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The values of the second dissociation constant, pK2, and related thermodynamic quantities of 3-[N,N-bis (2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO) have already been reported over the temperature range 5 to 55°C including 37°C. This paper reports the pH values of four NaCl-free buffer solutions and four buffer composition containing NaCl salt at I = 0.16 mol·kg−1. Conventional paH values are reported for all eight buffer solutions. The operational pH values have been calculated for four buffer solutions recommended as pH standards, at 25 and 37°C after correcting the liquid junction potentials with the flowing junction cell.
In order to obtain pH values for standard buffer solutions, it is useful to know the values of pK2 of the buffer compound DIPSO. Recently, the pK2 values of 3-[N,N-bis (2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO)  have been reported at temperatures from 5 to 55°C including 37°C. The structure of DIPSO is as follows:
This zwitterionic compound can be used as a physiological buffer for clinical fluids [2–3]. The pH of the NIST/NBS certified  physiological phosphate primary standard buffer is 7.415 at 25°C and 7.395 at 37°C.
There are some disadvantages [4–6] concerning the use of the phosphate buffer as an ideal pH standard for physiological fluids: (i) phosphates precipitate some polyvalent cations in the blood constituents such as Mg+2 and Ca+2, (ii) may also act as an inhibitor to enzymatic processes, and (iii) the temperature coefficient −0.0028 pH unit / °C does not closely approximate to the whole blood (−0.015 pH unit/°C) [5–6].
Wu and coworkers  have published the values of pK2 and pH of the zwitterionic buffer N-(2-hydroxyethyl)piperazine-N-2-ethanesulfonic acid (HEPES). For two-point calibrations of pH measurements for physiological fluids, 3-(N-morpholino)-2-hydroxypropanesulfonic acid (MOPSO), has been studied by Wu et al. . In 1973, the use of tris(hydroxymethyl)methylglycine (TRICINE) was suggested by Bates et al. [8–9] as a buffer standard for the physiological range of pH 7.2 to 8.5. The pH of 0.06 m TRICINE + 0.02 m sodium TRICINate buffer solution at 37°C is 7.407, matching very closely the pH of blood. In 1978, Bates et al.  also reported pH values of 2-amino-2-(hydroxymethyl)-1,3-propanediol (TRIS), N-tris-(hydroxymethyl) methyl-2-aminoethanesulfonic acid (TES), and N-(2-hydroxyethyl) piperazine-N’-2-ethanesulfonic acid (HEPES) for I = 0.16 mol·kg−1 at 25 and 37 °C. Roy et al.  reported results for pK2 and pH for 3-(N-morpholino)propanesulfonic acid (MOPS) in the temperature range 5 to 55°C. A glass-electrode pH meter assembly can be standardized by using the pH values of the standard buffer solutions with compositions: 0.08 m MOPS + 0.08 m NaMOPSate + 0.08 m NaCl. In 1976, Carmen and Vega  published pK2 values of 2-[N-morpholino]ethanesulfonic acid (MES), and N, N-bis-[2-hydroxyethyl]-2-aminoethanesulfonic acid (BES) at 5 to 55°C. The pH of these solutions closely matches that of the common biological media. In a continuation of research in this area, Roy et al.  have reported the second dissociation constants, pK2 and related thermodynamic quantities for buffer systems of DIPSO. Goldberg et al. , in their comprehensive review of the thermodynamic quantities of the biological buffers, indicated that no reliable data on p.4 are available for the DIPSO buffer.
In order to provide reliable pH values as standards in the pH region close to that of blood serum, we have now examined the buffer compound, DIPSO, with the following compositions in units of molality m, where m = mol·kg−1:
The preparations of these buffer solutions for DIPSO are described under the experimental section.
DIPSO was obtained from Sigma Chemical Co. (St. Louis, Missouri). The details of the purification by further crystallization as well as the determinations of the assay have been reported in earlier papers . The assays showed that the DIPSO was (99.94 ± 0.03) % pure. All buffer solutions were prepared by weighted amounts of DIPSO, NaCl, standard solutions of NaOH in quantity sufficient to prepare NaDIPSO, and calculated amounts of CO2-free doubly distilled water. Vacuum corrections were applied to all weighings.
The preparation of the hydrogen electrodes and the silver-silver chloride electrodes of the thermal electrolytic type whose design and function have been previously described [1, 12, 18]. The errors due to residual liquid junction effects are nearly eliminated if the ionic strength of the physiological phosphate standard buffer is matched with that of the buffer solution as noted in the previous paper (HEPES) in this issue. A correction for the liquid-junction potential has been made.
