The HI4004 calcium ion selective electrode is a potentiometric half-cell sensor housed within an epoxy body. The internal elements are isolated from the sample by an organic polymer sensing PVC membrane. This liquid membrane sensor requires a separate reference electrode to complete the measurement system.
Hanna’s liquid membrane half-cell ISEs are designed for use with the HI5315 reference half-cell. The reference half-cell features a cone style junction. The skirted cone geometry of the sensor forms the liquid junction with the test solution at the tip of the junction cone, producing a highly stable reference potential and consistent flow rates of electrolyte into the solution. A simple press of the top portion of the ISE makes it easy to empty the electrolyte fill solution and replace when necessary.
For the HI4004 to measure calcium accurately, it is important that the ionic strength of the standards and sample be adjusted to a high and constant value with the calcium ionic strength adjuster (ISA) solution. The ISA ensures that the activity coefficient is constant so the concentration of free calcium ions can be measured. Measuring all solutions with a constant ionic strength reduces the margin of error between measurements. The HI4004 calcium liquid membrane ISE can measure from 3µM (0.12 mg/L) to 1M (40, 080 mg/L).
Conversion Table for Ca2+
moles/L (M) to ppm (mg/L)
ppm (mg/L) to M (moles/L)
2.49 x 10-5
Replaceable Sensing Modules – Half-cell liquid membrane ISEs are electrodes that detect free ions in a solution. The HI4004 calcium half-cell ISE is designed to incorporate easy replacement of the sensing module.
Durable Epoxy Body – The probe body of the calcium half-cell ISE is composed of durable epoxy resin.
BNC Connection – The HI4004 has a universal BNC connector for easy connection to any benchtop meter with a female BNC probe input.
Theory of Operation
A liquid membrane ion selective electrode develops a voltage due to the ion exchange that occurs between the solution and the polymer membrane that contains organic ion exchange resin. When the ionic strength of the solution is fixed by the addition of ISA, the voltage is proportional to the concentration of free ions in solution.