URINE 497 



A normal alkali solution is exactly equivalent to a normal acid 

 solution, i.e., 'a liter of the alkali will neutralize a liter of the acid. 

 According to the reaction of neutralization, therefore, the 36.5 grams 

 of HC1 in a liter of this normal acid will require 40.0 grams of sodium 

 hydroxide to neutralize it, and a liter of normal sodium hydroxide 

 . must contain 40.0 grams of the alkali. 



HC1 + NaOH -> NaCl + H 2 O 



(36.5) (40.0) (58.5) (18) 



Having prepared solutions of acid and alkali of definitely known 

 strength, it is then possible to determine the strength of any unknown 

 acid or alkali by finding out how much of these standard solutions is 

 required to neutralize a definite volume of the unknown solution. 



In order to tell when the unknown solution has been exactly neu- 

 tralized, we use a small amount of one of a class of substances called 

 indicators. An indicator changes color at, or "hear, the neutral point, 

 and this color change indicates the end point of the titration. 



When strong acids (as HC1) are being titrated with strong alkalies 

 (as NaOH), almost any of the common indicators is satisfactory. If 

 weak acids (acetic acid) or weak bases (as ammonia) are being titrated, 

 it is necessary to be very careful in the choice of an indicator as all 

 indicators are not sufficiently sensitive to those. 1 



Preparation of N/io Oxalic Acid Solution. Weigh accurately a watch glass 

 or a piece of glazed paper. Then add to the weights on the balance pan 3.1512 

 gm. With a spatula transfer to the watch glass enough pure oxalic acid in the 

 form of clear crystals to counterbalance exactly the weights in the opposite 

 pan. Transfer completely to a 250 c.c. beaker with the aid of a camel's hair 

 brush. Add about 150 c.c. of distilled water and stir with a glass rod until 

 dissolved, warming gently if necessary. Transfer every particle of this solution 

 to a clean 500 c.c. volumetric flask, rinsing rod and beaker several times with 

 distilled water. Hold under the tap until cooled to room temperature. Then 

 add distilled water until the bottom of the meniscus is level with the mark on 

 the neck of the flask (the lower mark if there are two). Insert a stopper and 

 mix thoroughly by inverting the flask again and again. Transfer to a clean dry 

 bottle. Label. This solution will not keep indefinitely and is to be used only 

 in the standardization of N/io alkali. 



Preparation of N/io Sodium Hydroxide Solution. (a) Preparation of con- 

 centrated carbonate-free sodium hydroxide solution. Shake up about 120 gm. 

 best quality NaOH with 100 c.c. of distilled water in a 300 c.c. Erlenmeyer 

 flask (Pyrex) to make a saturated solution. Stopper and allow to stand for a 

 couple of days or until the sodium carbonate settles to the bottom leaving a clear 

 solution of NaOH practically free from carbonate. 



(b) Preparation of a Standard Sodium Hydroxide Solution. Measure out 

 6.3 c.c. of the saturated NaOH solution from a burette into a liter flask. Add 



1 For a further consideration of indicators see Chapter VII. 

 32 



