Measurement
29
For pH-measurement in untreated desalinated water a particular electrode is
necessary. This glass electrode is optionally available and has a 1 mol/l KCl
reference electrolyte. The diaphragm of this electrode must be stored in
1 mol/l KCl-solution.
Before usage the electrode must be checked on exterior damage and glass
crash. Crusts caused by leaking electrolyte can be removed easily by
rinsing. The sample volume should be 100 ml dialysate or 1 l of untreated
water. The sample volume is being led to the bottom of the vessel with a
tube or hose for coming as little as possible into contact with air. The pH-
value must be measured immediately in the same vessel. You have to make
sure that the display stabilizes before the value is read. In stirred solutions
the response rate is faster, the value, however, must be measured at resting
fluid.
The pH-electrode must be dabbed only and never rubbed dry. The rubbing
destroys the jelly layer on the glass surface which results in a longer
response time of the electrode.
Due to the special electrode, for pH-measurement in untreated water further
points need to be paid attention to: air bubbles in the membrane space of
the electrode can be removed easily by shaking movements similar to a
clinical thermometer. The reference electrolyte in this electrode can be
refilled. Make sure that the refill cap is sealed with the rubber hood. The
level of the reference electrolyte must always be sufficiently large.
Before the measurement the rubber hood must be removed, since otherwise
no internal pressure compensation occurs. On the lower shaft of the
electrode a plastic part is to be found which must always remain freely
movable on the shaft. If the mobility of the plastic part should be restricted
it can be released by rinsing with lukewarm water. This plastic part
conceals an opening in the glass which must be covered always.
Pressure and currents have considerable influence on the pH-
measurements. Therefore, it is necessary to accomplish the pH-
measurement in a resting solution at environmental pressure.
