Hach DR2400 - Page 791

Zinc
Zinc
Zinc_PP_Other_ZNC_Eng_Ody.fm Page 3 of 6
Interferences
Samples containing amino-tri(methylene phosphonic acid) (AMP) will exhibit a
negative interference. Perform a total phosphorus digestion (Method 8190) to
eliminate this interference. IMPORTANT: Adjust the pH of the sample after the
total phosphorus digestion to 4–5 with Sodium Hydroxide before analysis with
the zinc test.
Sample Collection, Preservation, and Storage
Collect samples in acid-cleaned plastic or glass bottles. If prompt analysis is
impossible, preserve the sample by adjusting to pH 2 or less with 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 4–5 with 5.0 N Sodium Hydroxide. Do not
exceed pH 5 as zinc may precipitate. 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.
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 Zinc Voluette
®
Ampule Standard, 25-mg/L Zn.
5. Prepare three sample spikes. Fill three mixing cylinders (Cat. No. 20886-40)
with 20 mL of sample and 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.
Interfering Substance Interference Levels and Treatments
Aluminum Greater than 6 mg/L
Cadmium Greater than 0.5 mg/L
Copper Greater than 5 mg/L
Iron (ferric) Greater than 7 mg/L
Manganese Greater than 5 mg/L
Nickel Greater than 5 mg/L
Organic Material Large amounts may interfere. Pretreat the sample with a mild digestion.
Highly buffered or extreme
sample pH
May exceed the buffering capacity of the reagents and require sample pretreatment. Adjust pH
to 4–5.