are designed as single-use versions and are based on the optical and potentiometric measuring principle 20. The optical determination of pH can be based on fluorescence or absorption, whereby fluorescence-based dyes such as fluorescence derivates or absorbing substances like phenol red are applied. The dyes are pH sensitive and show a pH value-dependent fluorescence or absorption in the solution. The advantage of optical pH sensors lies in their miniaturizability. However, the dyes can lose their sensitivity and are applicable only in a limited pH range of around 3 units. Potentiometric pH sensors measure the potentiometric deviation between the working and the reference electrode 41. They are also miniaturizable. Today, it is not yet possible to achieve comparably exact measurements as with conventional pH probes 62. See sensors, instrumentation.