Excerpt from The American Journal of Pharmacy, Vol. 59: Philadelphia College of Pharmacy
The mode of conducting a titration must have an important influence on the result. Some direct to allow the precipitate to subside after each addition of reagent, and test a drop of the clear supernatant fluid with a drop of the reagent. A more common practice, and that which I have adopted in most of my experiments, is to filter the fluid after each successive addition of reagent, using the same filter, which must be only large enough conveniently to hold the whole of the fluid. The titration is concluded when the addition to the filtered fluid - having a volume of 10 cc. - of two drops of reagent fails to produce a permanent turbidity. In some cases the reagent as it drops into the fluid produces a transient turbidity, clearing up at once, while addition of a larger quantity of the reagent produces a permanent precipitate. The formation of this precipitate being obviously conditioned by the presence of a large excess of reagent, it is proper to ignore it.
No doubt the results of a titration will coincide more nearly with what theory requires if a considerable length of time is allowed to elapse after each addition of reagent, but this manner of conducting the operation robs it of its single advantage - rapidity of execution - and is not to be recommended, since, after all, there will be considerable and often quite capricious departures from theoretical figures.
The method, however, of carrying out titrations with Mayer"s reagent that has been recently recommended by Frank S. Hereth secures the advantages of the plan just mentioned without extending the time actually consumed in the operation. The method is as follows: Knowing approximately the alkaloidal strength of the solution to be examined, provide half a dozen or more test tubes or vials, and into each measure 10 cc. of the solution. To the first add a quantity of reagent a little less than that likely to be required; to the second a somewhat larger quantity, and so on. Let the test tubes stand at least eight hours, then test a portion of the clear fluid from each with a drop of Mayer"s reagent. The first one which foils to react obviously has received a quantity of reagent a little more than sufficient for the precipitation, and it will not be difficult to judge by the amount of cloudiness produced in it just what excess of reagent is present. Of course the value of the titration equivalent will have to be determined anew experimentally, if this method is to be adopted.
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