6

REAGENTS

AND APPARATUS

The reagents chiefly employed are distilled water, physiological saline solution (0.6 per cent. NaCl), alcohol, ether, glycerine, the mineral acids, acetic, oxalic and tannic acids, potash, soda, ammonia, lime water, baryta water, silver nitrate, barium chloride, lead acetate, copper sulphate, mercuric chloride, sodium chloride, carbonate and sulphate, magnesium sulphate, the carbonate, chloride, molybdate, oxalate, sulphide and sulphate of ammonium, &c. &c.

Normal Solutions used in analysis are of such a strength that 1 litre at 15° C. contains the hydrogen equivalent of the reagent in grammes. A normal solution of hydrochloric acid, for instance, will contain (H=1+Cl=35·5=36·5) 36·5 grammes of the acid in a litre of water. In the case of a bivalent Or in the case of caustic soda, NaHO (Na=23+H=1+0=16), 40 grammes must be present in the litre. substance the equivalent is half the atomic or molecular weight; thus a normal solution of oxalic acid which is dibasic (C2H2O, +H2O=126) contains 63 instead of 126 grammes dissolved in the litre.

Decinormal solutions are, and centinormal solutions of the strength of normal solutions.

Empirical Standard Solutions are generally constructed so that 1 c.c. corresponds to 0.01 gramme (1 centigramme) of the substance to be estimated.

The Apparatus necessary consists of the ordinary appliances of the chemical laboratory: test-tubes, beakers, flasks, funnels, filters, dishes and crucibles, stirring rods, pestle and mortar, wash-bottles, retorts. pipettes, blow-pipe, balance, air and water-baths, exsiccators, measures, &c., and in addition certain forms of apparatus which are more fully described in subsequent chapters, such as microscope, spectroscope, polarimeter, dialyser, apparatus for gas analysis and for combustions, specific gravity bottles, urinometers, &c. &c. In addition, apparatus for the manufacture of carbonic acid, sulphuretted hydrogen, and other gases, is often needed.

7

CHAPTER II

ANALYTICAL METHODS

GRAVIMETRIC AND VOLUMETRIC ANALYSIS

GRAVIMETRIC analysis, or quantitative analysis by weight, consists in separating out the constituents of any compound in a pure state, or in the form of some new compound of known composition, and accurately weighing the products.

Volumetric processes are, as a rule, more quickly performed, and consist in submitting the substance to be estimated to certain characteristic reactions, employing for such reactions solutions of known strength, and from the volume of solution necessary for the production of such reaction determining the weight of the substance to be estimated.

Volumetric analysis consequently depends on the following conditions for its successful practice :

1. A solution of the reagent, the chemical power of which is accurately known, called the standard solution.'

2. A graduated vessel from which portions of it may be accurately delivered, called the burette (fig. 1).

3. The reaction produced by the test solution with any given substance must either by itself, or by an indicator, be such that its termination is unmistakable to the eye, and thereby the quantity of the substance with which it is combined accurately determined.

FIG. 1.-Two burettes on stand. (Sutton.)

The great advantage of volumetric processes is that the substance to be estimated need not be isolated in a pure condition, but the reaction chosen is generally one which is not interfered with by the presence of other substances.1

Suppose, for instance, one requires to know the amount of phos

1 The above introductory sentences are taken almost verbatim from Sutton's Volumetric Analysis.

phoric acid in the urine; a measured amount of urine is rendered acid and boiled, and to it is added from a burette, a solution of known strength of uranium acetate (which for accuracy has been previously titrated' with a standard solution of sodium phosphate); the result of this is the formation of a compound of the uranium with the phosphoric acid, and this compound, called uranium phosphate, is insoluble in hot acid urine, and so a precipitate occurs. The precipitate, which is yellowish-white in colour, continues to form until all phosphoric acid is combined. When the precipitate ceases to form, one knows that there is no more phosphoric acid in solution, and if one adds more uranium acetate it will be left uncombined and free. One's object then is to add just sufficient of the standard solution to precipitate all the phosphoric acid. The volume of urine originally taken is known, the strength of the standard solution is known, and the amount of the standard solution that has been used can be ascertained by reading the burette.2 It is, however, difficult to determine by the eye when precipitation is finished; an indicator is therefore used to detect any excess of the uranium salt; this is done by testing a drop of the mixture with a drop of potassium ferrocyanide on a white porcelain plate or testing-slab; this gives a reddish-brown precipitate with any uranium salt not combined with phosphoric acid. The appearance of such a brown precipitate indicates the end of the reaction.

