Acidity and Alkalinity
by Tim Loftus
Acidity levels in wastewater indicate its corrosive properties and can take a leading role in regulating biological processes as well as in chemical reactions (such as chemical coagulation and flocculation). Alkalinity, too, contributes to the properties of wastewater, many of which also affect biological processes (such as nitrification) and chemical reactions.
While both acidity and alkalinity are related to pH, they should not be confused with pH, nor should the terms be used interchangeably. Acidity is a measure of a solutionís capacity to react with a strong base (usually sodium hydroxide, NaOH) to a predetermined pH value. This measurement is based on the total acidic constituent of a solution (strong and weak acids, hydolyzing salts, etc.) It is possible to have highly acidic water but have moderate pH values. Likewise, the pH of a sample can be very low but have a relatively low acidity. Acidity is similar to a buffer in that the higher the acidity, the more neutralizer is needed to counteract it.
Alkalinity is the measure of a solutionís capacity to react with a strong acid (usually sulfuric acid H2SO4) to a predetermined pH. The alkalinity of a solution is usually made up of carbonate, bicarbonate, and hydroxides. Similar to acidity, the higher the alkalinity is, the more neutralizing agent is needed to counteract it. In general, a treatment plant and its collection system operates better with wastewater lower in acidity and higher in alkalinity.
According to EPA sampling and preservation guidelines, samples for both acidity and alkalinity measurements can be collected in glass or plastic bottles and stored at 4 degrees Celsius for up to fourteen days. However, sample handling probably plays a much more important role in preserving the integrity of the sample. Acidity and alkalinity are greatly affected by exposure to the atmosphere. Large surface areas or long contact times with the atmosphere, both in the sample container and during analysis, can either dissolve gases into the sample or allow dissolved gasses already in the sample to escape thereby changing the measurements. When sampling for acidity and alkalinity, it is best to leave no headspace in the container. Also, donít filter the sample (which creates high turbulence) or leave in an open beaker on the lab bench for hours. Pour an aliquot just before analysis so that you minimize exposure to the air. During analysis, keep mixing turbulence to a minimum. These measures are particularly important when determining low levels of acidity or alkalinity.
Acidity and alkalinity analyses each comprise of a simple titration. Basically, acidity is determined by titrating the sample with sodium hydroxide to a pH of 8.3 (often called the phenolphthalein acidity Ė this term dates back to the time before electronic pH meters). Alkalinity is determined by titration with sulfuric acid to a pH of 4.5. While these pH end-points are common in wastewater situations, other end-points are often used depending on certain test or sample conditions. Also, since both acidity and alkalinity are not analyses for specific chemicals but rather represent a solutionís specific properties, its measurements are equated with an equivalent amount of calcium carbonate. This is done so that measurements of different samples can be compared to each other. Therefore, always report acidity and alkalinity measurements as ď___ mg CaCO3/L to pH ___.Ē The mathematical formulas for calculating acidity and alkalinity, as well as descriptions of certain instances where sample pretreatment is necessary, can be found in Standard Methods.
In conclusion, acidity and alkalinity testing is easy to perform. And with careful sampling you will be far on your way toward accurately monitoring the effectiveness of your districtís domestic water corrosion control, monitoring part of the nitrification process in your facilityís activated sludge, or even troubleshooting the chemical addition process for phosphorus removal in the final effluent.
The recommendations in this article for acidity and alkalinity analyses are very general. Always check your state and local regulations. You may have additional requirements to meet.
If you have any questions, suggestions, or comments, please contact LPC Chair Paul Fitzgibbons at (401) 222-6780 ext 118 (firstname.lastname@example.org) or Tim Loftus at (508) 949-3865 (email@example.com). You can also visit our website at newea.org. Once on the website, press the Lab Practices button.