270 PREPARATION OF REAGENTS [CH.IX 



over it is better to weigh out the number of grams representing the percentage 

 and add to it the right amount of liquid in cubic centimeters. For example 

 if one were to make a 35% aqueous solution of caustic potash in water then 

 one would add 35 grams of caustic potash to 65 cc. of water. If one wished to 

 make a 10% alcoholic solution of caustic potash he would add 10 grams of 

 caustic potash to 90 cc. of alcohol. But here is a case where the alcohol being 

 of less specific gravity than water the mixture would not weigh 100 grams ; 

 and to make the mixture weigh.ioo grams giving therefore an exact percentage, 

 one should take 90 grams of alcohol and add to it 10 grams of caustic potash. 

 In practice in making solutions of collodion or celloidin one usually mixes 

 alcohol and 95% or absolute alcohol in equal volumes and then for a 10% 

 solution 10 grams of the dry soluble cotton or celloidin are added to 90 cc. of 

 the ether-alcohol mixture. But ether is much lighter than water and the alcohol 

 somewhat lighter, so that the percentage in this case would be more than 10% 

 because the 90 cc. of alcohol and ether would weigh considerably less than 90 

 grams. 



\ 377. Mixtures of Liquids to Obtain a desired Percentage. It frequently 

 happens that it is desired to obtain a lower percentage or strength of a liquid 

 than the one in stock. This is very readily done according to the general for- 

 mula: Divide the percentage of the strong solution by the percentage of the 

 desired solution and the quotient will give the number of times too strong the 

 solution is. To obtain the right strength take I of the strong solution, and of 

 the diluting liquid one less than the quotient obtained by dividing the per- 

 centage of the strong solution by the percentage of the weak solution, thus ; 

 Suppose it is desired to obtain a 5% solution of formaldehyde. As the strong 

 solution obtainable in the market is a 40% aqueous solution of formaldehyde 

 gas it is 8 times too strong for the desired solution. To get the proper 

 strength one takes i cc. of the 40% formaldehyde and adds to it 7 cc. of water 

 and the resulting mixture will be only *i the strength of the original solution 

 or 5% instead of 40%. 



378. Mixtures of Alcohol. For alcohol if one desires a 50% solution it 

 is usually near enough correct to add equal parts of 95% alcohol and water, but 

 this does not actually give a 50% solution. To find the real proportions 

 according to the general formula : 95%-=-5o% 1.9 i. e. y for every i cc. of 95% 

 alcohol should be added 0.9 cc. of water or for each 100 cc. of 95% alcohol, 90 

 cc. of water. This even will not give an exact mixture of alcohol for a mix- 

 ture of alcohol and water diminishes somewhat in volume. To get true per- 

 centages an alcoholometer for testing the specific gravity is used. 



A simple method of getting approximately correct mixtures of alcohol is 

 the following : Pour the strong alcohol into a graduate glass (Fig. 215-217) 

 until the volume is the same as the desired percentage, then add water until 

 the volume is the same as the original percentage of the alcohol. Example : 

 To get 50% from 95% alcohol put 50 cc. of 95% into a graduate and fill the 

 graduate to 95 cc. with water, and the resulting mixture will be 50% alcohol, 

 and so with all other strengths. Here the shrinkage is eliminated from con- 

 sideration because the water and alcohol are not measured separately and then 

 mixed, but one is added to the other until a given volume is attained. 



