Dispensing by Drops.

Other tomes: AJP1883

BY ALBERT HENRY KINSEY, PH.G.

Abstract from an Inaugural Essay.

The size of a drop generally depends upon and is influenced by at least four conditions. (See also paper by Prof. C. F. Himes, in "Amer. Jour, Pharm.," p. 394, 1883.—EDITOR.)

First: the self-attraction that the particles of liquids have for each other.

Second: its adhesion to the matter on which it is formed.

Third: the shape of this matter.

Fourth: the physical relations existing between the matter on which it is formed, the liquid constituting the drop itself, and the medium through which it passes.

In my experiments I have found that the greatest variance is caused by the third condition, viz.: the shape of the matter, to which may be added the amount of surface, as it is obvious that the more surface the greater will be the adhesion, and therefore will require more liquid to overcome tills force, and consequently will produce a larger drop. This is practically illustrated below, when, in dropping from a glass stopper the surface from which the liquid has been dropped has a U-shape and is formed on the convex side, while from a minim measure it is dropped from the concave side of a V-shaped surface, giving the drop only a very small point to form on, and therefore must be much smaller. This is further illustrated in dropping from a glass stopper held at different angles. When held horizontally the drop is about twice the size of one dropped at an angle of 45 degrees. The difference is still greater when a common cork is taken, as it has a more acute angle. In the case of tincture of opium, the drop from a common cork, when held in a horizontal position, was more than twice as large as when held at an angle of 45 degrees.

Another very important feature in the matter of dropping is the rapidity with which it is done. It is a well-known fact that the less the interval between successive drops, the larger they will be. This interval has been called the growing time, and it follows that if this growing time is constant in the same liquid, the size of the drop will be the same.

It has been shown by actual experiments, that when the growing time is decreased below 0.333 second (coconut oil was used in this instance) a continuous stream was the result, but of course the density of the liquid regulates this to a certain extent. It is also a curious fact that a stream so produced, delivers less in a given time than a series of large drops.

This rapidity of dropping is one of the greatest obstacles to overcome, for very few pharmacists will drop the same liquid in the same time, and if laws are to be laid down, governing dropping, the time certainly claims a large share of attention, for the same mistake is just as likely if not more so, to happen in this instance than in the previous one, for a pharmacist who dispenses 100 drops of a liquid at the rate of three drops a second, will give one half as much again as another who measures the same liquid at the rate of a drop every second and one half.

Prof. Guthrie has shown the effect of gradually decreasing the strength of saline solutions. Dropping, at the rate of two seconds, he found that decrease of solid constituents produced precisely the same effect upon the size of the drops, as a decrease in the growth rate in the drops of homogenous liquids. I find that these facts, however, have their greatest importance from a theoretical point of view, practically there is very little, if any, difference, although in some instances it does seem as though the matter in solution might be the cause of the decrease in size by increasing its specific gravity. The following table gives the result of my experiments, having chosen the glass stopper, minim measure and lip of the bottle in which the liquids are ordinarily kept, to drop from.

By comparing my table with those of Prof. Procter or Mr. Durand, it will be noticed, in a number of instances, that they vary very widely, about the only way I can account for this is, that the lip of the minim measure, which I used, must have been much smaller than theirs, but even when the same vessels are used, there is such a variety of results, that to get a medium size an average is required to be taken. This I have done in all of the unimportant liquids. How greatly they vary may be seen in the case of Acetum opii; in the first trial the result was 120 drops to a drachm, the second 85, and the third 103.

There are still other conditions which yield more or less influence on the size, and one which deserves mention, is the angle at which the vessel is held. I have already shown that a cork may be held so a drop can be obtained twice as large as another where the cork has been held at a different angle, the same is true with a bottle, but not quite in so great a degree.

The fulness of the bottle also exerts some influence, as tincture of aconite, when dropped from an ounce vial full, yielded 110 drops to the drachm, but when only one-fourth full gave 116 drops, also liquor potassii arsenitis, from a full ounce vial, gave 66 drops, and when one-third full, only 57. In the one case, decrease in the amount of liquid decreased the size, while in the other it was increased. The drop from an ounce vial was in most instances the same as from the shop bottle.

By a careful perusal of the above we can readily notice that the different classes of preparations can be grouped together, as for instance, the tinctures or alcoholic preparations may be classed as a group, whose drops are about one half the size of the aqueous liquids, while the oils and acids form an intermediate group between the two. Durand must have taken notice of this fact, when he laid down his two general rules concerning drops as follows:

First: that liquids, with a small proportion of water, afford a small drop, and vice versa.

Second: that amongst liquids containing a large proportion of water, those not charged with remedial substances, give a larger drop than those same liquids having extraneous bodies in solution.

In summing up my labors on this subject, there is only one general conclusion that I will mention, as it covers all of the others, and if properly heeded may be the means of saving considerable trouble, and I might say is also in harmony with those who before me have given the subject a still more thorough investigation. Having shown that the same liquid under different and even the same circumstances, varies in dropping so much, that no reliance whatever can be placed in this method of dispensing medicines, therefore their administration in this form is always attended with more or less danger.

The American Journal of Pharmacy, Vol. 56, 1884, was edited by John M. Maisch.