PICRIC ACID (Formula, ; synonym, carbazotic acid).—Prepared by the action of nitric acid on phenol. It takes the form of yellow plates or prisms, sparingly soluble in cold water, but more easily on heating, giving it a bright yellow color ; alcohol dissolves it readily. It has been used as a ray sensitizer in the orthochromatic process. It is sometimes used as a test for potassium, as its potassium salt is very slightly soluble in cold water.
Prof. W. K. Burton has pointed out the following uses of picric acid in photography: Staining plates to prevent halation.—It was from a French source that I gained the first hint as to this use of picric acid, but I cannot at this moment remember the authority.
A cold saturated solution of picric acid is made—picric acid is soluble in cold water to about the extent of one per cent.—and ammonia is added till the solution distinctly smells thereof. Plates dipped for two minutes or so in this solution and dried resist halation to a re markable extent. To give some idea of the amount of resistance given by this treatment, I may say that it is about as difficult to produce halation on picrated plates as on the Sandel double coated plates, but not nearly so difficult as on the triple coated plates of the same make.
The exposure is prolonged a little, but not much, if due precaution be taken in the devel opmmt, but such precaution must be taken. The reason is that the picrate of ammonium in the film acts as a very powerful restrainer, and that, if the usual developer be used, we get a very hard negative, giving the impression that it was much under-exposed. A developer that is com monly used with a restrainer—pyro with ammonia for example—may simply be used without the restrainer. A good developer is eikonogen with a large quantity of carbonate of soda and no restrainer. The following may be used: Eikonogen TO grains Soda (crystallized) 25 grains Sulphite of soda 25 grains to each ounce of developer.
There can be no doubt that a plate useful for many purposes could be made by adding picric acid to the emulsion before coating. The quantity of picric acid needed does not add
appreciably to the cost of the emulsion.
Picric acid for orthochromatizing plates. This is not one of the uses of picric acid, for picric acid has no orthochromatizing effect, yet I have several times seen it recommended for this purpose. I cannot very well understand why it should have been recommended, as it is the very last coloring matter that might be expected to have the desired effect. It has long been demon strated that dyes sensitize to those parts of the spectrum that they absorb. Now, as will be seen presently, picric acid, or picrate of ammonia, absorbs the extreme violet and violet-blue of the spectrum only, the very parts the sensitiveness of which we have to repress in orthochromatic work.
The fact that such dyes as have an orthochromatizing effect, i.ensitize to the parts of the spectrum that they absorb, is better illustrated in the case of cyanine than in that of any other dye that I have experimented with. This is probably because the absorption band is so very clearly marked in the case of this dye. If the light passing through a solution of it be examined by the spectroscope, it will be seen that there is a very well marked band of complete absorption extending from the orange-yellow, through the orange into the red, but not extending to the extreme red.
I may mention that for sensitizing to the whole of the visible spectrum (with the exception of the extreme red, to which ordinary plates are not sensitive, namely, to green, yellow, orange and red) a mixture of cyanine and eosine has a better effect than that of any one dye or other combination of dyes that I have tried. Plates so sensitized are very useful for copying pictures of some kinds. To get the maximum effect, the plates are first dipped in a very weak solution of nitrate of silver for five minutes, and then in a solution of the two dyes.