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Measuring Resistivity to Detect Ions

Electrical conductivity is a measure of a material’s ability to conduct an electric current. Water itself has a weak electrical conductivity. Electric current is transported in water by dissolved ions, making conductivity measurement a quick and reliable way to monitor the total amount of ionic contaminants in water. As conductivity also is related to ion mobility (which increases with temperature), conductivity values usually are reported compensated at 25 °C, in μS/cm @ 25 °C. Resistivity, expressed in MΩ•cm @ 25 °C, is the inverse of conductivity. Traditionally, conductivity is used for values above 1 μS/cm @ 25 °C, and resistivity for conductivity values below 1 μS/cm @ 25 °C. Water quality of high purity therefore will be expressed as resistivity in MΩ•cm @ 25 °C.

Merck Resistivity Meters

The resistivity meters designed by Merck for use with ultrapure water take specific needs into consideration:
  • Cell design is based on concentric electrodes, in order to generate a large surface and reduce the space between electrodes without risk of contact, therefore achieving the low cell constant (around 0.01 cm-1) recommended in ASTM D5391-99.
  • Electrodes made of 316 L stainless steel are used to minimize corrosion and ion release risks.
  • Flow-through cell design allows immediate detection of any ionic contamination in the water flow.
  • Temperature measurement performed by a thermistor with 0.1 °C sensitivity provides fast and accurate reaction to temperature variations in water.
  • Meters are able to display either resistivity or conductivity readings, compensated or noncompensated to 25°C.
  • The meter’s electronic circuit is regularly verified by an automatic process, and alarm messages will be sent if the electric or electronic parts are defective, ensuring that every result displayed is valid
  • The meters are calibrated with a method developed for use with high resistivity water, and are delivered with a certificate of calibration. Meters meet requirements for performance of USP § 645 suitability tests.
These meters provide continuous, reliable insurance that water produced by Elix, Milli-Q and AFS water purification systems will contain low levels of ionic contaminants.

Milli-Q® System High Precision Coaxial Resistivity Cell

The conductivity measurement is performed by dipping two electrodes in an aqueous solution. An electrical potential (below 1.23 V) is established between the two electrodes and the intensity of current passing through the circuit is measured. The conductivity of the solution will be proportional to the electrical current.

Measuring TOC to Detect Organics

TOC means “Total Oxidizable Carbon” and is sometimes referred as “Total Organic Carbon”. Water may contain hundreds of different organic substances at different levels of oxidation, and at different concentrations. TOC expresses the organic contamination of water with a single figure.


A10 TOC Monitor Photooxidation Cycle

Merck TOC Monitors

The Merck A10 TOC monitor contains a 0.5 ml quartz cell in which ultrapure water is captured. When the A10 UV lamp is powered on, photocatalytic oxidation of the organic compounds occurs. The final end product of the organic oxidation is carbon dioxide, which dissolves in the water, causing a conductivity increase. This change in conductivity is regularly monitored by the titanium electrodes (patented design) and temperature-compensated to 25 °C. A set of complex algorithms confirms complete oxidation and calculates the carbon level associated with this conductivity change.

Merck TOC monitors are available as stand alone units or integrated within some Milli-Q and Elix water purification systems.

The TOC monitor design offers several benefits:
  • As the oxidation and conductivity measurement occurs in the same cell, the instrument checks that all organics have been oxidized, and that a stable conductivity value has been reached before delivering a TOC value. Otherwise, the A10 monitor will indicate that it did not succeed in oxidizing all organics and will start a new test.
  • The A10 TOC monitor is able to detect TOC values over a range of 1 to 999 ppb TOC, with an accuracy of ± 1 ppb or 15 %, whichever is greatest.
  • Each A10 TOC monitor is calibrated for resistivity around two values: 18.0 MΩ•cm @ 25 °C and 1 MΩ•cm @ 25 °C, and for TOC with methanol over a range between 1 and 200 ppb TOC. The calibration results of each TOC monitor are delivered with the instrument."
  • Merck A10 TOC monitors meet the requirements for the performance of suitability tests as described in USP § 643. These tests can be performed with the support of Merck Service Engineers.
  • Merck A10 TOC monitors provide an accurate measure of the TOC value in the ultrapure water produced and therefore a warranty of the quality of that water. 

A10 TOC Monitor Photooxidation Cell

The A10 TOC monitor contains a quartz cell. Water flows through a channel at the base of the cell and then exits through a solenoid valve at the top. Titanium electrodes in the cell allow conductivity to be measured; a thermistor on the side measures temperature. A 185 /254 nm UV lamp located next to the cell can emit UV light.


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