498 PHYSIOLOGICAL CHEMISTRY 



750 c.c. of distilled water and miy thoroughly. Clean a burette by allowing it to 

 stand filled with cleaning mixture (potassium dichromate and sulphuric acid) for 

 a few minutes or longer if necessary. Empty, rinse several times with tap 

 water, finally with distilled water, and allow to drain. If necessary to use the 

 burette before it is perfectly dry, introduce a few c.c. of the NaOH solution, and 

 invert a couple of times to rinse the burette, discarding this NaOH. Then 

 fill the burette with the alkali solution, making sure that the tip contains no air 

 bubbles, and run out solution until the bottom of the meniscus is exactly at o. 



Into a clean Erlenmeyer flask (150-250 c.c.) now introduce 25 c.c. of N/io 

 oxalic acid solution measured from an accurate, clean pipette, previously rinsed 

 by means of a little of the acid solution drawn up into it. Allow the pipette to 

 drain about 15 seconds against the side of the flask. Add 2-3 drops of a i per 

 cent alcoholic solution of phenolphthalein. 



Now run in NaOH solution from the burette, rotating the flask. Ten c.c. 

 can be added quite rapidly ; then add more slowly, and finally drop by drop until 

 the last drop changes the color of the solution permanently throughout to a 

 definite pink. Taken the burette reading. Repeat the titration until two exact 

 duplicate readings are obtained. 



Calculate the strength of the NaOH solution. Divide 25 (the number of 

 c.c. of N/io oxalic acid used) by the burette reading and obtain the strength of 

 the NaOH in terms of N/io solution. For example, if 15.6 c.c. were required : 

 25 -T- 15.6 = 1.603 X o.i = 0.1603 N. 



(c) Preparation of the N/io NaOH Solution. Calculate how much of the 

 standard NaOH solution just prepared will be required to make a liter of 

 N/io solution. To do this divide 1000 c.c. by the strength of the NaOH in terms 

 of N/io solution. Thus in the example cited above: 1000 -r- 1.603 = 623.8 c.c. 

 required. Measure out the exact amount of alkali required (using the burette, 

 pipette, and volumetric flasks) into a 1000 c.c. flask. Dilute with distilled 

 water exactly to the mark. Mix very thoroughly and transfer to a clean, dry 

 bottle with a rubber (not glass) stopper. Check the strength of the solution 

 by again titrating 25 c.c. portions of oxalic acid solution. 1 



Preparation of N/io Hydrochloric Acid. Concentrated hydrochloric acid is 

 about 10 N or 36.5 per cent HC1. Approximately N/io HC1 may, therefore, be 

 prepared by diluting 10 c.c. of the concentrated acid to i liter hi a volumetric 

 flask. This must be standardized by titration with N/io alkali, using preferably 

 alizarin red as an indicator. 



Or introduce into a liter flask 12 c.c. of concentrated HC1 and 750 c.c. of 

 distilled water. Mix well and titrate 10, 15, or 25 c.c. portions of the acid solu- 

 tion with N/io NaOH, using alizarin as an indicator. Dividing the number of c.c. 

 of acid used by the number of c.c. of N/io NaOH required gives the strength of 

 the HC1 in terms of N/io solution. Dividing 1000 by this quotient gives the 

 number of c.c. of HC1 solution to be measured into a volumetric flask and made 

 up to 1000 c.c. 



This diluted solution will be N/io HC1. It should be mixed thoroughly and 

 25 c.c. portions of it checked by titration with the N/io NaOH. 2 



*If a very high degree of accuracy is desired, the alkali may be checked against pure acid 

 potassium phthalate. 



2 The acid solution may be standardized directly in the following manner: Introduce a 

 platinum dish containing very pure sodium bicarbonate or the highest grade anhydrous 

 sodium carbonate into a hot air o^en previously heated to 2ooC. Raise the temperature 



