Honeywell UDA2182 manual Importance of pH control, Reliable measurement with gold electrode

Appendices

Importance of pH control

As shown in Figure 15-4, pH has a direct effect on the ORP potential and must be closely controlled to achieve consistent ORP control, especially if hypochlorite is used as the oxidizing agent. Hypochlorite raises pH, which lowers the ORP potential, which in turn calls for additional hypochlorite -- a runaway situation. To avoid this situation, use close pH control and locate the ORP electrode at a distance from the hypochlorite addition point.

Reliable measurement with gold electrode

For this application, a gold ORP electrode gives a more reliable measurement than does a platinum electrode, because platinum may catalyze additional reactions at its surface and is more subject to coating than gold. Note that the solubility of gold in cyanide solutions does not present a problem as it is in contact, primarily, with cyanide. In fact, a slight loss of gold serves to keep the electrode clean.

Second Stage of Cyanide Destruction

Neutralize and further oxidize cyanate

The wastewater is neutralized in order to promote additional oxidation and to meet the discharge pH limits. Typically, sulfuric acid is added to lower the pH to about 8.5. At this pH the second oxidation occurs more rapidly.

WARNING

Failure to comply with these instructions could result in death or serious injury.

An interlock must be provided to prevent the addition of acid before the positive oxidation of ALL cyanide. Failure to observe this precaution can result in the generation of highly toxic hydrogen cyanide.

Additional chlorine or sodium hypochlorite (NaOCl) can be added in proportion to that added in the first stage, or by separate ORP control to complete the oxidation to sodium bicarbonate (NaHCO3) in the following reaction:

2NaCNO + 3NaOCl + H2O → 2NaHCO3 + N2 + 3NaCl

ORP control in the second stage is very similar to that in the first stage, except that the control point is near +600 mV. Control of pH in the second stage is more difficult than in the first stage, because the control point is closer to the sensitive neutral area. Proportional type pH control is often used.

Removal of suspended metal hydroxides

Following the second stage, a settling tank and/or a filter can be used to remove suspended metal hydroxides. However, further treatment may be required to lower concentrations of some metals below their hydroxide solubilities.

206 UDA2182 Universal Dual Analyzer Product Manual January 2009