Different pH Scales
There are several types of pH scales, including:
- Total pH scale: A scale that accounts for all of the dissolved inorganic carbon species in seawater.
- Free pH scale: A scale that accounts for only the free proton concentration in seawater.
- Seawater pH scale: A scale that is specific to seawater and is based on the total hydrogen ion concentration in seawater.
- IUPAC pH scale: A scale that is based on the activity of the hydrogen ion, which is a measure of the effective concentration of hydrogen ions in solution.
- Sørensen pH scale: A scale that is based on the concentration of hydrogen ions in a solution, with a pH of 7 corresponding to neutral.
- Glass electrode pH scale: A scale that is based on the voltage generated by a glass electrode in contact with a solution and is calibrated using buffer solutions of known pH.
The different pH scales are used for different applications, and choosing the appropriate pH scale depends on the specific circumstances and needs of the experiment or measurement.
Measuring pH in seawater
Free pH Scale
-The free pH scale is a pH scale that considers only the concentration of free hydrogen ions in a solution. It assumes that all other acid-base equilibria in the solution are at equilibrium and that the activity coefficients of all the species in the solution are unity. This pH scale is used in dilute solutions, where the activity coefficients can be assumed to be unity, and is based on the thermodynamic dissociation constant (pKa) of the acid in question.
-IUPAC (International Union of Pure and Applied Chemistry) buffers are used to calibrate pH measurements using the free pH scale.
-The free pH scale is the most commonly used pH scale in chemistry, and is also known as the standard hydrogen electrode (SHE) scale.
-IUPAC buffer system uses a mixture of weak acids and their corresponding conjugate bases, which are chosen based on their pKa values. The pH of the IUPAC buffer is determined by the Henderson-Hasselbalch equation, which relates the pH of the buffer to the pKa of the weak acid and the ratio of the concentrations of the weak acid and its conjugate base.
-The IUPAC pH scale is not exactly the same as the free pH scale, although they are related. The IUPAC pH scale is based on the activity of hydrogen ions in a solution, while the free pH scale is based on the concentration of hydrogen ions in a solution assuming that all other chemical species are at their natural concentrations in seawater. In seawater, the difference between the IUPAC pH scale and the free pH scale is relatively small, typically less than 0.1 pH units, but it can become larger in more extreme conditions.
Why is the free pH scale not accurate for seawater?
The free pH scale is not good for seawater because it assumes that seawater is a simple aqueous solution and does not take into account the presence of other ions and compounds that are present in seawater. Seawater is a complex mixture of different salts, including carbonates, bicarbonates, borates, sulfates, and chlorides, which can interact with hydrogen ions and affect the pH measurement. Therefore, the free pH scale cannot provide an accurate measurement of the pH of seawater. For this reason, the total pH scale or other pH scales that assume equilibrium conditions are preferred for measuring the pH of seawater.
Total pH Scale
The total pH scale involves measuring the concentration of H+ ions in seawater by using a pH electrode calibrated with Dickson buffers. The Dickson buffer system is a mixture of a weak acid (H2CO3) and its conjugate base (HCO3-) in seawater, which helps to maintain a stable pH. This method takes into account all dissolved inorganic carbon species in seawater, including carbon dioxide, bicarbonate, and carbonate. The total pH scale is widely used in oceanography and marine chemistry research because it provides a comprehensive measure of seawater pH.
The total pH equation is based on the dissociation constants of carbonic acid, bicarbonate, and carbonate ions in seawater, and it includes the contribution of other dissolved ions as well. The equation was first proposed by Dickson and Millero in 1987 and has been modified by several researchers since then, including the Hansson and Mehrbach modification. The equation is given as:
pH(total) = pH(H+) + log{(K(HSO4-) / [H+])(1 + K(HB) / [B] + K(HSO4-) / [HSO4-])}
where
- pH(total) is the total pH of seawater,
- pH(H+) is the pH of the hydrogen ion,
- [H+] is the concentration of hydrogen ions,
- [B] is the concentration of total boron,
- [HSO4-] is the concentration of bisulfate,
- K(HB) is the dissociation constant of boric acid,
- K(HSO4-) is the dissociation constant of bisulfate.
