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.

KunststoffdiaphragmaPlastic diaphragm

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

Platinum diaphragm

The platinum diaphragm is an
SI Analytics® development. It
consists of fi ne, twisted platinum
fi laments between which the
electrolyte fl ows out along
precisely defi ned channels. The
platinum diaphragm does not
easily become blocked and
therefore features a very constant
outfl ow. 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 superfl uous, 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.