Errors buffer selection

The pH scale has been defined operationally, and standard reference solutions based on a conventional scale of hydrogen ion activity have been selected (i, 2). Measurements of the pH of seawater made with different electrodes and instruments are satisfactorily reproducible when standardized in the same way (3). The results obtained, however, do not always have a clear interpretation. Formally, this diflSculty can be attributed to the residual liquid junction potential involved in the measurement. The primary standards are necessarily dilute buffer solutions (ionic strength, I 0.1) whereas seawater normally has an ionic strength exceeding 0.6. This difference in the concentrations and mobilities of the ions coming in contact with the concentrated solution of potassium chloride of which the salt bridge-liquid junction is composed gives rise to a potential difference that is indeterminate. Consequently, the meas-m ed pH is in error by an unknown amount and does not fall exactly on the scale fixed by the primary standards. 

Many chemicals are available that can be used without further purification. Their selection is made on a trial and error basis, and even different lots from the same supplier may differ in purity. Volatile buffers and solvents such as pyridine, acetic acid, formic acid, and n-propanol, which are employed for chromatography, can be purified by distillation over ninhydrin. Constant-boiling hydrochloric acid is rountinely prepared over sodium dichromate (Schwabe and Catlin, 1974), and water of high purity is obtained from a system composed of a 0.2-fim particle filter, an activated charcoal cartridge, and two deionizer cartridges (Hydro Service and Supplies, Durham, North Carolina). The solvents are tested for purity in the following manner. A sample of the solvent is dried in vacuo. The residue is dissolved in pH 2.2 citrate buffer and injected onto the amino acid analyzer column. The distilled solvents are typically found to contain aspartic acid, serine, and glycine at 20-30 pmol/ml as the major contaminants. 

Thus, in addition to the extrapolation to zero intensity of the pumping light which must be made to eliminate systematic errors due to the light shifts discussed in section 17.9.7, a further extrapolation of the measured hyperfine resonance frequencies to zero buffer-gas density must be made. Zero-field hyperfine intervals measured by the optical pumping method are listed in Table 18.3, together with a selection of the results obtained by the atomic beam technique. Generally the optical pumping results are more precise than the older atomic beam results. The table... 

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