Conversion from Total pH Scale to Free pH Scale
While the two buffer systems differ in their composition and calibration methods, they both serve the same purpose of providing a stable pH reference for accurate pH measurements. Therefore, the pH values obtained from these two buffer systems can be related through calibration and conversion equations.
To convert from the total pH scale to the free pH scale in marine chemistry, one needs to take into account the ionic strength of the seawater. The total pH scale considers the activity of all chemical species in seawater that can influence the pH, while the free pH scale only considers the activity of the hydrogen ion (H+) and hydroxide ion (OH-) that contribute directly to the acidity or alkalinity of the seawater.
The conversion from total pH to free pH involves using the activity coefficients of seawater to determine the activity of H+ and OH- ions. This is typically done using an equation known as the “Hansson and Mehrbach” equation.
Hansson and Mehrbach are two different pH scales used in marine chemistry. They were developed to address the limitations of the total pH scale and the free pH scale, respectively.
The Hansson pH scale is an improvement over the total pH scale and takes into account the complex chemical equilibria that occur in seawater. It uses the same hydrogen ion activity coefficient as the free pH scale, but incorporates additional constants to account for the effect of ionic strength and the presence of major seawater constituents. The Hansson pH scale has been found to provide more accurate measurements of seawater pH than the total pH scale.
The Mehrbach pH scale, on the other hand, is an improvement over the free pH scale and takes into account the effects of temperature and salinity on pH measurements. It uses a different set of dissociation constants and activity coefficients than the free pH scale, which are specific to seawater conditions. The Mehrbach pH scale has been found to provide more accurate measurements of pH in seawater than the free pH scale, especially at high salinities and temperatures.
The Hansson and Mehrbach equation is an equation that allows the calculation of the pH of seawater as a function of its chemical composition. The equation takes into account the activity coefficients of the various ions present in seawater and is based on the dissociation constants of carbonic acid and boric acid. The equation is given as:
pH = pK1 + pK2 + log {[HCO3-]/(α1K1)([H2CO3] + [CO32-]) + [B(OH)4-]/(α2K2)(1 + [H+]/Kb) + [H+]/Kw}
where:
- pH is the pH of seawater,
- pK1 and pK2 are the dissociation constants of carbonic acid,
- [HCO3-] is the concentration of bicarbonate ions,
- α1K1 is the product of the activity coefficient of bicarbonate ions and the dissociation constant of carbonic acid,
- [H2CO3] is the concentration of dissolved carbonic acid,
- [CO32-] is the concentration of carbonate ions,
- [B(OH)4-] is the concentration of borate ions,
- α2K2 is the product of the activity coefficient of borate ions and the dissociation constant of boric acid,
- [H+] is the concentration of hydrogen ions,
- Kw is the dissociation constant of water,
- Kb is the dissociation constant of boric acid.
Once the activity coefficients have been calculated, the free pH scale can be obtained by calculating the negative logarithm of the activity of H+. This provides a more accurate measure of seawater acidity and alkalinity as it only considers the contributions of H+ and OH- ions, which are directly related to the concentration of carbon dioxide in seawater and its influence on ocean acidification.
Overall, the Hansson and Mehrbach pH scales are both improvements over the total pH and free pH scales, respectively, and are widely used in marine chemistry research.
To convert from free pH to total pH, you need to take into account the dissociation of carbonic acid (H2CO3) in seawater. The relationship between free pH and total pH is given by the following equation:
pH(total) = pH(free) + log([HCO3-]/[H2CO3] + 1)
where
- [HCO3-] is the concentration of bicarbonate,
- [H2CO3] is the concentration of carbonic acid.
This equation is based on the assumption that the activities of all other dissolved species in seawater remain constant.
What about Colorimetry for seawater?
Colorimetric methods use chemical indicators that change color in response to changes in pH. These methods are often used in water quality testing and monitoring, but are less accurate than the total pH scale. Colorimetric methods only measure the pH of the free hydrogen ions (H+) in seawater and do not take into account other dissolved inorganic carbon species, which can result in inaccurate readings.
How is ANB’s sensor calibrated?
In our sensor, we have two different types of electrode. The pH sensing electrode and the reference tracking electrode.