Alkalinity

Alkalinity of Natural Waters

Alkalinity of natural waters is typically a combination of bicarbonate, carbonate, and hydroxide ions, the proportions of which vary depending on pH.

Bedrock, typically limestone, soil type, weathering processes, and precipitation will influence the predominant form of alkalinity present. For example, waters flowing through limestone regions tends to have a higher alkalinity than waters flowing through granite, conglomerate, and sandstone regions.

As pH rises, more bicarbonate will change to carbonate. Sewage and wastewaters usually exhibit higher alkalinities due to the presence of silicates and phosphates. Bicarbonate and Hydroxide Alkalinity cannot coexist in general terms and occur at different pHs. At pH 7, the proportion of carbonate is very small.

At or below pH 4.5, all bases have been reduced to water and carbon dioxide. Waters between pH 4.5 and 8.3 contain weak acids such as carbonic acid, and below pH 4.5 contain mineral acids. Carbon dioxide and carbonic acid exist in a chemical equilibrium in solution.

Bicarbonate salts of calcium, magnesium and sodium, when heated, typically in solution to boiling point, break down to carbonate salt, CO₂ and water. CaCO₃ and MgCO₃ are insoluble, so they are precipitated out of solution. Mg(HCO₃)₂ and The Ca(HCO₃)₂ salts are sometimes therefore referred to as 'temporary hardness' as they can be removed from solution by boiling and filtering, thereby lowering the alkalinity of the solution (assuming no change in total volume).

Alkalinity Measurement

The alkalinity of water is a measurement of its buffering capacity. It is the ability of a solution to neutralise acid or to resist resist acidification. In contrast, pH is a logarithmic scale for expressing Hydrogen (H⁺) ion concentration. A solution of pH 7.0 is considered to be neutral, whereas a solution above pH 4.5 is considered to have alkalinity.

Alkalinity is measured by titrating a water sample with a strong acid to a designated titration end point. The end point is commonly determined using pH (visual) indicators. Alternatively, a pH meter can be used.

Both bromocresol green / methyl red and phenolphthalein indicators are specified in the American Public Health Association (APHA)'s Standard Methods, Method 2320 B for the determination of total alkalinity and phenolphthalein alkalinity, respectively.

Alkalinity is expressed in terms of its base concentration, usually as equivalent mg/l CaCO₃. There are 3 main forms of alkalinity that are distinguished by their end points. The term 'alkalinity' is usually used to refer to total alkalinity (T alkalinity) or methyl orange alkalinity, otherwise known as M alkalinity. Phenophthalein alkalinity is usually abbreviated to P Alkalinity. Hydrate alkalinity is also known as hydroxide alkalinity, OH alkalinity or just O alkalinity.

Below pH 4.5 there is said to be no total alkalinity; and below pH 8.3 there is said to be no phenolphthalein or hydrate alkalinity. Phenolphthalein alkalinity is the sum of all carbonate (CO₃^2-), Bicarbonate (HCO₃^-) and Hydroxide (OH^-) alkalinity above pH 8.3. Hydrate Alkalinity on the other hand is just the Hydroxide (OH^-) component of alkalinity above pH 8.3.

CHEMetrics offers three Total Alkalinity Test Kits employing industry standard pH indicators to deliver sensitivity and accuracy within minutes, covering the ranges 10-100 ppm (K-9810), 50-500 ppm (K-9815) & 100-1,000 ppm CaCO₃ (K-9820).

Titrets ampoules use a reverse titration technique to measure analyte concentration levels. This means that the titrant volume inside the ampoule is fixed while the sample volume is varied. After snapping the ampoule tip, sample is drawn into the test ampoule in small doses until a colour change signals that the end point has been reached. The titration is stopped at the end point, and the liquid level in the ampoule corresponds to the concentration printed on a scale on the ampoule's outer surface.

The end point of the titration for CHEMetrics’ Total Alkalinity Titrets Test Kits is signaled by a color change from pink to bright green. If the test ampoule is filled with sample but remains pink, the total alkalinity is below the test range. If the solution changes to bright green immediately upon introduction of the first small dose of sample, the total alkalinity is above the test range. If the sample itself turns pink immediately upon addition of the indicator (activator) solution (prior to introduction of the sample into the test ampoule), the sample pH is less than or equal to 4.5, which indicates that the alkalinity of the sample is 0 ppm.

Alkalinity Calculations

By determining both the T and P alkalinity values of a sample, an analyst can actually calculate the individual concentrations of CO₃^2-, HCO₃^- and OH^-, which are the components that are typically attributed to a sample's alkalinity, using the table below. This table presupposes incompatibility of OH^- and HCO₃^- alkalinities, which is essentially correct, although in reality there is a tiny overlap in pH transition between the two.

T Alkalinity ≈ 2 [CO₃=] + [HCO₃-] + [OH-]

P Alkalinity ≈ [CO₃=] + [OH-]

Source: APHA Standard Methods, 22^nd ed., Method 2320 B: Table 2320:II (1997).

Combine the above two equations to calculate the total CO₃^2- and HCO₃^- concentration:

T - P alkalinity ≈ [HCO₃^-] + [CO₃^2-]

References

1. APHA (1997). Standard Methods, 22^nd ed., Method 2320 B. Washington DC: APHA.
2. CHEMetrics (2018). Total Alkalinity Titrets Kit. Rev.11. Calverton, VA: CHEMetrics