Metrohm 791 VA Detector - Manual (Page 27)

3.2 Electrodes
791 VA Detector
21
Carbon paste electrode CPE
Ordering designation 6.0807.000
Sensor material
paste composed of spectroscopic grade car-
bon powder and paraffin
Chemical resistance good resistance to aqueous solutions; limited
resistance only to water/acetonitrile and wa-
ter/methanol mixtures
Voltage range (–200) … 0 … +1200 mV (pH < 7)
Regeneration
The sensor surface of the CPE is more vulnerable to both me-
chanical damage and chemical attack than that of the GCE, it is
more easily regenerated: often it suffices merely to rub the tip of
the electrode backwards and forwards a few times against the
surface of a sheet of ordinary, smooth paper, or alternatively the
upper layer of carbon paste can be renewed (see below) and
then polished in this manner.
Filling
Carefully remove the old carbon paste from the approximately
2.5 mm deep cavity with a suitable sharp instrument. Take care
not to damage the platinum contact wire at the bottom of the cav-
ity. Using a small spatula, fill bit by bit with the fresh 6.2801.020
carbon paste and tamp well down using the 6.2826.000 stopper.
The cavity should be filled over the brim with paste. Now rub off
the excess paste by continuous circular motion against a sheet of
paper with a hard, smooth, non-porous surface; the circular mo-
tion plus the light pressure will polish the surface of the paste,
which must be smooth and shiny when the operation is finished.
There should be no cracks in the peripheral zone where the paste
is up against the plastic material of the electrode shaft.
Metal electrodes
Ordering designations 6.1204.120 Pt tip
6.1204.130 Ag tip
6.1204.140 Au tip
+ 6.2103.110 Contact pin
Chemical resistance good resistance to acetonitrile, methanol, wa-
ter, etc.
Voltage range Pt: (–1200) … 0 … +200 mV (pH < 7)
Ag: (–800) … 0 … +400 mV (pH > 7)
Au: (–400) … 0 … +1200 mV (pH < 7)
Regeneration
Sprinkle a little aluminium oxide powder (Al
2
O
3
, grain size 0.3 µ m)
on the polishing cloth (ordering number of both these accesso-
ries: 6.2802.000) and moisten with distilled water. Hold the elec-
trode vertically and polish with a circular motion for about
20 … 60 s, exerting only light pressure. Rinse with distilled water
and clean with dry cloth. A metal electrode should always be pol-
ished for a short while immediately before being fitted to the de-
tector cell.