Filter
Top Products
Appendices
January 2009 UDA2182 Universal Dual Analyzer Product Manual 209
First Stage of Chrome Removal
Lower pH and add reducing agent
Sulfuric acid is used to lower the pH to about 2.5, which promotes the reduction reaction
and ensures complete treatment. The reducing agent may be sulfur dioxide, sodium
sulfite, sodium bisulfite, sodium metabisulfite, sodium hydrosulfite, or ferrous sulfate.
The reaction is given below. The chromate is expressed as chromic acid, CrO
3
, with a +6
charge on the chromium and the reducing agent is expressed as sulfurous acid, H
2
SO
3
,
which is generated by sulfites at low pH. The result is chromium sulfate, Cr
2
(SO
4
)
3
, with
a +3 charge on the chromium. The reaction is expressed as:
23 3
3232432
CrO H SO Cr SO H O
+
→
+
()
This first stage reaction is analyzed and controlled by independent control loops: acid
addition by pH control; reducing-agent addition by redox potential or ORP (oxidation-
reduction potential) control. Often an ON-OFF type of control using solenoid valves or
metering pumps can be used. The pH controller simply calls for additional acid whenever
the pH rises above 2.5. The ORP controller calls for additional reducing agent whenever
the ORP potential rises above about +250 mV. (The metal ORP electrode is positive with
respect to the reference electrode.)
Titration curve
The ORP titration curve in Figure 15-6 shows the entire millivolt range if Cr
+6
chrome is
treated as a batch. With continuous treatment, operation is maintained in the fully
reduced portion of the curve near the +250 mV setpoint. The ORP setpoint can vary
between installations, depending on pH, reducing agent, presence of additional
contaminants and dissolved oxygen, and the type of reference electrode used. Determine
the exact setpoint empirically. This ORP setpoint should be at a potential where all of the
Cr
+6
has been reduced without excess sulfite consumption, which can release sulfur
dioxide gas. This point can be verified with a sensitive colorimetric test kit or similar
check.
VOLUME OF BISULFATE ADDED
REDOX POTENTIAL (mV)
0
1
2
3
4
5
6
7
100
200
300
400
500
600
pH= 2
pH= 3
700
Figure 15-6 Chrome Reduction - Typical Titration Curve