Three cell types were employed in the present investigation. The schematic diagram of the Harned-type cell is given below:
where the molalities m1, m2, and m3 indicate the respective species, and 1 atm = 101.325 kPa in SI units. The cell (B) is the flowing junction cell.
For cell (C), the NIST/NBS physiological phosphate buffer was used. This primary reference, as previously mentioned in the HEPES paper in this issue, was used. The schematic design of cell (C) is:
The values of the standard electrode potential, E°SCE, and of the saturated calomel electrode were taken as: −0.2415 V, and −0.2335 V at 25 and 37°C [14–17], respectively. The value of the liquid junction potential, Ej, was obtained [7, 11–12] by employing a flowing junction cell. The values of Ej for the physiological phosphate as well as for the other buffer solutions for cell (B) were calculated using the following equation [6–7, 11]:
where k = 0.059156 (as defined in the HEPES paper of this issue) and pH = 7.415 (physiological phosphate buffer solution) at 25°; k = 0.061538 and pH = 7.395 at 37°C. The operational definition of pH, namely pH (x), is:
where x refers to the unknown buffers (DIPSO + NaDIPSO), s is the primary standard reference solution (NBS/NIST physiological phosphate buffer) of known pH, and δEj = Ej(s) − Ej(x). If the residual liquid junction-potential is assumed to be zero for the glass electrode pH meter assembly, then Eq. (2) reduces to
Accurate values of the second dissociation constants, pK2, and related thermodynamic quantities have been reported earlier . The emf values for cell (A) containing four equimolal chloride ion-free buffer solutions, and four buffer solutions in which NaCl had been added to make I = 0.16 mol·kg−1, have been corrected to a hydrogen pressure of 1 atm. The values of the emf at 25°C are the average of at least two readings (at the beginning and the middle) and sometimes at the end of the temperature sequence. The silver-silver chloride electrodes were used throughout the experiments in order to determine pH values of buffer solutions. Duplicate cells usually gave a reading on the average within (0.02 ±0.01) mV in the temperature range 5 to 55 °C. All these results are listed in Table 1 and Table 2, respectively.
The values of the paH for all buffer solutions were calculated using the method [7, 9–12]. The specific compositions are listed in the introduction. Values of the acidity function p(aHγCl) and pαH were derived at from the following equation:
where k is the Nernst slope.
The selection of a reasonable estimate of γCl for the calculation of paH by Eq. (5) has been described elsewhere [6–7, 10–12, 18]. The pH values obtained from the liquid junction cell are designated as pH, the ‘conventional’ pH is designated as paH. A convention  based on an extended Debye-Hückel equation was used for the calculation of with the following form:
The empirical equation for the calculation of the parameter C  is given below:
where C25 = 0.032 .
The values of p(aHγCl)° and p(aHγCl) are listed in Table 3 and Table 4, respectively. The values of paH, listed in Table 5–Table 6, for buffer solutions of DIPSO without and with NaCl were computed from Eq. (4)–Eq. (7) and are represented by the following equations:
The values of pH at 25 and 37°C were also determined from cells with liquid-junction (cells B and C). The flowing junction cell was donated to the author’s laboratory by the National Institute of Standards and Technology (NIST/NBS). The emf values of the cells (B) and (C) at 25 and 37°, are given in Table 7. Also, the values of the liquid junction potential Ej shown in Table 8 were obtained by using Eq. (1). Finally, the values of pH listed in Table 9 were obtained by using Eq. (2) and Eq. (3). The error in pH values can be attributed to the assumptions in the convention used for single-ion-activity coefficient calculation, as well as an error in liquid junction measurement. However, the excellent agreement between the calculated pH values and the values obtained from Ej corrections are within ± 0.002 pH units. The overall uncertainty for the pH values was estimated by combining the systematic uncertainties due to (i) assumption for the calculation of the single-ion-activity coefficient (± 0.003 pH unit), (ii) extrapolation to p(aHγCl)° by plotting to mCl = 0, (iii) liquid junction potential measurement, and (iv) error in the experimental emf measurement (± 0.03 mV). It is evident from table 9 that the pH range lies in the physiological region, for all four buffer solutions of DIPSO. These are recommended as pH buffer standards in the range of biophysiological application.
The authors are grateful to the late Dr. R.G. Bates for useful discussions and important suggestions incorporated in this revised manuscript; and the grant fund from the National Institute of Health (AREA Fund), under grant R15 GM 066866-02, and R15 GM 066866-02S1 (NIH Supplemental Grant).
This original publication is available at springerlink.com by using the following link: http://www.springerlink.com/content/1008357531314322/?p=ec17a7a4ec2a40f492be277a1c205450&pi=6