The standard “ASTM D513-06 Standard test methods for total and dissolved carbon dioxide in water” published by the American Society for Testing and Materials (ASTM) describes test methods for measuring total or dissolved carbon dioxide present in water as carbon dioxide (CO2), carbonic acid, bicarbonate ion and carbonate ion:
Test method A - The gas sensing electrode test method determines total or dissolved carbon dioxide present in water as CO2, carbonic acid, bicarbonate ion, and carbonate ion, within specified interference limitations.
Test method B - CO2 evolution, coulometric titration test method, determines total or dissolved carbon dioxide present in water as carbon dioxide, carbonic acid, bicarbonate ion and carbonate ion within specified interference limitations. Carbon dioxide is also detected from carbonates present in particles in samples.
Carbon dioxide can also be detected from carbonates present in particles in samples. Test method A is applicable to a variety of natural waters and brines. Test method B is applicable to natural waters, brines and a variety of industrial waters specified in this standard. It is the responsibility of the user to verify the validity of these test methods in waters from matrices that have not been tested.
For the terms and definitions used in these test methods, the “ASTM D1129-13 Standard Terminology for Water” standard applies.
Carbon dioxide is the primary product of respiration of plants and animals and of the decomposition of organic matter and certain minerals. The atmosphere contains an average of 0,04 vol. percent CO2. Surface waters generally contain less than 10 mg/L, except at localized points of abnormal organic or mineral weathering, but groundwater, especially deep waters, may contain several hundred mg/L.
When dissolved in water, CO2 contributes significantly to the corrosion of water treatment systems. This is particularly problematic in steam condensate systems. The loss of CO2 from an aqueous system can upset the carbonate balance and cause calcite encrustation on confining surfaces. The scaling of water heaters is a good example. Because of the delicate balance between corrosion and encrustation tendencies, great care must be taken to control CO2 and related species in water systems. For this purpose, recarbonation of municipal sources is applied in the final stages of softening and amine neutralization of steam condensate.
All tests use reagent grade chemicals. Unless otherwise stated, all reagents are intended to conform to established specifications. Other grades may be used if the reagent is found to be of high purity.
The sample is collected in accordance with ASTM D1066-18 and ASTM D3370-07, as applicable. If only dissolved species are to be determined and particles that may contain carbonate are present, the samples are filtered upon collection. When aliquots of samples are taken from sample bottles containing particles, the bottle is shaken or otherwise homogenized to ensure a representative sample is obtained. When particles form in the sample after collection due to changes in temperature, pH, or similar, these particles should be included in the sample tests. Care should be taken to avoid loss of CO2 during homogenization or filtration of the samples. Samples are not filtered unless it is necessary to remove potentially interfering particles.
A rigid, glass, chemically resistant bottle is used to collect the sample. The sample bottle is completely filled so that no air pockets remain under the cap, and the sample is stored at a temperature below that at which it was collected until determination is made.
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