Hach DR2400 - Page 239

Chromium, Total
Chromium, Total
ChromiumTot_UniCell_Other_DCH_Eng_Ody.fm Page 3 of 4
Larger amounts of iron, copper, reducing or oxidizing agents give low results.
Lead, mercury, and tin give high results.
Note: Undissolved chromium is not determined with the determination of chromium VI. Total
Chromium can only be determined after digestion.
Note: Concentrations above 20 mg/L produce result displays within the ranges given above. In
such cases, verify the results by dilution.
Sample Collection, Preservation, and Storage
Collect samples in acid-cleaned glass or plastic containers. No acid addition is
necessary if analyzing the samples immediately or if the samples contain only
chromium(VI) ions. To preserve the sample for Total Chromium, adjust the
pH to 2 or less with concentrated nitric acid (about 2 mL per liter). Preserved
samples may be stored up to six months at room temperature.
Before analysis, adjust the pH to between 3 and 9 with 5.0 N sodium hydroxide
standard solution.
Accuracy Check
Standard Additions Method (Sample Spike)
1. After reading test results, leave the sample vial (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. Pipet 0.1 mL, 0.2 mL, and 0.3 mL of 50-mg/L Cr (VI) standard, respectively, to
three 25-mL samples and mix thoroughly.
5. Accept each standard additions reading by touching
Read. Each addition
should reflect approximately 100% recovery.
6. 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 0.70-mg/L Cr
6+
standard solution daily, as follows:
1. Using a pipet to transfer 1.40 mL of Hexavalent Chromium Standard
Solution, 50-mg/L, into a Class A 100-mL volumetric flask.
2. Dilute to the mark with deionized water.
Ag
+
, Pb
2+
25 mg/L
Cu
2+
, Fe
3+
10 mg/L
Sn
2+
1 mg/L
(continued)
Interfering Substance Interference Levels