Interferences, Run a check standard – Hach-Lange ISECL18103 User Manual User Manual
Page 6
•
The sample pH is in the range that is given in the specifications.
•
Omission of the ISA is accepted by the regulatory reporting agency (if the measurement is
for regulatory reporting).
Interferences
The sensing element responds to chloride as well as other ions. Typically, probe
response to another ion increases the potential, and causes a positive error. The
response to other ions can be semi-quantitatively determined through the Nikolsky
equation, an extended Nernst equation:
E = Eº + (RT/(zF))ln[aN
a
+ KN
a
x × ax]
Where
•
ax—the activity of the interfering ion
•
KN
a
x—the selectivity coefficient for the interfering ion relative to chloride
High levels of ions which form insoluble salts of silver can deposit a layer of salt on the
sensing surface, and cause electrode malfunction. Strong reducing solutions can also
form a surface layer of silver. In either case, clean the sensing element. Refer to
on page 10.
Mercury must be absent from samples.
Measurements can be made in solutions that contain oxidizing agents such as Copper
(Cu
2+
), Iron (ferrous) (Fe
3+
) and Maganate (MnO
4
-
).
If Chloride ISA is added to the standards and samples, the effect of interfering ions is
minimized.
The selectivity coefficient is the approximate apparent increase in the measured
concentration caused by one unit of the interfering ion (e.g., 1 unit of I
-
raises the chloride
concentration by 0.1). The approximate selectivity coefficients for some ions with the
IntelliCAL
®
Chloride ISE are shown in
.
Table 1 Interferences
Interference
Selectivity coefficient
Iodide (I
-
)
> 0.1
Bromide (Br
–
)
> 0.1
Silver (Ag)
> 0.1
Sulfide (S
2-
)
> 0.1
Cyanide (CN
-
)
> 0.1
Run a check standard
The run check standard feature validates instrument performance between sample
measurements. Use the run check standard feature for periodic or user-defined interval
measurements of a traceable standard solution. Set the criteria for check standards from
the ISECl181 Settings menu.
Note: Access control must be off or a valid password must be entered before any of the check
standard method options can be changed.
1. Push . The Full Access Options menu is shown.
2. Select Run Check Standard.
Note: Select the correct probe if two probes are connected to the meter.
3. Prepare the standard solution shown on the display. Add one powder pillow per
25 mL of standard solution.
4. Put the probe in the standard solution and push Read. The display will show
"Stabilizing" and a progress bar as the reading stabilizes. The display shows the
value of the check standard and either Check Standard Passed or Check Standard
Failed.
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