Buffers primary standard

Calibrating the electrode presents a third complication since a standard with an accurately known activity for H+ needs to be used. Unfortunately, it is not possible to calculate rigorously the activity of a single ion. For this reason pH electrodes are calibrated using a standard buffer whose composition is chosen such that the defined pH is as close as possible to that given by equation 11.18. Table 11.6 gives pH values for several primary standard buffer solutions accepted by the National Institute of Standards and Technology. 

Now consider a primary standard buffer containing 0.025 0 m KH2P04 and 0.025 0 m Na2HP04. Its pH at 25°C is 6.865 0.006.4 The concentration unit, m, is molality, which means moles of solute per kilogram of solvent. For precise chemical measurements, concentrations are often expressed in molality, rather than molarity, because molality is independent of temperature. Tabulated equilibrium constants usually apply to molality, not molarity. Uncertainties in equilibrium constants are usually sufficiently great so that the 0.3% difference between molality and molarity of dilute solutions is unimportant. 

HH A solution containing 0.008 695 m KH2P04 and 0.030 43 m Na2HP04 is a primary standard buffer with a stated pH of 7.413 at 25°C. Calculate the pH of this solution by using the systematic treatment of equilibrium with activity coefficients from... 

The quantity F is most variable when the experimental design calls for titrations over a relatively wide range of pH and is best known at those points on the pH scale where the pH meter has been calibrated with primary standard buffers. 

For this calculation some arbitrary assumption must be made, such as 7h3o+ = y the value of 7 can be estimated from the Debye-Hiickel equation. Alternatively, the known pH of an NBS primary standard buffer can be used in Eq. (5.30). This latter procedure probably is the most practical and, if the pH of the solutions lies between 2 and 12 and they contain only simple ions in concentrations less than 0.2 M, good constancy of Ej is found. This value of ( + Ej) consequently relates specifically to solutions that are similarly constituted. 

The pH measurement system is calibrated against primary standard buffers admitted either manually or automatically into the sample chamber. The buffers are phosphate solutions that should meet National Institutes of Standards and Technology (NIST) specifications. Calibration buffers meeting NIST specifications are available from the manufacturer of an instrument, usually in containers of appropriate size and shape for mounting as a reservoir on the instrument. The pH values of the low and high calibrator buffers axe set by the manufacturer but always lie close to 6.8 and 7.4 at 37 °C. The tolerance of the specified values should be less than or equal to an SD of 0.003 to achieve SDs of 0.005 to 0.01 in measuring blood pH. Unopened containers should be stored at room temperature. When visual observation or an instrument display warns of low concentrations in the reservoir, recommended practice is to replace the reservoir with a newly opened container rather than to replenish fluid in the current one. Pooling several almost empty containers is not recommended. 

Workers at NIST and elsewhere have used cells without liquid junctions to study primary-standard buffers extensively. Some of the properties of these buffers are discussed in detail elsewhere. Note that the NIST buffers are described by their molal concentrations (mol solute/kg solvent) for accuracy and precision of preparation. For general use, the buffers can be prepared from relatively inexpensive laboratory reagents for careful work, however, certified buffers can be purchased from the NIST. 

The required attributes listed above effectively limit the range of primary buffers available to between pH 3 and 10 (at 25 °C). Calcium hydroxide and potassium tetraoxalate tire excluded because the contribution of hydroxide or hydrogen ions to the ionic strength is significant. Also excluded are the nitrogen bases of the type BH+ (such as tris(hydroxymethyl)aminomethane and piperazine phosphate) and the zwitterionic buffers (e.g. HEPES and MOPS (10)). These do not comply because either the Bates-Gu enheim convention is not applicable, or the liquid junction potentials are high. This means the choice of primary standards is restricted to buffers derived from oxy-carbon, -phosphorus, -boron and mono, di- and tri-protic carboxylic acids. The uncertainties (11) associated with Harned cell measurements are calculated (1) to be 0.004 in pH at NMIs, with typical variation between batches of primary standard buffers of 0.003. 

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