NORMAL ALKALI AND ACID SOLUTIONS. 255 



tion of sodium hydroxid. 1 Add this solution, drop by drop, 



1 To make strictly accurate normal solution of NaOH and HC1 re- 

 quires familiarity with quantitative chemistry. Solutions accurate 

 enough for bacteriological work may be made as follows : 



Normal NaOH. Dissolve 40 grams of fresh, dry NaOH in one liter 

 of distilled water. This will not be a strictly accurate normal solu- 

 tion, but very nearly so. The solution should not be kept in a glass- 

 stoppered bottle. It loses its strength and it is best to use solutions 

 which are fresh. To make a one tenth normal solution dilute any 

 quantity of the normal solution with ten times its bulk of distilled 

 water. This should be made fresh from a normal solution at the 

 time of using. 



Normal HCl. To 700 c.c. distilled water add 100 c.c. of concen- 

 trated c.p. HCl which, assuming that the HCl is 30 per cent, acid, 

 would give 30 grams of HCl in 800 c.c. of the solution. This gives 

 about the right strength (i. e., 36.5 c.c. per liter), but since the HCl 

 varies in strength, the solution must be standardized with a normal 

 NaOH prepared as described above. To do this proceed as follows : 

 Place 10 c.c. of the HCl solution in an evaporating dish. Add 40 c.c. 

 of distilled water and I c.c. of phenolphthalein solution. Fill a 

 burette (see Fig. 39) with some freshly prepared normal NaOH, 

 and allow the NaOH to flow from the burette into the evaporating 

 dish, drop by drop, until the faintest pink color appears in the acid 

 solution and remains. This indicates the neutral point. Read from 

 the burette the amount of NaOH which was required to neutralize 

 the 10 c.c. of acid. If the HCl solution were exactly normal, 10 c.c. 

 would require exactly 10 c.c. of NaOH solution for neutralization. 

 The solution prepared as above described (100 c.c. of HCl in 700 c.c. 

 of water) is usually too strong and requires more than 10 c.c. of 

 normal NaOH for neutralization. It must, therefore, be diluted with 

 water. The amount of water that must be added can be calculated as 

 follows: Suppose there were required n c.c. of the NaOH to 

 neutralize 10 c.c. of the HCl solution. This would indicate that the 

 HCl was eleven tenths of its proper strength. To make it normal 

 there should be added to it one part of water to every ten parts of 

 solution. The solution prepared now contains 79 c - c -> an d 79 X 

 1/10 = 79 c.c. Hence 79 c.c. of distilled water should be added 

 to the 790 c.c. of HCl to give a normal HCl solution. Add 79 

 c.c. of water and test again to correct any error. If the original HCl 

 solution should prove to be too weak it is easier to make another 

 solution a little stronger than to calculate the amount of acid neces- 

 sary to bring the solution to a normal strength. 



The normal HCl solution once made will keep a long time without 

 deterioration if kept in a stoppered bottle. A one tenth normal HCl 

 solution may be made by diluting the normal solution with ten times 

 its bulk of water at the time of using. 



