Manual

14
5 Chart recorder output
The Digital Conductometer CG 855 has a chart recorder output (see Fig. 2, Pos. 3) with a source resi-
stance of R
i
= 2 k , for connecting a servo-type chart recorder whose input is floating with respect to
ground. The available chart recorder output voltage is 1 mV/digit of the liquid crystal display for
conductivity measurements. This output voltage corresponds to the "abs" function in all conductivity
measuring ranges.
Measuring range: "200 S/cm" Output voltage: 1 mV/0.1 S·cm
-1
"2000 S/cm" 1 mV/1 S·cm
-1
"20 mS/cm" 1 mV/0.01 mS·cm
-1
6 Instructions and explanations for making electric conductivity
measurements
The metal electrode surfaces of the measuring cell must be in complete contact with the sample solution.
Thus ensure that the measuring cell is dipped into the sample solution deeply enough. Air bubbles falsify
the measurement readings.
For all measurements made with dip-in measuring cells, wait until the reading is constant (this takes about
3 seconds) before taking it as the measurement result.
Do not use mechanical scrubbing methods to clean electrodes which are coated with platinum black. If
rinsing in water is inadequate, dip the measuring cell into 1 % sodium hydroxide solution, hydrochloric
acid or a suitable organic solvent, depending on the kind of contamination. For plastic measuring cells
with platinized nickel electrodes, employ acid cleaning only very briefly or preferably not at all, because
there is a danger that these electrodes will dissolve in acid.
7 Coating the conductivity measuring cells with platinum black
The Digital Conductometer CG 855 has a constant current source for coating the electrodes of the
measuring cells with platinum black: Connect the measuring cell and the platinum wire to the three 4 mm
sockets on the rear of the unit (Fig., Pos.). Dip the anode wire (platinum) and the measuring cell in the
platinizing solution. The platinum coating of the measuring cell then takes place immediately, of its own
accord, without actuating any manual control knobs. The process of coating the electrodes with platinum
black does not need to be carried out very often, so that it is convenient to use a "one way platinizing
solution" which is discarded after single use: Dilute solution of hydrochloroplatinic acid containing lead
acetate (0.5 g hydrochloroplatinic acid and 0.2 g lead acetate in 1 liter of distilled water). The coating
process takes about
3 minutes for a measuring cell with cell constant K = 1.00 cm
-1
. A good coating of platinum black is im-
portant especially when measuring high conductivities. Platinum as well as nickel measuring cells can be
coated with platinum black. The platinizing solution is available from SCHOTT-GERÄTE ready for use.
Abbreviations and formulae used in conductometry:
(kappa): Electrical conductivity of an electrolyte solution, specified in (S · m
-
1
) or (mS · cm
-
1
)
or ( S · cm
-
1
)
K: Measuring cell constant (cell factor) in (cm
-
1
)
G: Conductance in Siemens (S); G (S) = 1/R (1/ )