Calcium (Ca) is an abundant element in the Earth's crust. The most abundant forms of calcium are calcium carbonate (CaCO3), the main constituent of marble and travertine and calcium sulfate (CaSO4), the main component of plaster. Calcium in water supplies comes from water passing over deposits of limestone, dolomite, gypsum, and gypsiferous shale. The concentration may extend from zero to several hundred milligrams per liter, depending on the source and treatment.
Calcium in water as carbonate is a primary component of water hardness that can cause pipe or tube scaling. Classifying the degree of hardness of water is a meausrement of the parameter "total hadness" which is the total sum of the Ca2+ and Mg2+ ions.
A main use of calcium is in the production of cements and mortars for the construction of buildings. It is also used as a deoxidizing, desulphurizing, and decarburizing agent for various ferrous alloys, as well as a binding agent in the production of aluminum, lead, magnesium, and beryllium alloys. The Ca2+ ion plays a crucial role in cell physiology and biochemistry. As a building block of animal bones and shells, calcium is essential for animal nutrition. Calcium also helps plant roots develop and increases the resistance and strength of plant tissues and stems
Hanna Instruments offers a variety of methods to measure calcium. Products include portable and benchtop pH/ISE meters that can use a liquid membrane calcium Ion-Selective Electrode (ISE) for direct measurement. Titrations for calcium can be performed with the calcium ISE or a photometric probe using EDTA as a titrant.
Calcium is also measured photometrically with reagents. Photometric analysis is based on the Beer-Lambert principle of absorbance. Photometric analysis products include handheld colorimeters, portable and benchtop photometers, and spectrophotometers. Photometric methods include reagent chemistries based on the Calmagite method found in the Standard Methods for the Examination of Water and Wastewater, Oxalate method, and the Zincon method used for measuring calcium in the saltwater aquarium for reef maintenance.
Calcium is commonly expressed as mg/L of calcium carbonate (CaCO3). The portable and benchtop photometers allow calcium to be expressed as French degrees (of), German degrees (odH) and English degrees (oE).
Below are the most common products used for measuring calcium.
Titration systems with precision dosing pumps are available to measure calcium. These systems can use either the calcium ISE or a photometric probe to determine the endpoint and results can be displayed in a variety of formats based on the specific need.
Benchtop pH/ISE meters
Benchtop pH/ISE meters include a two channel option in which a pH probe can be connected to one channel and the calcium ISE to the other. The benchtop meters have the ion charge and formula weight pre-programmed for use with the calcium ISE. The benchtop meters also have incremental methods pre-programmed including the known addition, known subtraction, analyte addition and analyte subtraction methods
Portable pH/ISE Meters
Portable pH/ISE allow the flexibility to perform an ISE measurement with a portable even though the ISE measurement method is more suitable to a benchtop environment due to the addition of the ISA to fix the ion activity in the sample and the use of a magnetic stirrer to keep the sample homogenous. The portable pH/ISE meter has the ion charge and formula weight pre-programmed for concentration measurements.
The calcium ISE is a liquid membrane sensor in which an ionophore is embedded in a matrix that reacts with calcium in a lock and key fashion. The calcium ISE is available as either a single half-cell or combination design that includes a reference cell. The ISE is a convenient way to measure calcium.
A photometric probe is a sensor that generates a steep millivolt potential when a solution with an indicator changes color after adding a titrant. Calcium hardness is determined with EDTA as a titrant and calconcarboxylic acid (Patton-Reeder) indicator. In this complexometric reaction calcium binds to the indicator to change the color of the solution from blue to red. As EDTA is added to the solution the calcium binds with it instead of the indicator. The solution will change from red to blue when there is no free calcium available to react with the indicator. The photometric probe has a wavelength of 625 nm in which there is high transmittance when the solution is red but has a low transmittance when the solution turns blue.
Solutions for calcium measurement include ISE standards and ISA for calibration and measurement. Other solutions include the CAL Check standards for verifying and, if needed, calibrating portable photometers. Each CAL Check standard is supplied with a Certificate of Analysis stating the accuracy and traceability of the standard.
The spectrophotometer offers the highest precision due to the quality of the optical system that has a wavelength accuracy of +/- 1.5 nm. The spectrophotometer allows for custom methods and can display calcium results in a variety of chemical forms and units of measure.
Benchtop photometers include multiparameter versions for water quality, aquaculture and nutrient analysis. Each photometer is customized to have the parameters used by a specific industry. All benchtop have a digital pH electrode input allowing it to be used as a traditional pH meter. All benchtop photometers can display results in multiple chemical forms.
Single parameter portable photometers are available with two modes to measure either calcium or magnesium hardness. The portable photometers have a CAL Check feature that allows for performance verification and if needed, recalibration using the CAL Check standards. The portable meters are available as a meter only option or as a kit. The kit version include a rugged carrying case and CAL Check standards
Checker HC, like a chemical test kit, are reagent based in which there is a color change based on concentration. The handheld colorimeters, like portable and benchtop instrumentation, use the Beer-Lambert principle to determine the color change. The color change is measured by a photoelectric cell so detection of the color change is more accurate than subjective visual interpretation.
Reagents include those used with Checker HC and photometers.
Accessories include spare cuvettes and caps for photometers, magnetic stirrers to ensure adequate mixing when using an ISE and the spare parts for the calcium ISE.