Hach AutoCAT 9000, Instruction Manual

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Amperometric Titration Theory
Amperometric Titration Theory
Page 148
50081_Titration.fm
conditioned electrodes. Since the electrodes contact the sample, certain species in the
sample may plate out or coat the electrode's metallic surface. Metal ions such as copper
(+2), silver (+1) and iron (+3) have been reported to interfere with amperometric
titrations. In some waters, foaming or surface-active agents will coat the metallic
electrodes, decreasing sensitivity.
1.2.3 Manganese Compounds
Manganese (Mn) exists in oxidation states of +2 t7. The higher oxidation states,
typ3 to +7, will interfere with all the common chlorine methods. Free chlorine
reacts to oxidize soluble manganese compounds, for example:
Mn
2+
+ HOCl + 3OH
– →
MnO
2
+ Cl
–
+ H
2
O
If present, oxidized forms of manganese (+4 to +7) will titrate with PAO in the forward
titration procedure for free chlorine. This will cause the measured chlorine concentration
to be artificially high.
Iodide (I
–
) can be oxidized by Mn (+4 to +7) to I
2
, which also interferes with the
titrimetric methods for total chlorine. Oxidized forms of manganese will react with
iodide at pH 4 producing iodine, which then titrates with PAO causing an interference.
The interference of oxidized manganese in the back titration method appears to be a
function of iodide concentration and the test pH
*
.
1.2.4 Nitrite Interference
Nitrite (NO
2
–
) can exist as a transitory compound found in certain waters due to the
biological oxidation of ammonia:
NH
4
+
+ 3O
2
→
2NO
2
–
+ 4H
+
+ 2H
2
O
There is conflicting information about the interference of nitrites in forward or backward
amperometric methods for total chlorine.
Standard Methods , section 4500-Cl D.1. b ,
states that nitrites do not interfere in the forward titration methods. Section 4500-Cl
C.1.
b , the Iodometric Method II, states that nitrite interference can be minimized by
buffering to pH 4.0 before addition of iodide. It also states that interference from more
than 0.2 mg/L nitrites can be controlled by the use of a phosphoric acid-sulfamic acid
reagent. This reagent is used with iodate (IO
3
–
) as titrant, since a higher acidity is
required to liberate free iodine.
Research scientists at Hach have confirmed that nitrites can oxidize KI to iodine at low
pH values, similar to the reaction of KI with chlorine or chloramines. The reaction of KI
with nitrite is dependent on the titration pH and the order of reagent addition.
In the forward titration for total chlorine, nitrites appear to cause positive and negative
interferences, depending on the order of reagent addition. If iodide is added to the sample
before the pH 4 buffer, the error increases as a function of nitrite concentration. If buffer
is added before the iodide, a large negative error occurs, which is independent of the
nitrite level.
In the back titration using iodate ( Standard Methods 4500-Cl C), a lower solution pH
occurs due to the addition of the phosphoric acid-sulfamic acid mixture. In this case,
nitrites as low as 1 mg/L will react quickly with the added iodide, liberating free iodine.
The preferred procedure, which shows little interference from nitrites, is the back
titration at pH 4 using standard iodine titrant. The iodometric procedure in which KI is
added first, then buffered, is least affected with increasing amounts of nitrites. The back
*. EPA Report 600/7-77-053 , U.S. Environmental Protection Agency, 1977.