Calibration; Formazin Stock Solution; Dilution Water - Hach 2100N Instruction Manual

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SECTION 3,
continued

3.2 Calibration

Note: Alternately, obtain optimum
calibration accuracy using the
StablCal™ Calibration Kit for the
2100N Turbidimeter. This kit
contains prepared, stabilized
formazin suspensions of <0.1-, 20-,
200-, 1000-, and 4000-NTU. Order
Cat. No. 26621-00 (500 mL each) or
Cat. No. 26621-05 (ampules).

3.2.1 Formazin Stock Solution

3.2.2 Dilution Water

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The electronic and optical design of the 2100N Turbidimeter provides long-term
stability and minimizes the need for frequent calibration. The three-detector
ratioing optical system compensates for electronic and optical system variations
between calibrations. When data is used for USEPA reporting, recalibrate at least
every 90 days, or as stipulated by the regulating authority.
The calibration is based on a first order linear equation consisting of up to three
independent variables. Unpredictable results may occur if standards other than the
recommended calibration points are used. The factory suggested calibration points
are those determined by Hach Company chemists and engineers to provide the
best calibration accuracy. Use of Formazin standards other than those specified in
Section 3.2.3 on page 38 may result in less accurate calibrations.
Periodically, as experience or regulating authorities indicate, verify the instrument
calibration using pre-calibrated Gelex Secondary Standards (refer to Section 3.2.5
on page 42). If the reading in the range of use is not within 5% of the standard's
assigned value, recalibrate using Formazin primary standards (refer to Section
3.2.4 on page 39).
Make Formazin dilutions for instrument calibration from a 4000-NTU stock
solution equivalent to the Hach Cat. No. 2461-49 turbidity standard supplied with
the instrument. This prepared stock solution is stable for up to one year when
properly stored. Thoroughly mix the 4000-NTU stock solution prior to use for
making standards.
If desired, prepare a 4000-NTU stock solution from hydrazine sulfate and
hexamethylene-tetramine (also available from Hach). Refer to Section 3.2.6 on
page 43 for preparation instructions.
Use high-quality, low-turbidity water (<0.5 NTU) to prepare the Formazin
dilutions required to calibrate the instrument. The 2100N Turbidimeter provides
automatic correction for <0.5 NTU turbidity contributed by dilution water (refer
to Section 3.3.3 on page 44). Distilled, demineralized or deionized water usually is
sufficient, as is most filtered tap water. If the purified water exceeds 0.5 NTU,
filter it to meet the turbidity requirements (see Section 2.8.1 on page 29).
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