On Citric Acid. 107 



which is generally used for acids and alkalies. For this end, I 

 know of none better than the common blue litmus paper, which 

 may be bought in London by the sheet. This paper should be 

 kept in a stoppered bottle, or closely folded in paper, to preserve 

 it from the action of the atmosphere, as well as from the light, 

 and this will always prove an excellent test to discover the pre- 

 sence of an acid, which changes the blue to a red ; and the same 

 paper, being slightly tinged by means of a very dilute acid, be- 

 comes equallv delicate as a test for alkalies. A verv weak mix- 

 ture of vinegar and water answers this purpose. 



The use which the printer will have to make of theses, papers, 

 is to ascertain when the portion of lemon-juice under examina- 

 tion is exactly saturated with the alkali. He may, therefore, 

 proceed with it in the following manner : 



Let one thousand grains, or any given quantity, of the lemon- 

 juice be weighed out ; and having previously balanced a phial of 

 the solution of_alkali in an accurate pair of scales, let a portion 

 of the alkali be gradually added to the juice, which should be 

 well stirred with a glass rod, or poured from one vessel to an- 

 other, between every addition of the alkali. When the effer- 

 vescence seems to grow less, a slip of the blue test-paper should 

 be dipped in to ascertain whether the mixture approaches the 

 point of saturation. If the paper comes out red, it indicates that 

 the acid still predominates, and more alkali must therefore be cau- 

 tiously dropped in. After waiting a minute or two, to allow 

 time for them to unite, another slip of test-paper is to be ap- 

 plied, and in this way repeated until the liquor has lost the power 

 of occasioning any change in the colour of the test. 



Wlien this is the case, a slip of the reddened test-paper should 

 be dipped in, which will acquire a blue colour if too much of 

 the alkali has been used ; but if the liquor be brought to that 

 state that it will cause no visible change of colour in either the red 

 or the blue litmus paper, the lemon-juice may then be considered 

 to be accurately saturated, and brought to the neutral state. 



Now, replace the phial of dilute alkali in the scale, note down 

 how many grains in Aveight are requisite to balance it, and call 

 this the <iuantity of alkali expended in saturating the juice. When 

 the operator has by previous experiment determined how much 

 of his standard alkali is necessary to saturate any given quantity 

 of the pure crystals of citric acid, he may at once, by the com- 

 mon rule of proportion, say how much solid citric acid is con- 

 tained in any particular sample of lemon-juice which has been 

 submitted to the tests already described. 



The method now reconnnended of examining the goodness of 

 lemon-juice, will answer the purpose of any individual so far as 

 respects his own purchases ; but as the pearl ash of commerce 



contain.^ 



