alcoholic solutions with chloral. While this chloral alcoholate acts as an hypnotic, it is harsh in its action, leaving unpleasant secondary effects. Alcoholic prepara

tions should not be prescribed with chloral, especially not in connection with the bromides of sodium and potassium, because, if the solutions used are at all concentrated, the chloral will separate as alcoholate, float on the surface, and a great risk will be incurred of giving a large overdose if the patient is not very careful to shake the bottle before taking a dose.

ounces of water and mixed, and the wine of opium should be diluted with an ounce of water and added, followed by the tincture of catechu, likewise diluted. This yields a brownish colored liquid, holding a fine precipitate in suspension.

Lead Acetate is incompatible with such preparations as contain organic acids, as it forms insoluble compounds with these. In the above instance it unites, forming an insoluble compound, with the meconic acid of the opium and the catechuic acid of the catechu.

Lead Subacetate, being of an alkaline reaction, precipitates alkaloids, organic acids, solutions of gummy and coloring matters, tannins, albumen, etc.

CHAPTER XXXVII.

VOLUMETRIC QUANTITATIVE ANALYSIS.

VOLUMETRIC ANALYSIS is the method by which we determine the quantity of certain substances by means of the volume of selected reagents which are required to perform a given reaction. The process of adding the liquid reagent from graduated measures is called Titration.

The strength of a volumetric solution as compared with that of another, or, in other words, its ratio to the latter, is called its Titer.

A VOLUMETRIC OR STANDARD SOLUTION is a solution of definite strength, which is made by dissolving a given weight (in grammes) of a reagent, in a definite volume (in cubic centimeters) of water. These solutions are usually made by dissolving the molecular weight of a reagent in grammes or a fraction thereof in 1000 cubic centimeters (one liter) of water. The following abbreviations are in use:

N or

N

10

N

1

(normal). "Volumetric solutions are designated as normal) when they contain in 1 liter the molecular weight of the active reagent, expressed in grammes, and reduced to the valency corresponding to one atom of replaceable hydrogen or its equivalent.

"Thus, hydrochloric acid, HCl = 36.37, having but one H atom replaceable by a basic element, has 36.37 Gm. of HCl in 1000 Cc. of the normal volumetric solution; while sulphuric acid, H2SO, = 97.82, having two replaceable H atoms, contains only one-half this number, or 48.91 grammes of H2SO4 in 1000 Cc. of its normal solution. Potassium hydrate, KOH= 55.99, has but one K to replace one H in acids, hence its normal solution contains 55.95 grammes of KOH in one liter.”

=

(decinormal) Solutions which contain in one liter one-tenth of the quantity of the active reagent in the normal solution. (centinormal) Solutions which contain in one liter one hun100 dredth of the quantity of the active reagent in the normal

2

N

N

N

2

solution.

=

(double normal) = Solutions which contain in one liter twice the quantity of the active reagent in the normal solution. (semi-normal) = Solutions which contain in one liter one-half of the quantity of the active reagent in the normal solution.

INDICATORS.

An indicator is a substance which is added to the solutions, during titration, for the purpose of showing, by a change of color (or other visible change), the exact point at which the reaction is complete.

The principal indicators employed are:

(1) Solution of Litmus (Test solution, U. S. P.). This turns red with acids and blue with alkalies.

(2) Alcoholic Solution of Phenolphtalein (Test solution U. S. P.). -This is colorless with acids and turns deep red with alkali hydrates and carbonates; bicarbonates and most other salts do not produce this color. It is not reliable when alkaline salts of ammonium or phosphoric acid* are present.

(3) Solution of Methyl-Orange (Test solution U.S. P.).—This solution acquires a yellow color in contact with alkali hydrates, carbonates, or bicarbonates. With the inorganic acids the solution acquires a crimson color. It is indifferent to carbonic acid and should not be used with organic acids.

(4) Solution of Rosolic Acid in diluted alcohol (Test solution U. S. P.). The solution turns violet with alkalies and yellow with acids.

(5) Decoction of Brazil- Wood (Test solution U. S. P.). This is a very sensitive indicator, especially adapted in the estimation. of alkaloids. The solution turns purplish-red with alkalies, and yellow with acids.

Tincture of Cochineal (Test solution U. S. P.).-The solution turns violet with alkalies and yellowish-red with acids. It is employed as an indicator when ammonia or alkaline earths are present.

(7) Starch Mucilage (Test solution U. S. P.).-The cold solution turns blue in the presence of free iodine.

(8) Solution of Potassium Chromate (Test solution U. S. P.).—The solution gives a red color with silver nitrate. When any halogen salts are present, the red color does not appear until every trace of the halogen has combined with the silver. (9) Solution of Potassium Ferricyanide (Test solution U. S. P.).— This gives a blue color or precipitate with ferrous, and only a clear brown solution with ferric salts. This indicates that all of the ferrous salt has been oxidized to ferric, when it ceases to impart a blue color to the solution.

* Excepting in case of an orthophosphate MHPO4-See Estimation of Phosphoric Acid.

APPARATUS EMPLOYED.

MEASURING FLASKS AND CYLINDERS.-For the preparation of volumetric solutions we employ accurately graduated flasks or cylinders. The flasks (Fig. 384) are made of various sizes, usually of the capacity of 250, 500 and 1000 cubic centimeters (at 15° C. or 59° F.), which is indicated by a mark on the neck. Graduated cylinders (Fig. 385) are also employed for this purpose.

PIPETTES. These are instruments (see page 164) graduated to deliver definite volumes of fluids. They are constructed to hold,

when the fluid is drawn up to the mark indicated on the stem, various volumes, as 1 cubic centimeter or fraction thereof, 5, 10, 15, 20, 25 or 50 cubic centimeters.

BURETTES. These are long tubes, usually of a capacity of 25, 50 or 100 cubic centimeters, and graduated into divisions of 1 Cc. or fractions thereof (one-fifths or one-tenths). These are employed for measuring out standard solutions. They are graduated between two points on the cylinder, the upper one (0) beginning a short distance below the upper extremity, the lower one (50 or 100 Cc.) being situated a short distance above the point where the tube begins to narrow toward the outlet; the fluid can be measured only between these points (0 and 50, Fig. 387). The

burettes are usually fitted below with a pinch-cock arrangement (Fig. 386), or with a glass stop-cock (Fig. 388, a-b); the former, when pressed, or the latter when turned, allows the fluid to flow out at any speed desired. The pinch-cock arrangement is applied to the simpler forms of burettes in which a short heavy piece of rubber tube is slipped over the narrow neck (a, Fig. 386) of the lower end of the burette, and tied, if necessary (b, Fig. 386). Into the other end of the rubber tube is slipped a short piece of glass tubing (d, Fig. 386) of small caliber, drawn to a fine orifice; between the extremities of the rubber, a pinch-cock* (c, Fig. 386) is placed,

and by pressing the two tips it is caused to open and permit the flow of liquid from the burette. Fig. 387 illustrates the same form, in which the pinch-cock is replaced by a small glass marble, or short rounded piece of glass rod of sufficient size so that when it is inserted inside the tube, it will retain its position and not permit the liquid to pass through. It is operated by pressing the rubber tube slightly together at this point, forming a narrow channel, which permits the fluid to flow out at any rate desired.

Beginners find it at first somewhat difficult to read off the height of the column of fluid correctly. On holding the burette so that the surface of the fluid is on a level with the eye, it

*See also page 90, Figs. 138, 139.