74 



ALKALIMETRY 



to form a short narrow tube, over which is secured a piece of caoutchouc tubing. This 

 is terminated by a fine glass jet made by drawing out a piece of quill tubing, so as to 

 deliver the solution contained in the alkalimeter in a fine stream, or by single drops, as 

 required. A spring clamp s (Jig. 27) is used to regulate the flow of the liquid. A 

 screw clamp, with one end attached to the support of the burette, is still better, as it 

 enables the operator to deliver the test liquor at any desired rate, and leaves both 

 hands free. Instead of a clamp a short piece of glass rod, of a diameter slightly 

 greater than the bore of the caoutchouc tube, may be introduced between the end of the 

 alkalimeter and the jet. By slightly pinching the rod between the thumb and fore- 

 finger, the caoutchouc tube is expanded laterally, and allows the liquid to escape 

 between the glass rod and its inner surface, effectually closing again the moment the 

 pressure is relieved. A float (Jig. 28) may be used with advantage with these 

 burettes. It consists of a short glass tube, closed at both ends, and weighted with 

 mercury, so as to float upright and rise but little out of the liquid. A lino is etched 

 round the middle of the float, and from this all readings off are made, without regard 

 to the upper surface of the fluid. The float should fit pretty closely, but easily, and 

 the line round it should be perfectly horizontal, or inaccuracies will be introduced into 

 the readings. There are several other forms of alkalimeter, but whichever of them is 

 employed the process is the same namely, pouring carefully an acid of a known 

 strength into a known weight of the alkali under examination, until the neutralising 

 point is obtained, as will be fully explained presently. 



11. Blue litmus-papor being immediately reddened by acids is the reagent used for 

 ascertaining the exact point of the neutralisation of the alkali to be tested. It is prepared 

 by pulverising one part of commercial litmus, and digesting 

 it in six parts of cold water, filtering, and dividing the blue 

 liquid into two equal portions, adding carefully to one of the 

 portions, and one drop at a time, as much very dilute sulphuric 

 acid as is sufficient to impart to it a slight red colour, and 

 pouring the portion so treated into the second portion, which 

 is intensely blue, and stirring the whole together. The mix- 

 ture so obtained is neutral, and by immersing slips of white 



blotting-paper into it, and carefully drying them by hanging them on a stretched 

 piece of thread, an exceedingly sensitive test paper of a light blue colour is obtained, 

 which should be kept in a wide-mouth glass-stoppered bottle, and sheltered from the 

 air and light. 



12. Since the principle on which alkalimetry is based consists in determining the 

 amount of acid which a known weight of alkali can saturate or neutralise, it is clear 

 that any acid having this power can be employed. 



13. The test acid, however, generally preferred for the purpose, is^ sulplraric acid, 

 because the normal solution of that acid is more easily prepared, is less liable to 

 change its strength by keeping, and has a stronger reaction on litmus-paper than any 

 ether acid. It is true that other acids tartaric acid, for example can be procured of 

 greater purity, and that as it is dry and not caustic, the quantities required can be 

 more comfortably and accurately weighed off; and on this account some chemists, 

 after Buchnor, recommended its use, but the facility with which its aqueous solution be- 

 comes mouldy is so serious a drawback, that it is hardly ever resorted to for that object. 



14. -When sulphuric acid is employed, the^wre acid in the maximum state of con- 

 centration, or, as it is called by chemists, the pure hydrate of sulphuric acid, specific 

 gravity 1'8485, is preferable. Such an acid, however, is never met with in commerce, 

 for the ordinary English oil of vitriol is seldom pure, and never to the maximum state 

 of concentration ; the operator, however, may prepare it by distilling ordinary oil of 

 vitriol, but as the specific heat of the vapour of sulphuric acid is very small, the 



