CLASSIFICATION OF BACTERIA 291 



a pure aqueous solution of NaOH containing exactly 40 grams per 

 litre. Beginners often attempt to make the solution by weighing 

 .40 grams of sodium hydrate sticks, dissolving in 700 — 800 c.c. 

 -water and making up to 1 litre, or even by adding 40 grams directly 

 to 1 litre of water. Unfortunately, sodium hydrate is so hygros- 

 copic that in the very process of weighing it will absorb water from 

 the atmosphere sufficient to change its weight, so that it is impossible 

 thus to prepare an accurate solution. A sufficiently accurate normal 

 NaOH solution may be prepared (if pure oxalic acid is available) 

 ~by dissolving 62.5 grams of oxalic acid in 700 — 800 c.c. water, 

 making up exactly to 1 litre. This constitutes a normal acid solu- 

 tion. Dissolve about 50 to 60 grams NaOH, weighed quickly and 

 roughly in 700 to 800 c.c. of recently boiled water (thus securing a 

 solution C0 2 -free, and somewhat stronger than normal). Titrate 

 the inexact NaOH solution against the exact oxalic acid solution, 

 using the phenolphthalein solution as an indicator ; it will be found 

 that 5 c.c.s (say) of the NaOH require for neutralization (say) 

 7 c.c.s of the oxalic acid solution; hence 5 c.c.s of NaOH solution 

 should be diluted to 7 c.c.s with water, in order that it should exactly 

 equal in strength the oxalic acid solution. Hence 800 c.c.s (say) of 

 the NaOH solution should be diluted to (7/5 of 800) 1,120 c.c.s in 

 order to equal in strength, c.c. for c.c, the oxalic acid solution. 

 Dilute with great care to a point somewhat short of the calculated 

 figure (say, to 1,050 c.c.s,) and again titrate. If the NaOH solu- 

 tion still proves too strong (say that 5 c.c. of the NaOH solution 

 now require 5.5 c.c.s of oxalic acid solution for neutralization), 

 dilute again in the proportions thus indicated, 5.5/5 of 1,045 ( al_ 

 lowing for the 5 c.c.s withdrawn for titration), i. <?., dilute the re- 

 maining 1,045 c - c - s °f NaOH solution with water to 1,149.5 c - c - s 

 and check again. N 20 solutions are prepared by accurately diluting 

 1 part of a normal solution to make 20 parts in all — i. c, dilute 50 

 c.c.s normal NaOH with distilled water to make one litre. 



It is best when possible to have the normal NaOH prepared 



