ALKALIMETRY . 75 



distillation is a somewhat hazardous operation, unless peculiar precaution be taken. 

 Tho following apparatus, however, allows of the acid being distilled in a perfectly safo 

 and convenient manner ; it consists of a plain glass retort, charged with oil of vitriol : 

 a little protosulphate of iron is added, for the purpose of destroying any nitrous pro- 

 ducts which the acid may evolve, and it is then placed into a cylinder of iron, the 

 bottom of which is perforated with holes about three-quarters of an inch in diameter, 

 except in the middle, where a large hole is cut of a suitable size for the retort to rest 

 upon ; the sides of the cylinder are likewise perforated, as represented in Jig. 29. 

 Ignited charcoal is then placed all round the retort, the bottom of which protruding 

 out of the influence of the heat, allows the ebullition to proceed from the sides only. 

 It is well to put into the retort a few fragments of quartz or a few lengths of platinum 

 wire, the effect of which is to render the ebullition more regular. 



15. In order to prevent the acid fumes from condensing in the neck of the retort, it 

 should be covered with a cover of sheet iron, as represented in fig. 29. 



16. The first fourth part which distils over should be rejected, because it is too 

 weak ; the next two-fourths are kept, and the operation is then stopped, leaving the 

 last fourth part of the acid in the retort. The neck of the retort should be about four 

 feet long and about one and & half inches in the bore, and be connected with a large 

 receiver ; and as the necks of retorts are generally much too short for the purpose, an 

 adapter tube should be adjusted to it and to the receiver, but very loosely ; this precau- 

 tion is absolutely necessary, for otherwise the hot acid falling on the sides of the 

 receiver would crack it ; things, in fact, should be so arranged that the hot drops of 

 the distilling acid may fall into the acid which has already distilled over. Do not 

 surround the receiver with cold water, for the hot acid dropping'on the refrigerated 

 surface would also certainly crack it. Tho acid so obtained is pure oil of vitriol, or 

 monohydrated sulphuric acid, SO 3 , HO, and it should be kept in a well-stoppered and 

 dry flask. 



17. For commercial assays, however, and, indeed, for every purpose, the ordinary 

 concentrated sulphuric acid answers very well : when used for the determination of 

 the value of potashes, it is made of such a strength that each division (or 10 water- 

 grains' measure) of the alkalimeter saturates exactly one grain of pure potash ; an 

 acid of that particular strength is prepared as follows : 



18. Take 11276 grains of pure neutral and anhydrous carbonate of soda, and dis- 

 solve them in about 5 fluid ounces of hot water.' This quantity, namely, 112'76 

 grains, of neutral carbonate of soda will exactly saturate the same quantity of pure 

 sulphuric acid (SO 3 ) that 100 grains of pure potash would. It is advisable, however, 

 to prepare at once a larger quantity of test solution of carbonate of soda, which is of 

 course easily done, as will be shown presently. 



19. Mix, now, 1 part, by measure, of concentrated sulphuric acid with 10 parts of 

 water, or rather as it is advisable, where alkalimetrical assays have frequently to be 

 made, to keep a stock of test acid mix 1,000 water-grains' measure of concentrated 

 sulphuric acid with 10,000 grains of water, or any other larger proportions of concen- 

 trated sulphuric acid and water, in the above respective proportions ; stir the whole 

 well, and allow it to cool. The mixture of the acid with the water should be made 

 by first putting a certain quantity of the water into a glass beaker or matrass of a 

 suitable size, then pouring the concentrated acid slowly therein, while a gyratory 

 motion is imparted to the liquid. The vessel containing the acid is then rinsed with 

 the water, and both the rinsing and the rest of the water are then added to the whole 

 mass. When quite cold, fill the graduated alkalimeter with a portion of it up to the 

 point marked 0, taking the under line of the liquid as the true level ; and whilst 

 stirring briskly with a glass rod the aqueous solution of the 112'76 grains of neutral 

 carbonate of soda above alluded to, drop the test acid from the alkalimeter into the 

 vortex produced by stirring, until, by testing the alkaline solution with a strip of 

 reddened litmus-paper after every addition of acid, it is found that it no longer shows 

 an alkaline reaction (which is known by the slip of reddened litmus-paper not being 

 rendered blue), but, on tho contrary, indicates that a very slight excess of acid is 

 present (which is known by testing with a slip of blue litmus-paper, which will then 

 turn slightly red). 



20. If, after having exhausted the whole of the 100 divisions (1,000 water-grains' 

 measure) of the diluted acid in the alkalimeter, the neutralisation is found to be 

 exactly attained, it is a proof that the test acid is right. 



21. But suppose, on the contrary (and this is a much more probable case) suppose 

 that only 80 divisions of the acid in the alkalimeter have been required to neutralise 

 the alkaline solution, it is then a proof that tho test acid is too strong, and accord- 



' Anhydrous, or dry, neutral carbonate of soda may be obtained by keeping a certain quantity of 

 pure bicarbonate of soda for a short time at a dull red heat, in a platinum crucible : the bicarbonate 

 is converted into its neutral carbonate, of course free from water. 



