Hach DR2400 - Page 621

Phosphorus, Reactive (Orthophosphate)
Phosphorus, Reactive (Orthophosphate)
PhosphorusReac_None_Other_AMI_Eng_Ody.fm Page 3 of 6
Interferences
Sample Collection, Storage, and Preservation
Collect samples in clean plastic or glass bottles that have been cleaned with
1:1 Hydrochloric Acid Solution (Cat. No. 884-49) and rinsed with deionized
water. Do not use a commercial phosphate-based detergent for cleaning
glassware because the phosphate content will contaminate the sample.
Analyze samples immediately for best results. If prompt analysis is not possible,
preserve samples by filtering immediately and store at 4 °C (39 °F) for up to 48
hours. The sample should have a neutral pH (6–8) and be at room temperature
before analysis.
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.
Interfering Substance Interference Levels and Treatments
Calcium (Ca
2+
) Greater than 10,000 mg/L as CaCO
3
Chloride Greater than 150,000 mg/L Cl
–
Colored samples
Add 1 mL of 10 N Sulfuric Acid Standard Solution (Cat. No. 931-53) to another 25-mL sample.
Use this instead of untreated sample as the blank to zero the instrument. Use a pipet and pipet
filler to measure the sulfuric acid standard.
High salt levels (Na
+
)
May cause low results. To eliminate this interference, dilute the sample until two successive
dilutions yield about the same result.
Magnesium Greater than 40,000 mg/L as CaCO
3
Nitrites (NO
2
–
)
Bleach the blue color. Remove nitrite interference by adding 0.05 g of sulfamic acid (Cat. No.
2344-14) to the sample. Swirl to mix. Continue with Step 4.
Phosphates, high levels
(PO
4
3–
)
As the concentration of phosphate increases, the color changes from blue to green, then
to yellow and finally to brown. The brown color may suggest a concentration as high as
100,000 mg/L PO
4
3–
. If a color other than blue is formed, dilute the sample and retest.
Sulfide (S
2–
)
Sulfide interferes. For samples with sulfide concentration less than 5 mg/L sulfide interference
may be removed by oxidation with Bromine Water as follows:
1. Measure 25 mL of sample into a sample cell.
2. Add Bromine Water (Cat. No. 2211-20) drop-wise with constant swirling until permanent
yellow color develops.
3. Add Phenol Solution (Cat. No. 2112-20) drop-wise until the yellow color just disappears.
Proceed with step 4.
Temperature For best results, sample temperature should be 21 ±3 °C (70 ±5 °F).
Tu rb i di ty
May give inconsistent results for two reasons. Some suspended particles may dissolve because
of the acid used in the test. Also, desorption of orthophosphate from particles may occur. For
highly turbid samples, add 1 mL of 10 N Sulfuric Acid Standard Solution (Cat. No. 931-53) to
another 25-mL sample. Use this instead of untreated sample as the blank to zero the instrument.
Use a pipet and pipet filler to measure the sulfuric acid standard.
Highly buffered samples or
extreme sample pH
May exceed the buffering capacity of the reagents and require sample pretreatment.