preface to fifteenth edition

8.106 SECTION 8
TABLE 8.15 Compositions of Standard pH Buffer Solutions [National Bureau of Standards (U.S.)]
Air weight of material per liter of buffer solution.
Standard
Weight, g
KH 3 (C 2 O 4 ) 2 ·2H 2 O, 0.05M 12.61
Potassium hydrogen tartrate, about 0.034M Saturated at 25C
Potassium hydrogen phthalate, 0.05M 10.12
Phosphate:
KH 2 PO 4 , 0.025M 3.39
Na 2 HPO 4 , 0.025M 3.53
Phosphate:
KH 2 PO 4 , 0.008665M 1.179
Na 2 HPO 4 , 0.03032M 4.30
Na 2 B 4 O 7 · 10H 2 O, 0.01M 3.80
Carbonate:
NaHCO 3 , 0.025M 2.10
Na 2 CO 3 , 0.025M 2.65
Ca(OH) 2 , about 0.0203M
Saturated at 25C
The buffer values for the NBS reference pH buffer solutions are given below:
Buffer solution
KH
tartrate
0.05 M
KH 2
citrate
0.05 M
KH
phthalate
0.025 M
KH 2 PO 4 ,
0.25 M
Na 2 HPO 4
0.0087 M
KH 2 PO 4 ,
0.0302 M
Na 2 HPO 4
0.01 M
Na 2 B 4 O 7
0.025 M
NaHCO 3 ,
0.025 M
Na 2 CO 3
Buffer value 0.027 0.034 0.016 0.029 0.016 0.020 0.029
For the secondary pH reference standards, the buffer value is 0.070 for potassium tetroxalate and
0.09 for calcium hydroxide.
To prepare the standard pH buffer solutions recommended by the National Bureau of Standards
(U.S.), the indicated weights ofthe pure materials in Table 8.15 should be dissolved in water of
specific conductivity not greater than 5 micromhos. The tartrate, phthalate, and phosphates can be
dried for 2 h at 100C before use. Potassium tetroxalate and calcium hydroxide need not be dried.
Fresh-looking crystals ofborax should be used. Before use, excess solid potassium hydrogen tartrate
and calcium hydroxide must be removed. Buffer solutions pH 6 or above should be stored in plastic
containers and should be protected from carbon doxide with soda-lime traps. The solutions should
be replaced within 2 to 3 weeks, or sooner ifformation ofmold is noticed. A crystal ofthymol may
be added as a preservative.
8.3.2 Standards for pH Measurement of Blood and Biological Media
Blood is a well-buffered medium. In addition to the NBS phosphate standard of 0.025 M (pH s
6.480 at 38C), another reference solution containing the same salts, but in the molal ratio 1 : 4, has
an ionic strength of0.13. It is prepared by dissolving 1.360 g ofKH 2 PO 4 and 5.677 g ofNa 2 HPO 4
(air weights) in carbon dioxide-free water to make 1 liter of solution. The pH s is 7.416 0.004 at
37.5 and 38C.
The compositions and pH s values of tris(hydroxymethyl)aminomethane, covering the pH range
7.0 to 8.9, are listed in Table 8.16.