72 



ALKALIMETRY 



21 



22 



45 



importance), not merely the mode of preparation, but also the easiest method of 

 detection. C. G. W. 



ALKAI.IMETER. There are various kinds of alkalimeters, but it will be more 

 convenient to explain their construction and use in the article on ALKALIMETRY, to 

 which the reader is referred. 



AZJCAXiXMETIfsr. 1. The object of alkalimetry is to determine the quantity of 

 caustic alkali or of carbonate of alkali contained in the potash or soda of commerce. 

 The principle of the method is, as in acidimetry, based upon Dalton's law of chemical 

 combining ratios that is, on the fact that in order to produce a complete reaction a 

 certain definite weight of reagent is required, or, in other words, in order to saturate 

 or completely neutralise, for example, one equivalent of a base, exactly one equivalent 

 of acid must be employed, and vice versd. This having been thoroughly explained in 

 the article on ACIDIMETHY, the reader is referred thereto. 



2. The composition of the potash and of the soda met with in commerce presents 

 very great variations ; and the value of these substances being, of course, 

 in proportion to the quantity of real alkali which they contain, an easy 

 and rapid method of determining that quantity is obviously of the greatest 

 importance both to the manufacturer and to the buyer. The process by 

 which this object is attained, though originally contrived exclusively for 

 the determination of the intrinsic value of these two alkalies (whence its 

 name Alkalimetry), has since been ex- 

 tended to that of ammonia and of earthy 

 bases and their carbonates, as will be 

 shown presently. 



3. Before, however, entering into a de- 

 scription of the process itself, we will give 

 that of the instrument employed in this 

 method of analysis, which instrument is 

 called an alkalimeter. 



4. The common alkalimeter is a tube 

 closed at one end (see fg. 21), of about 

 fths of an inch internal diameter, about 

 9-J inches long, and is thus capable of 

 containing 1,000 grains of pure distilled 

 water. The space occupied by the water is 

 divided accurately into 100 divisions, num- 

 bering from above downwards, each of 

 which, therefore, represents 10 grains of 

 distilled water. 



6. When this alkalimeter is used, the 

 operator must carefully pour the acid from 

 it by closing the tube with his thumb, so 

 as to allow the acid to trickle in drops as 

 occasion may require ; and it is well also to 

 emear the edge of the tube with tallow, in order to prevent any portion of the test 

 acid from being wasted by running over the outside after pouring, which accident 

 would, of course, render the analysis altogether inaccurate and worthless ; and, for 

 the same reason, after having once begun to pour the acid from the alkalimeter by 

 allowing it to trickle between the thumb and the edge of the tube, as above men- 

 tioned, the thumb must not be removed from the tube till the end of the experiment, 

 for otherwise the portion of acid which adheres to it would, of course, be wasted 

 and vitiate the result. This uncomfortable precaution is obviated in the other forms 

 of alkalimeter now to be described. 



6. That represented \nfig. 22 is Gay-Lussac's alkalimeter; it is a glass tube about 

 14 inches high, and an inch in diameter, capable of holding more than 1,000 grains 

 of distilled water; it is accurately graduated from the top downwards into 100 

 divisions in such a way that each division may contain exactly 10 grains of water. 

 It has a small tube, b, communicating with the larger one, which small tube is bent and 

 bevelled at the top, c. This very ingenious instrument, known also under the names 

 of ' burette ' and 'pouret,' was contrived by Gay-Lussac, and is by far more convenient 

 ^han the common alkalimeter, as by it the test acid can be unerringly poured drop by 

 drop, as wanted. The only drawback is the fragility of the small side-tube, b, on 

 which account the common alkalimeter, represented in Jiff. 21, was generally used, 

 especially by workmen, because, as it has no side-tube, it is less liable to be broken ; 

 but it gives less accurate results, a portion of the acid being wasted in various ways, 

 and it is besides less manageable. Gay-Lussac's ' burette ' is therefore preferable ; 

 and if melted wax be run between the space of the large and of the small tube, the 



