NBS pH scale

Culberson (1988) writes The major problem with the NBS pH scale for equilibrium measurements in sea water is the unreproducibility of its liquid junction potential, not its lack of theoretical meaning. ... 

Note that the constants given in Appendix 6.2, Table A6.3, are slightly different because a different pH convention (NBS pH scale) is used. 

The pressure dependence of the various carbonate equilibrium constants is less well defined than the temperature dependence, with the result that less attention has been devoted to empirical equations for the pressure dependence of pH. Millero (1979) quotes the following equation for the NBS pH scale as a function of pressure P ... 

It is appropriate at this point to discuss the "apparent" pH, which results from the sad fact that electrodes do not truly measure hydrogen ion activity. Influences such as the surface chemistry of the glass electrode and liquid junction potential between the reference electrode filling solution and seawater contribute to this complexity (see for example Bates, 1973). Also, commonly used NBS buffer standards have a much lower ionic strength than seawater, which further complicates the problem. One way in which this last problem has been attacked is to make up buffered artificial seawater solutions and very carefully determine the relation between measurements and actual hydrogen ion activities or concentrations. The most widely accepted approach is based on the work of Hansson (1973). pH values measured in seawater on his scale are generally close to 0.15 pH units lower than those based on NBS standards. These two different pH scales also demand their own sets of apparent constants. It is now clear that for very precise work in seawater the Hansson approach is best. 

An operational definition endorsed by the International Union of Pure and Applied Chemistry (lUPAC) and based on the work of Bates determines pH relative to that of a standard buffer (where pH has been estimated in terms of p"H) from measurements on cells with liquid junctions the NBS (National Bureau of Standards) pH scale. This operational pH is not rigorously identical to p H defined in equation 30 because liquid junction potentials and single ion activities cannot be evaluated without nonthermodynamic assumptions. In dilute solutions of simple electrolytes (ionic strength, I < 0.1) the measured pH corresponds to within 0.02 to p H. Measurement of pH by emf methods is discussed in Chapter 8. 

The accuracy and usefulness of these relationships is dependent on the mathematical definition of/ . This coefficient is estimated from various modifications of the Debye-Hiickel equation [23,44,45] [Eqs. (13-17) Section 2.2.5]. The definition given by Eq. (1) is certified by British Standard no. 2586 (a revision of Br. Std. no. 1647) to be accurate to 0.02 pH imit in the temperature region 0 to 95 °C. The British Standard [60] is stated [45] to be "consistent in nearly all respects with the NBS conventional scale "... 

Irvin JL and Irvin EM, Apparent dissociation exponents of quinine, pamaquine, and a quinolylpiperidylcarbinol Application of an extended pH scale,/. Biol. Chem., 174, 577-587 (1939). Cited in Perrin Bases no. 2958 ref. 16. NB Also reported pJ<2 of 4.33 by a spectrophotometric meBiod, same conditions (U). 

The NBS concentration scale has also been applied to seawater measurements. The measured values of pH(NBS) are, however, dependent on the change in liquid junction potential between the buffer(s) and sample, which needs to be taken into account if the measured pH values are to be used in thermodynamic calculations. This effect is often summarized as an apparent activity coefficient /h for the hydrogen ion ... 

Table 8-1 lists the constants which are recommended for use on the mol/(kg-seawater) concentration scale and the total hydrogen ion concentration pH scale pH(T). The determinations by Roy et al. (1993 1994) and Dickson (1990) listed in Table 8-1 supersede the careful determinations of Hansson (1973), which covered a mine limited salinity range and that had a somewhat higher standard deviation. Careful stability crmstant measurements have also been made on the seawater pH(SWS) scale (Xi, Kz, Goyet and Poisson, 1989 K, Dickson and Riley, 1979) and on the pH(NBS) scale (K, Kz, Mehrbach etaL, 1973 K- Culberson and Pytkowicz, 1973). In addition, Dickson and Millero (1987) recalculated the Ki and Kz values of Hansson (1973) and Mehrbach et aL (1973) to the pH(SWS) scale and found no consistent difference between them. 