As an example, suppose the amount of urine taken was 50 C.C., and the amount of standard solution required for the appearance of the terminal reaction 24 c.c. The standard uranium solution used is of such a strength that 1 c.c. will exactly precipitate 0·005 gramme of phosphoric acid. 24 c.c. will precipitate 24 times 0·005=0·12 grammes. This is the amount of phosphoric acid in 50 c.c. of urine: the amount in 100 c.c. of urine will therefore be twice as great-c.c. 0·24 per cent.

This is an operation which can be completed in a few minutes, whereas if it had been necessary to separate out the phosphoric acid in a pure condition and weigh it, the processes would have extended over several days.

There are, however, certain substances to which a volumetric method cannot be applied, and it is then necessary to use the gravi

A titrated solution is one of which the strength has been accurately found by experiment. When a solution is directed to be titrated, the meaning is that it is to be quantitatively tested for the amount of pure substance it contains by the help of standard or previously titrated solutions.

2 It is usual to note the position of the lowest point of the curved surface (meniscus) of the fluid in a burette. Sometimes accuracy is obtained by using a glass float which rises and falls with the fluid in the burette without wavering; this has a horizontal line drawn on it, and the coincidence of this line with the graduation mark on the burette is accepted as the true reading.

metric method. The precautions to be used in such a method may be best illustrated by taking an example, and we may again select our example from the urine in certain forms of morbid urine, proteid matter is present; it is often desirable to estimate it, and the best method for doing so accurately, is to precipitate it, and weigh the pre

FIG. 2.-Sprengel's filter pump. Water enters by tube ir, and escapes by d, drawing air with it from the rest of the apparatus, so producing a partial vacuum in the flask i, into which filtration is performed. The gauge f indicates the pressure, and the clips a and b regulate the rate of flow of water. (Gschleidlen.)

cipitate. Among the many precipitants of proteids, alcohol is on the whole the most convenient. A known volume of urine is evaporated to a small bulk, and if alkaline is rendered faintly acid; about ten times the amount of alcohol is added, and the mixture boiled. The precipitate is collected on a previously dried and weighed filter. The filter used must either contain no ash, or the amount of ash (i.e. mineral constituents) must be known.

The precipitate is then thoroughly washed with alcohol and ether to remove all the other constituents of the urine; the filter, with the precipitate on it, is dried at 110° C., cooled in an exsiccator, and again weighed. The increase in weight is the amount of albuminous substance in the volume of urine originally taken. Proteid, however, carries down with it a certain amount of ash; this is estimated as follows: A crucible is dried and weighed; in this the filter and precipitate are carefully burnt, until ash only remains, allowance being made for the ash of the filter, this amount of ash must be deducted from that of the proteid previously found.

FILTRATION

The filter should be smaller than the funnel into which it is inserted, and generally should be moistened with water before being used. Filtration may be hastened by the use of ribbed filters; hot liquids also filter more quickly than cold. Filtration under pressure may be accomplished by using one of the many forms of filter pump of which one is here figured (fig. 2).1

E

In order to keep a liquid hot during filtration, the ordinary glass funnel is enclosed in a hollow copper funnel filled with hot water (fig. 3).

In the case of fine precipitates, a small portion of the precipitate may at first pass through the filter; soon, however, the larger pores of the paper become plugged, and the portions, which first passed through, must be returned to the filter.

With voluminous, dense, or gelatinous deposits the separation of the greater part of the precipitate may be accomplished FIG. 3.-Sectional view of hot water funnel. by filtering through muslin or linen.

(Gschleidlen.)

Precipitates are now often collected on asbestos filters; these are readily made by perforating the bottom of a platinum crucible with fine holes and filling up the bottom of the crucible with finely divided asbestos. They are advantageous, as they can be easily dried and weighed, are permanent, and where incineration is necessary are free from ash.

WASHING PRECIPITATES

Precipitates may be washed on the filter; it is best to use a wash-bottle, and care must be taken that the wash-liquid penetrates to every part of the precipitate, which may be gently disturbed for the purpose with a glass rod.

Washing by decantation is only applicable to precipitates that are heavy and subside readily; the precipitate is well mixed with the wash-liquid and allowed to settle; the wash-liquid is then poured, syphoned, or pipetted off, more liquid added, and the process repeated as often as necessary.

1 Simple forms of filter pump can now be purchased for a few shillings, and can be attached to any ordinary water-tap.