pH Electrodes

pH electrodes are analytical sensors for measuring potential of hydrogen (pH), the negative logarithm of the hydrogen ion activity in solution. The pH value of a substance is directly related to the ratio of the hydrogen ion [H+] and the hydroxyl ion [OH-] concentrations. This is one of the most common laboratory measurements because so many chemical processes are dependent on pH. With pH electrodes then, there are three main performance specifications: pH range, pH accuracy, and response time. Wide-range pH electrodes function over the activity range 1 M H⁺ (pH 0) to 10^-14 M H⁺ (pH 14). Response time is usually expressed as a value such as 95% in 10 seconds. 

pH electrodes carry physical specifications for electrode configuration and reference solution options. Single cells or electrode pairs require a separate reference electrode. This type of device is the best choice for colloidal suspensions, iodides in sample, and high-percentage solids in fluid. Combination pH electrodes are also commonly available. These devices have two parts: the measuring electrode and the reference electrode. In terms of reference solution options, reference electrodes are either refillable or sealed. The trade-off between the two types if the amount of maintenance versus length of product life. Refillable pH electrodes require more maintenance, but also last longer and typically have higher accuracies. Sealed pH electrodes doe not require refilling, but have a limited life since the chemicals inside are used up and are not replaceable. 

Junction type is important physical specification to consider when selecting pH electrodes. There are two basic types: single junction and double junction. As their name suggests, single junction reference electrodes contain a single electrolyte. The electrolyte provides a constant level of the ion sensed by the reversible reference element and forms a low potential liquid junction with the sample solution. Double junction reference electrodes have an additional reference cell in front of the potassium chloride cell. This cell screens the sample from the potassium chloride reference cell. Typically, double junction cells have a longer time. pH electrodes with other, unlisted junction types are also available. 

pH electrodes differ in terms of body material, mounting options, and features. There are two common choices for body material: glass, and epoxy or polymer. Mounting options are defined as handheld or portable, in-line (insertion), and flow-through (pipeline). Handheld or portable configurations allow pH electrodes to be used in laboratories where an operator may need to test several different samples. Insertion-style pH electrodes are often inserted into process piping through a tapped hole in a pipe or bulkhead. Flow-through styles fit directly into the pipeline and become an integral part via some connection, such as flanges or other fittings. Typical features for pH electrodes include temperature compensation, built-in temperature sensing, a submersible or water resistant design, and a design that is specially-suited for food or sanitary applications.

• Shelf Life of pH Electrodes (.pdf)
  Electrodes have an average life span of 2 - 5 years. Due to the chemical reaction inside the electrode, the electrode begins "working" on the first day it is manufactured. Improper storage of the...
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• Guide to Minimizing User Errors in pH Measurements
  Over one hundred different compositions available from many different manufacturers None exhibit completely linear pH response in overall pH range specified Maximum available pH range: -2 to 16 pH in...
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• Advances in pH Measurement in High-Temperature Biotech Processes
  By CD Feng, Emerson Process Management, Rosemount Analytical Liquid The A electrodes were made from a new high-temperature glass formulation based upon the research showing that aging occurs primarily...
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Part Numbers for pH Electrodes