Table 9.9 The effect of temperature on pH of the standardizing buffers NBS Standard pH scale (Bates, 1962)...

Effect of pH on the addition reactions was studied from pH 4 to 10. For pH values below 7, the reaction was buffered with a 25 mM sodium acetate solution whereas for pH values above 7, a 25 mM disodium tetraborate solution was used the pH was adjusted by adding either HC1 or NaOH. The combination electrode used for the determination of experimental pH, was calibrated with National Bureau of Standards (NBS) buffers for Milli-Q water (36) and tris(hydroxymethyl)aminomethane (TRIS) buffers for seawater (37) and NaCl solutions (38) on the pHF (free proton) scale. Since the addition of reactants caused a small pH change in the buffered medium, the experimental pH values shown in the results were measured after the reactants were added to reaction bottle. Samples from low pH reaction series were adjusted topH 9 by addition of a strong borate buffer just prior to HPLC analysis. Inis was necessary because thiol analysis using o-phthalaldehyde requires this pH for optimum derivatization. 

As a practical matter, pH values of seawater are generally measured at 25°C and 1 atmosphere total pressure. To apply these pH measurements to in situ conditions it is necessary to correct for pressure and temperature changes. For salinities between 30 and 40, temperatures of 0 to 40°C, and pH=7.6 to 8.2, the temperature correction on the NBS scale is (Millero, 1979) ... 

The halohydroxylation method has been developed for exploiting the reactivity of the double bond present in dialkyl vinyl- and propenylphosphonates. Preparation of the halohydrins of diethyl vinylphosphonate failed using NBS or NBA. Treatment of diethyl vinylphosphonate on large scale with NaOCl or NaOBr in aqueous medium at pH < 3 gives the halohydrins with the hydroxyl group in the P-position (anti-Markovnikoff addition) with 95% or 65-85% purity, respectively (Scheme 4.9). As a consequence of the acid conditions, the preparation of halohydrins is generally accompanied by the formation of undesired diethyl 1,2-dihalogenophosphonates. ... 

Unfortunately, since individual ionic activity coefficients cannot be evaluated without extrathermodynamic assumptions, the theoretical thermodynamic elegance and desirabflity of this pH definition cannot be rigorously related to experimental quantities. For this reason, the modem operational NBS (National Bureau of Standards) scale of acidity has been developed. 

The disodium salt of l,2-diaminoethane-N,N,N ,N -tetraacetic acid (Na-zHzEDTA), potassium hydrogen phthaate, HN03, NaOH (Yitrisol), andtheni-trate salts of Na+, Ni2+, Cu2+ and Zn+ (all pro analysi) were from Merck AG, Darmstadt, FRG. All solutions for the potentiometric pH titrations were prepared with ultra pure C02-free water.The buffer solutions (pH 4.00, 7.00, 9-00 based on the NBS scale now NIST) used for calibration of the pH-measuring instruments were from Metrohm AG, Herisau, Switzerland. 

However, conversion from the pH(NBS) scale depends on values of for the electrode pair used, and conversion between the pH(T) and pH(SWS) scales depends on the values of the protonation constants XHSO4 wd hf (see Section 7.4). Hie parameter IIThf is not particularly well defined, and conversion between these scales is therefore not recommended for accurate calculations. 

A range of other cationic niobium and tantalum tetrakis(dithiocarbamate) complexes, [M(S2CNR R )4]X (M = Nb, Ta R = R = Me, f-Bu R = Me, R = I -Pr, Ph, Cy, Bz), have been prepared from the metal pentachlorides and anhydrous dithiocarbamate salts (719). They are fluxional on the NMR time scale, a polytopal mechanism being proposed via a cubic or square antiprismatic transition state. The kinetics of this process has been studied by total line shape analysis, giving AG values of 42-50 kJ moP. ... 

Johnson NB, Lennon IC, Moran PH, Ramsden JA. Industrial-scale synthesis and apphcations of asymmetric hydrogenation catalysts. Acc. Chem. Res. 2007 40(12) 1291-1219. 

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