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Alkalinity

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. pH in contrast 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.

Because typically a premeasured sample whose alkalinity is to be determined has a strong acid titrant added to it, alkalinity is therefore a measurement of the amount of strong acid required to neutralise all dissolved bases to a designated titration end point. Alkalinity is expressed in terms of its base concentration, usually as equivalent mg/l CaCO3. There are 3 main measurements of alkalinity:

Type Indicator Definition
Total Alkalinity (T) Bromocresol Green / Methyl Red Alkalinity above pH 4.5
Phenolphthalein Alkalinity (P) Phenolphthalein Alkalinity above pH 8.3
Hydrate Alkalinity (OH) Phenolphthalein OH- Alkalinity above pH 8.3

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.

Both Bromocresol Green / Methyl Red and Phenolpthalein are 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.

The term 'Alkalinity' is usually used to refer to Total Alkalinity (T Alkalinity). Methyl Orange Alkalinity is often referred to as M Alkalinity, and the term is frequently used interchangeably with T Alkalinity. Total Alkalinity titrations may also be carried out using a pH meter rather than an indicator solution. Phenophthalein Alkalinity is usually abbreviated to P Alkalinity. Hydrate Alkalinity is also known as Hydroxide Alkalinity, OH Alkalinity or just O Alkalinity.

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, 50-500 ppm & 100-1000 ppm CaCO3. Based on CHEMetrics patented self-filling reagent ampoule technology. Premixed. Premeasured. Precise. Each kit contains 30 tests.

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. The alkalinity of natural waters is determined by the bedrock and soil through which it passes through containing these compounds, typically limestone, and the action of chemical weathering. For example, waters flowing through limestone regions tends to have a higher alkalinity and waters flowing through granite, conglomerate and sandstone regions tend to have a lower alkalinity.

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 co-exist in general terms and occur at different pHs. At pH 7, the proportion of carbonate is very small.

pH Composition
1.0 - 4.5 Free mineral acidity, CO2 escapes (i.e. no HCO3-, CO3=)
4.5 - 9.6 Bicarbonate alkalinity (HCO3-)
8.3 - 14.0 Carbonate alkalinity (CO3=)
9.5 - 14.0 Hydroxide alkalinity (OH-)

At or below pH 4.5, all bases have been reduced to water (and CO2). 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).

Phenolphthalein Alkalinity is the sum of all Carbonate (CO3=), 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.

Alkalinity Calculations

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

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

P Alkalinity ≈ [CO3=] + [OH-]

Titration
Result
OH- Alkalinity
as CaCO3
CO3= Alkalinity
as CaCO3
HCO3- Alkalinity
as CaCO3
P=0 0 0 T
P<0.5T 0 2P T-2P
P=0.5T 0 2P 0
P>0.5T 2P-T 2(T-P) 0
P=T T 0 0

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

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

T - P alkalinity ≈ [HCO3-] + [CO3=]

Patrik Askert, Galgo, May 2019

References:

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

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