Storage solution 6.2323.000 for pH glass electrodes

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General

Glass has a silicate skeleton that forms a thin hydrated layer on contact with aqueous media. The glass structure is softened in this hydrated layer, i.e. ions can penetrate this thin layer and alter the electrochemical properties of the glass. This hydration of the glass surface is essential for the use of glass as the material for pH glass electrodes, as without this hydrated layer no pH measurement would be possible. The structure of glasses used for pH glass electrodes (mainly lithium silicate glasses) has been optimized so that, as far as possible, only protons can penetrate the glass membrane. Because of the very slow but continuous hydration process it is impossible to prevent other ions from penetrating the glass. Sodium and potassium ions are particularly important here. At higher concentrations these result in the so-called alkali error of glass electrodes, i.e. the measured value is incorrect at relatively low proton con-centrations. If a glass electrode is stored for a long time in a solution containing plenty of sodium or potassium ions then these will penetrate the glass membrane and result in an increased response time of the glass membrane as the protons must first replace the “foreign ions” from the hydrated layer.

Fig. 1: Cross-section through a pH glass membrane. If several cations are contained in the solution to be measured then these will compete for the free places in the softened hydrated layer. Potassium and sodium ions are the main ions that can penetrate the glass membrane and cause increased response times.

One of the most widely used electrolytes for pH measurement is c(KCl) = 3 mol/L, as KCl produces a very small diffusion potential at the diaphragm and is also very favorably priced. Normally a combined pH glass electrode is only stored in c(KCl) = 3 mol/L because it is required to be available for immediate use without having to first condition the diaphragm. However, long-term storage in KCl damages the glass as it results in ever-increasing response times. For the glass itself the optimal storage medium would be distilled water, although after such storage the diaphragm must first be conditioned for several hours. The Storage solution for combined pH glass electrode has been developed to solve this very problem. If a combined pH glass electrode is kept in the Storage solution then the performance of the glass membrane does not change with respect to the response time and the alkali error. If c(KCl) = 3 mol/L is additionally used as the reference electrolyte then the optimized com-position of the Storage solution guarantees that the pH glass electrode is always ready for immediate use. Conditioning before the measurement is not necessary, no matter how long the electrode has been stored.

Fig. 2: pH measurement in c(NaHCO3) = 0.05 mmol/L. Aquatrode electrode glass kept in the Storage solution shows a considerably shorter response time compared with the same type of electrode glass stored in KCl for the same length of time.

Field of application

• Aquatrode Plus (6.0253.100 and 6.0257.000): The outstanding property is the short response time in low-ionic solutions. Storage in the Storage solution retains this advantage permanently.
• Storage of a pH glass electrode for a period of several weeks or months.
• For the occasional use of a pH glass electrode for measurements, particularly when measurements have to be compared with one another.
• Storage between individual series of measurements, primarily in sam-ple changer systems. The pH glass electrode can be automatically “parked” in the Storage solution overnight or after a series has been completed and is again ready for immediate use without conditioning.
• pH glass electrodes for SET titrations: A short electrode response time is particularly important in order to avoid over-titrating the solution.

Use

• The bridge electrolyte should not be replaced by the Storage solution.
• If a combined pH glass electrode with c(KCl) = 3 mol/L as the bridge electrolyte is stored then the filling opening for the reference electrolyte must be closed. Mixing the Storage solution with the reference electrolyte in the diaphragm has no influence on the func-tioning of the electrode.
• For the long-term storage of a combined pH glass electrode 2 – 3 ml Storage solution is placed in the electrode sheath instead of c(KCl) = 3 mol/L. The filling opening for the reference electrolyte must be closed.
• For storage in a different container the electrode with the diaphragm must be immersed in the Storage solution. The filling opening for the reference electrolyte must be closed.