Hach DR2400 - Page 235

Chromium, Total
Chromium, Total
ChromiumTot_PP_Other_AHY_Eng_Ody.fm Page 3 of 4
Interferences
Sample Collection, Storage and Preservation
Collect samples in acid-washed glass or plastic containers. To preserve samples,
adjust the pH to 2 or less with nitric acid (Cat. No. 152-49). This requires
approximately 2 mL per liter of the acid. Store preserved samples at room
temperature up to six months. Adjust the pH to about 4 with 5.0 N Sodium
Hydroxide (Cat. No. 2450-26) before analysis. Correct the test result for volume
additions; see Section 3.1.3 Correcting for Volume Additions on page 29.
Accuracy Check
Standard Additions Method (Sample Spike)
1. After reading test results, leave the sample cell (unspiked sample) in the
instrument. Verify the chemical form.
2. Touch
Options. Touch Standard Additions. A summary of the standard
additions procedure will appear.
3. Touch
OK to accept the default values for standard concentration, sample
volume, and spike volumes. Touch
Edit to change these values. After values
are accepted, the unspiked sample reading will appear in the top row. See
Standard Additions in the instrument manual for more information.
4. Snap the neck off a Trivalent Chromium Voluette
®
Ampule Standard, 12.5-
mg/L as Cr
3+
.
5. Prepare three sample spikes. Fill three mixing cylinders (Cat. No. 1896-40)
with 25 mL of sample. Use the TenSette
®
Pipet to add 0.1 mL, 0.2 mL, and
0.3 mL of standard, respectively, to each sample and mix thoroughly.
6. Analyze each sample spike as described in the procedure above, starting with
the 0.1 mL sample spike. Accept each standard additions reading by touching
Read. Each addition should reflect approximately 100% recovery.
7. After completing the sequence, touch
Graph to view the best-fit line through
the standard additions data points, accounting for matrix interferences. Touch
View: Fit, then select Ideal Line and touch OK to view the relationship between
the sample spikes and the “Ideal Line” of 100% recovery.
See Section 3.2.2 Standard Additions on page 32 for more information.
Standard Solution Method
Prepare a 0.25-mg/L trivalent chromium standard as follows:
1. Dilute 5.00 mL of Trivalent Chromium Standard Solution, 50-mg/L as Cr
3+
,
to 1000 mL with deionized water. Prepare this solution daily.
2. To adjust the calibration curve using the reading obtained with the
0.25-mg/L standard solution, touch
Options on the current program menu.
Touch
Standard Adjust.
Interfering Substance Interference Levels and Treatments
Highly buffered samples or
extreme sample pH
May exceed the buffering capacity of the reagents and require sample pretreatment; see Section
3.3 Interferences on page 41.
Organic material
May inhibit complete oxidation of trivalent chromium. If high levels of organic material are
present, see Section 4 for instructions on sample digestion. Perform the analysis as described
on the digested sample.
Turbidity For turbid samples, treat the 25-mL blank and the sample the same during steps 2–8.