exist in chemical oceanography. As part of its operational definition of the pH scale, the IUPAC defines a series of buffer solutions across a range of pH values (often denoted with National Bureau of Standards (NBS) or National Institute of Standards and Technology (NIST) designation). These solutions have a relatively low ionic strength (≈0.1) compared to that of seawater (≈0.7), and, as a consequence, are not recommended for use in characterizing the pH of seawater, since the ionic strength differences cause changes in electrode potential. To resolve this problem, an alternative series of buffers based on artificial seawater was developed. This new series resolves the problem of ionic strength differences between samples and the buffers, and the new pH scale is referred to as the 'total scale', often denoted as pHT. The total scale was defined using a medium containing sulfate ions. These ions experience protonation, + , such that the total scale includes the effect of both protons (free hydrogen ions) and hydrogen sulfate ions: []T = []F + [] An alternative scale, the 'free scale', often denoted 'pHF', omits this consideration and focuses solely on []F, in principle making it a simpler representation of hydrogen ion concentration. Only []T can be determined, therefore []F must be estimated using the [] and the stability constant of , : []F = []T − [] = []T ( 1 + [] / K )−1 However, it is difficult to estimate K in seawater, limiting the utility of the otherwise more straightforward free scale. Another scale, known as the 'seawater scale', often denoted 'pHSWS', takes account of a further protonation relationship between hydrogen ions and fluoride ions, + ⇌ HF. Resulting in the following expression for []SWS: []SWS = []F + [] + [HF] However, the advantage of considering this additional complexity is dependent upon the abundance of fluoride in the medium. In seawater, for instance, sulfate ions occur at much greater concentrations (>400 times) than those of fluoride. As a consequence, for most practical purposes, the difference between the total and seawater scales is very small. The following three equations summarise the three scales of pH: pHF = −log []F pHT = −log([]F + []) = −log[]T pHSWS = −log(]F + [] + [HF]) = −log[v]SWS In practical terms, the three seawater pH scales differ in their values by up to 0.10 pH units, differences that are much larger than the