Electrodes, Sensors, and Measurement Cells

These corresponding sockets / connections are then located on your measuring device:

Optical methods

These methods use pH-dependent color changes of specific organic pigments, so-called color indicators. So for example as the pH value increases, the color of methyl red in an aqueous solution changes from red to yellow at a pH of 4.9. Phenolphthalein for example turns reddish at a pH of 9.5. The best known of these is the pH indicator paper or pH test strips, which are prepared with indicator solutions of these organic pigments. The pH value is estimated by means of a visual comparison of the color against a color scale. However the precision is only sufficient to provide a rough estimate.

Photometric pH measurement

The color change of the indicator pigments can also be photometrically determined by shining a light and measuring the absorbance. These methods are referred to either as colorimetric or spectrophotometric, depending on the equipment and light source used. In theory it is possible to take pH measurements in this way. However the method is very prone to interference and the equipment needed is large.

Potentiometric determination of the pH value

This method uses the electrical potential of pH-sensitive electrodes as a measurement signal. A distinction is made between hydrogen, metal and glass electrodes. The glass electrode is the most commonly used sensor today. Not having the disadvantages of the optical methods, it can be used almost universally. It is one of the most sensitive and at the same time most selective sensors there is and has an unmatched measurement of pH 0 to 14, means from percent to ppq (= parts per quadrillion = one molecule in one quadrillion other molcules).

Ceramic diaphragm 

The ceramic diaphragm uses the porosity of unglazed ceramic. It's KCl outflow rate is approximately 0.2 ml / 24 h (p = 1m water column). Its electrical resistance is relatively high at 1 kΩ. In measurement solutions with greater ionic strength, the concentration gradient at the diaphragm is very large, meaning diffusion potentials are very easily created. At lower ionic strengths the resistance of the test material may be too high for exact measurements. Both effects are amplified by low outflow rates, and so ceramic diaphragms are less suitable in such cases. Due to the high risk of blockage of its pores, it is also not suitable for solutions containing suspended particles. Only in measurement solutions that contain oxidizing substances is it clearly superior to the platinum diaphragm.

Ground-joint diaphragm

The ground-joint diaphragm works with the thin gap of the unlubricated ground glass as an outfl ow opening for the electrolyte. The outfl ow rate is 3 ml/24 h (p = 1m water column) and greater. Its electrical resistance is very low at 0.1 kΩ. It is suitable for measurements in contaminated solutions, as it is easy to clean. Due to the high outfl ow rate, it is suitable for both high and lowion solutions. In versions without a screw connection, the ground gap must be manually adjusted in order to set a consistent fl ow rate.

Plastic diaphragm

For special applications there are also diaphragms made from plastic fibers. For example, single-rod measuring chains with a plastic shaft often have diaphragms made from nylon fibers so as to avoid contamination of the connection hole. For process measurements in solutions that contain fluoride, electrolyte keys with PTFE diaphragms are used.

Platinum diaphragm

The platinum diaphragm is an SI Analytics® development. It consists of fine, twisted platinum filaments between which the electrolyte flows out along precisely defined channels. The platinum diaphragm does not easily become blocked and therefore features a very constant outflow. With approximately 1 ml / 24 h (p = 1m water column) and approximately 0.5 kΩ  electrical resistance, it has advantages over ceramic diaphragms. However it is more sensitive to mechanical stress. It is also less than  optimal for strongly oxidizing or reducing solutions due to the occurrence of disruptive potentials.

Hole or annular gap diaphragm

With polymer electrolyte electrodes a conventional diaphragm becomes superfluous, as the solid surface serves as an interface. In combination electrodes, this is utilized e.g. in the form of an annular gap diaphragm. It consists of an annular interface drawn around the sensor between the membrane and the outer tube. A relatively low resistance is achieved due to the relatively large electrolyte/measuring medium interface and its small distance from the sensor. The ring-shaped arrangement around the sensor eliminates geometrically induced interference effects.

The different electrode glasses differ in their composition and are optimized for different applications.

• H Glass: for high temperatures, in the acidic and alkaline range, even with high sodium ion concentrations
• S Glass: in hot alkaline media with good reproducibility and fast response time
• L Glass: for low temperatures and general applications
• A Glass: Fast response time in drinking, domestic, and waste water and for general use and in low-ion media

Standard buffers in accordance with DIN 19266 are used for the calibration of pH measurements. The so-called technical buffers
are governed by DIN 19267. DIN buffers are manufactured in accordance with DIN 19266 and can be traced back to primary or
secondary reference material. The primary reference material (powder form) is manufactured by NIST (National Institute of Standards and Technology).

Technical buffer solutions are based on DIN 19267 and differ in several respects from DIN buffer solutions manufactured in accordance with DIN 19266. They are often colored, so as not to be confused during every day use, are based on whole numbers and are more stable. The composition varies depending on the manufacturer.

Particular emphasis must be placed on the maintenance and care of electrodes in order to optimize useful life. We can make the following recommendations with respect to handling and care:

• Remove the watering cap above the membrane and diaphragm. It contains electrolyte solution (potassium chloride solution 3 mol/l). The electrode is ready for measuring.
• If the electrodes are accidentally stored dry, please first soak them for 24 hours in electrolyte solution (typically the same solution with which the reference electrode is filled, i.e. 3 mol/l KCl).
• In the electrolyte space of the reference system, any missing potassium chloride solution should be refilled.
• The fill level of the electrolyte solutions should be above the level of the measuring medium, ideally 3 cm or more.
• To calibrate and measure liquid electrolyte electrodes, the cap of the refill opening must be opened.
• The diaphragm must be immersed in the measurement solution.
• For low-maintenance electrodes with gel filling, Duralid® or Referid®, there is no need to refill.
• Electrodes should ideally be stored in electrolyte solution, i.e. 3 molar KCl or the solution that is also in the reference electrode if it does not contain KCl. Always use KCl for gel and polymer electrolytes because their electrolyte is also saturated with KCl.

• Contamination on the membrane/Pt sensor and diaphragm leads to measurement deviations. Coatings can be removed with diluted mineral acids (e.g. hydrochloric acid 1:1), organic soiling can be dissolved with suitable solvents, grease can be removed with surfactant solutions and protein can be dissolved with hydrochloric acid pepsin solution (cleaning solution L510). After cleaning, rinse the electrode with distilled water, do not rub dry.
• Ceramic diaphragms clogged from the outside can be made functional again by carefully rubbing them with fine sandpaper or a diamond file. The pH glass membrane must not be scratched in the process!
• Platinum diaphragms must not be treated mechanically. Chemical cleaning (e.g. with diluted hydrochloric acid) can be followed by rinsing (e.g. suction with vacuum).
• Ground-joint diaphragms are made ready for use before measurement by lifting them slightly and then placing the ground-joint sleeve on the ground-joint core. The refill opening should be open. Caution: this increases the flow of electrolyte so that the surface of the ground joint is properly wetted.
• The glass membrane can be cleaned by rubbing it with a lint-free cloth soaked in ethanol.

Each electrode must meet the strict quality requirements of our final inspection in order to deliver the optimum in measurement reliability, setting speed and service life. The life of an electrode is highly dependent on the conditions in which it is used.

Extreme conditions can affect the lifetime. For example:

• • High or frequently changing temperatures
  • Strong acids and alkalis
  • Protein
  • Heavily contaminated solutions
  • Electrode poisons such as sulphide, bromide and iodide
  • Hydrofluoric acid and hot phosphoric acid (which attack glass)