the lid. This can be avoided by always wiping the edge of the jar to remove any drops of the solution before replacing the lid. If the seal under the lid is intact and in good condition, and the lid firmly tightened, there should be no evaporation or crusting. Similarly, ensure that wicking does not occur by never allowing a string or other absorbent material to be elevated above the surface of the picric acid, and never have it draped over the edge of the container. Doing so may enable solution to wick up the material and crystallise on the end of it in a dry state.

Should some dried picric acid be encountered, do not panic. It is unlikely to explode. The easiest way to deal with such a problem is to fill a large bucket or sink with tap water, then place the crusted jar into it so that it sinks. The acid will then dissolve off. After an appropriate time the jar may be removed and cleaned. The lid may then be undone and resealed properly.

If you are not comfortable with that, then involve your institution's safety officers and have them deal with it.

Susan Budavari, Editor, (1996)
The Merck Index, Ed. 12
Merck & Co., Inc., Whitehouse Station, NJ, USA



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