OHAUS STARTER 5000, Руководство по обслуживанию

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CHAPTER 2 DIAGNOSIS
Ohaus Corporation www.ohaus.com 2-4
ST5000 Bench Top pH Meter
Service Manual
2.7 DIAPHRAGM CONTAMINATION ISSUE.
Most of the problems during pH measurements are caused through a contaminated diaphragm,
Symptoms:

Dependancy of the stirrer speed

Drifting result

Shift of offset

Decrease of slope

Contaminated diaphragm = Number one troublemaker!
Diaphragm contamination can occur with:

solutions that contaminate the diaphragm
(e.g. solutions containing proteins, paints, AgS,…)

suspensions, emulsions

highly acidic solutions

strong alkaline solutions

low ionic samples (distilled water, ultrapure water)

solutions with high salt concentration

non-aqueous solutions
Cleaning the diaphragm

blocked with AgCl
-
with concentrated ammonia

blocked with Ag
2
S
-
with 8 % thiourea in 0.1 molar HCl

blocked with proteins
-
with 5 % pepsin in 0.1 molar HCl

other contaminations
-
in ultrasonic bath with water or 0.1 molar HCl
Or just replace with a new one
2.8 RECONDITIONING OF THE DRY GLASS MEMBRANE.
Dry membrane:
Re-conditioning in 0.1 mol/L HCl during 12 hours.
2.9 LIFETIME OF AN ELECTRODE.

The lifetime of an electrode is limited. Apart from maintenance and sample consistency,
one of the reasons is the aging of the membrane.

The aging o the pH sensitive membrane and therefore the decrease of the H
+
-sensitivity
depends on the temperautre: