522 SIMPLE CHEMICAL MANIPULATIONS 



only substance which can immediately be safely used to obtain 

 accurate normal solutions is the perfectly dry, absolutely pure sodium 

 carbonate, Na 2 CO 3 . Of this, 53 grams are weighed out, placed in 

 a 1000 c.c. volumetric flask, dissolved with several hundred cubic 

 centimeters of distilled water, and made up to 1000 c.c. at 16 C. The 

 next solution to be prepared is one of sulphuric or hydrochloric 

 acid. This must be done in such a manner that 1 c.c. of the alkaline 

 carbonate of sodium solution will completely neutralize 1 c.c. of the acid 

 solution, and vice versa. This, however, is very difficult in practice, 

 and generally there is a slight discrepancy. After much manipulation 

 in the preparation of the acid normal solution it may, for instance, 

 be found that 49.8 c.c. of the latter will just neutralize 50 c.c. of the 

 alkaline normal solution. Such a slight difference can be entirely 

 neglected in bacteriological work; in very exact chemical work, 

 however, correction of the result would be necessary by multiplication 

 of the number -of cubic centimeters used out of the burette contain- 

 ing the slightly too strong acid normal solution by T |\ = 100.4. 

 As a rule, normal solutions are not used in full strength, but they 

 are diluted with distilled water in proportions of 1 in 2, 1 in 4, 1 in 5, 

 1 in 10, 1 in 20, 1 in 100. Such dilutions are called one-quarter normal 

 solution, decinormal solution, centinormal solution, etc., and desig- 

 nated in writing as ^ T> TIP ~2T> TTO > e ^ c - ^ n bacteriology decinormal 

 solutions (YQ sol.) are generally used and prepared by taking 100 c.c. 

 of the full strength normal solution, pouring it into a 1000 c.c. volum- 

 etric flask and making it up with distilled water to 1000 c.c. at 16 C. 



When a normal or dilute normal solution is used to find out the 

 exact amount of a certain substance in another solution the latter 

 solution is said to be titrated. 1 In doing so the normal solution is 

 allowed to flow gradually from a burette into an exactly measured 

 amount, say 10, 20, or 50 c.c. of the solution which is being tested. 

 The steps of such a titration have been explained in detail on page 502 

 in the chapter on Milk in the Quantitative Estimation of Lactic Acid. 

 A titration may also be made by placing a definite amount of the 

 normal solution in a beaker and filling the other solution which con- 

 tains the substance to be determined quantitatively into a burette and 

 allowing it to discharge gradually into the normal solution until the 

 reaction is complete. 



Indicators. Whenever a titration is made there must be something 

 to indicate when the reaction is complete. If, for instance, a fluid is 

 titrated for the amount of acid which it contains it is necessary to 

 know when enough of the normal alkaline solution has been added 

 to neutralize the acid present. A reagent added to the acid solution 



1 The words titrate and titration are derived from the French word "titre," which means 

 title, power, or strength. Titrating means, therefore, to find out the strength or concentration 

 of a substance in a solution. This word is also used a good deal in serum investigations, as, 

 for instance, to ascertain the titre of an immune serum, etc. 



