Legal Chemistry / A Guide to the Detection of Poisons, Examination of Tea, Stains, Etc., as Applied to Chemical Jurisprudence

by Alfred Naquet

DETECTION OF PHOSPHORUS.

ORFILA'S METHOD.

The solid substances found in the alimentary canal are mechanically separated from the fluids present by means of a linen cloth. They are then examined by aid of a magnifying glass, and any fragments of phosphorus found separated and preserved under water. If none are discovered, the presence of phosphorescent vapors may possibly be detected by examining the materials in the dark. In any case, a portion of the suspected materials should be treated with nitrate of silver: in presence of phosphorus the materials acquire, first, a reddish-brown, then, a black color. The remaining portion is spread upon a shovel and heated: a white flame, burning at various points of the mass, and originating from the combustion of phosphorus, is observed, if this body be contained in the substances under examination. This method is evidently far from perfect.

MITSCHERLICH'S METHOD.

Mistcherlich's method is based upon the luminosity of the vapors of phosphorus. The suspected materials are moistened with dilute sulphuric acid, and heated, in a flask communicating with a glass worm which passes through a glass cooler into a receiver. If the apparatus is placed in the dark, and the materials contain phosphorus, luminous vapors will be observed in the flask and receiver. When the quantity of the poison present is considerable, the phosphorous acid formed can be collected and its properties tested.

DUSART'S METHOD, AS MODIFIED BY BLONDLOT.

Fig. 10.

Dusart's process takes advantage of the facility with which hydrogen combines with phosphorus. The substances under examination are placed between two asbestus stoppers in a tube, one end of which tapers to a point, and a current of pure hydrogen conducted over them. In presence of phosphorus the evolved gas will burn with a green flame, and, upon bringing this in contact with a porcelain plate, red spots will be deposited upon the latter. Blondlot prefers to introduce the suspected materials into the flask in which the hydrogen is generated. He employs the apparatus represented in Fig. 10: a is a flask for evolving hydrogen; b is a U tube, filled with fragments of pumice stone which are saturated with a concentrated solution of potassa; c is a Mohr clamp; d a screw-clamp; e a platinum jet. This jet is necessary in order to avoid a yellow coloration of the flame by the soda contained in the glass. Pure hydrogen is at first evolved, in order to ascertain that the flame is colorless and red spots are not produced when it is intersected by a cold plate. The purity of the reagents used having thus been confirmed, the clamp d is closed until the acid is forced back into f; and the materials to be examined are then added to the fluid. Upon opening the clamp the liquid passes from f into a, and the evolution of gas recommences. The gas is then ignited: the flame possesses the characteristic properties mentioned above, if the suspected substances contain phosphorus.

METHOD PROPOSED BY FRESENIUS AND NEUBAUER.

According to this method, the materials are brought into a flask provided with a doubly-perforated stopper, and water, acidulated with sulphuric acid, added. The flask is then heated over a water-bath, and a current of carbonic acid conducted through the mixture for at least two hours. The gas, on leaving the flask, passes into a solution of nitrate of silver. Should no precipitate form in this solution, the absence of free phosphorus is established, for, were this body present, a portion would be volatilized, and a black precipitate, consisting of phosphide of silver, together with phosphoric acid, produced. The formation of a black precipitate is, however, not necessarily a proof of the presence of phosphorus. In order to conclusively determine the character of the precipitate, it is collected on a filter and examined by the method of Dusart and Blondlot.

This process has given result in cases where none were obtained by Mistcherlich's method. It possesses, moreover, an advantage over the latter process, in not being influenced by the presence of foreign bodies; whereas, in Mistcherlich's method, some time must elapse before the luminosity of the vapors becomes apparent if ether or alcohol is contained in the solutions, and this phenomenon totally fails to appear in presence of oil of turpentine.

DETECTION OF PHOSPHORUS BY THE USE OF BISULPHIDE OF CARBON.

In a report read before the Academy of Sciences in 1856, presented by an examining commission, of which MM. Dumas, Pelouze and Claude Bernard were the reporters, the following results were contained: Phosphorus may remain, in the free state, in the organs fifteen days after death, and even then its isolation can easily be accomplished. For this purpose the stomach or intestines, and the articles of food contained therein, are cut into pieces and treated with bisulphide of carbon. Upon filtering the liquid, a solution is obtained containing all the phosphorus present, which exhibits the following properties: 1st, When ignited, it burns with a very luminous flame; 2nd, if allowed to spontaneously evaporate (the combustion of the phosphorus being prevented by the organic matter present [Naquet]) an inflammable residue is obtained, which, if dissolved in boiling monohydrated nitric acid, gives a solution that, after saturation with ammonia, produces a precipitate soluble in acids in solutions of barium salts. If the solution is mixed with perchloride of iron, and the sesquioxide of this metal subsequently eliminated by the addition of ammonia, it no longer causes a precipitation in barium solutions. The fluid acquires a yellow coloration when boiled with a solution of molybdate of ammonia.

According to our personal experience, the apparatus employed by Flandin and Danger for the detection of arsenic, can also be made use of in the examination of the bisulphide of carbon solution. To this end, the fluid supposed to contain phosphorus is mixed with perfectly pure alcohol, and the mixture placed in a small spirit-lamp provided with a very loose asbestus wick. The lamp is then ignited and the flame introduced in the combustion tube D (Fig. 11).

Fig. 11.

By the combustion of the mixture, sulphurous, carbonic, phosphorous acids and water are formed. The water condenses in c, and, falling into the dish F, carries with it the sulphurous and phosphorous acids. The acid liquid collected in this way is evaporated to dryness, some nitric acid added, and the solution again evaporated. The remaining mass is then dissolved in water to which some ammonia is added, and the solution tested for phosphoric acid. This method is an advantageous one as the phosphoric acid formed must originate from phosphorus in the free state, and not from any phosphates which, owing to the presence of organic matter, might be contained in the bisulphide of carbon solution. It would, however, lead the analyst into error if the person, supposed to have been poisoned had eaten cerebral substances or eggs previous to death, as these contain glycero-phosphoric acid; it is therefore advisable to compare the results given by this process with those obtained by the use of other methods.

DETECTION OF PHOSPHOROUS ACID.

Provided free phosphorus has not been detected, it is necessary to search for phosphorous acid. To this end, the residue remaining in the flask, in either Mistcherlich's or Fresenius and Neubauer's method, is introduced into the apparatus of Dusard and Blondlot. If the phosphorus reaction appears, it is sufficient; otherwise, its production may have been hindered by the presence of organic matter. In case, therefore, the flame is colorless, the evolved gas is conducted into a neutral solution of nitrate of silver. If the materials contain phosphorous acid, a precipitate of phosphide of silver is formed which should be collected and washed. The precipitate, which is now free from organic matter, is then examined for phosphorous acid by means of the apparatus of Dusard and Blondlot.

ESTIMATION OF PHOSPHORUS.

The best process for determining quantitatively the amount of phosphorus present is the one recommended by Fresenius and Neubauer. The gaseous current is continued until a fresh nitrate of silver solution is no longer precipitated. The solution is filtered, the precipitate washed and then dissolved in nitric acid. The silver is next precipitated by addition of hydrochloric acid, the fluid again filtered, and the precipitate well washed. The washings are added to the filtrate, and the liquid concentrated in a porcelain capsule. A solution of sulphate of magnesia, containing ammonia, is next added to the fluid, and the phosphoric acid determined as pyrophosphate of magnesia: the precipitate formed, is washed, heated to redness, in order to convert it into the pyrophosphate, and then weighed.