Volume Increment; Predose Volume; Auto-Scaling; End Point Determination - Hach AutoCAT 9000 Instruction Manual

Collecting and Accessing Data

5.2.1.2 Volume Increment

5.2.1.3 Predose Volume

5.2.1.4 Auto-scaling

5.2.2 End Point Determination

Collecting and Accessing Data
Page 44
The AutoCAT 9000 delivers titrant to the sample solution in discrete volume increments.
During data collection, the AutoCAT 9000 injects one volume increment of titrant
solution into the sample once every 3 seconds. The volume increment is an adjustable
parameter that controls the volume (mL) of titrant dispensed for each addition. The
smallest increment setting is 0.0003 mL.
In general, for forward titrations, a reasonable volume increment can be calculated based
on the estimated analyte concentration (in mg/L) by dividing the end point volume in
milliliters (numerically equivalent to the chlorine concentration when using
0.00564 N PAO as a titrant) by the number of points to be collected before the end point.
For example, if you wish to collect 50 points before the end point then the appropriate
volume increment would be given by the following equation:
Volume Increment (mL)
=
A table is provided in each embedded method procedure to help determine the
appropriate volume increment to use for data collection.
Predose defines a volume of titrant added to the sample prior to data collection.
If 1.50 mL of titrant is required to reach the end point, the entire titration can be run using
a 0.03 mL volume increment and no predose. Alternatively, a titration can run using a
0.01 mL volume increment and a 1.00 mL predose. In this case 1.00 mL of titrant is
added prior to data collection and the final 0.50 mL is delivered during data collection.
The same number of points will be collected and the same amount of time will be
required. In the first titration a 20 µA current range will be required to avoid "clipping"
the top of the titration curve. A 2 µA current range may be used in the second case.
The AutoCAT 9000 has an auto-scaling feature. When Auto-scaling is enabled the
AutoCAT 9000 will attempt to set the current range and volume increment automatically.
This feature is useful for titration of samples containing over 25 ppb total chlorine. Auto-
scaling is less useful for free chlorine titrations and cannot be used with back titrations.
Auto-scaling can be used with or without a titration predose.
Two associated parameters, No. points before EP and Fraction before EP, become
available when Auto-scaling is activated.
The No. points before EP sets the approximate number of points desired before the end
point.
The Fraction before EP sets the fraction of the total number of points collected. For
example if the fraction is set to "0.50" then approximately 50% of the data points are
collected before the end point and 50% afterwards.
The AutoCAT9000 incorporates two routines for end point determination. Both routines
(described in Section 5.2.2.1 and 5.2.2.2) attempt to determine the volume of titrant
needed to reach the break in the titration curve.
(Estimated Chlorine Concentration (mg/L))
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