52 PHYSICAL CHEMISTRY 



"HC1" = A o.i n solution of HC1 titrated with silver nitrate. 



"NaOH" = A o.i n solution of NaOH made by adding an 

 excess of sodium or sodium amalgam to C0 2 -free H 2 and 

 titrating with HC1 and diluting to .1 n. Or decanting a clear 

 saturated solution of NaOH and diluting to required volume 

 with C0 2 -free H 2 0. 



"Citrate" = A o.i m solution of secondary sodium citrate 

 made by dissolving 21.008 g citric acid in 800 cc of .25 n NaOH 

 and diluting to a liter. A layer of ether on top will preserve it. 



"H 3 P0 4 " = A 1/15 m solution of H 3 P0 4 accurately stand- 

 ardized. 



"KH 2 P0 4 " = A 1/15 m solution of KH 2 P0 4 made by dis- 

 solving 9.078 g of the recrystallized anhydrous (desiccated) salt 

 to the liter. 



"Na 2 HP0 4 " = A m/15 solution of Na 2 HP0 4 made by dis- 

 solving 11.876 g to the liter, of Na 2 HP0 4 2H 2 that has been 

 recrystallized and dried until it has the right water content by 

 analysis. 



"Borate" = A solution of 12.404 g of recrystallized desiccated 

 boric acid + 100 cc n NaOH solution made up to 1 liter. 



The distilled water used in making these solutions should be 

 free from NH 3 and C0 2 . The C0 2 may be practically eliminated 

 by boiling 15 minutes in a tin vessel and cooling under a soda 

 lime tube. All solutions should be kept in "nonsol" or "pyrex" 

 glass vessels or in paraffined bottles and those with PH above 5 

 should be provided with soda lime tubes. The reagents should 

 be the purest. In dropping metallic sodium into water 2 should 

 be excluded. NaOH made from Na may be purchased but 

 should be freed from carbonate by making it first an 80 per cent 

 solution in a stoppered bottle and allowing the carbonate to pre- 

 cipitate, then pipetting off the clear solution, diluting with C0 2 - 

 free water and standardizing it. All of the salts should be dis- 

 solved in boiling H 2 0, filtered and crystallized at least once. It 

 is difficult to obtain pure phosphates. The Na 2 HP0 4 solution 

 should turn phenolphthalein a deep red. In the chart, Fig. 20, 

 impurities in the reagents affect the ends of the curves more than 

 the middle. 



The buffer mixtures of Clark and Lubs (1916) are as follows: 



