Phosphorus, Reactive (Orthophosphate)

Scope and Application

For drinking water, wastewater, and seawater analysis.

• USEPA Accepted for reporting for wastewater analyses. Procedure is equivalent to USEPA and Standard Method 4500-P-E for wastewater.
• USEPA Accepted for reporting for drinking water analysis. Procedure is an acceptable version of EPA Method 365.1, approved at 40 CFR part 141 NPDWR compliance monitoring.
• Adapted from Standard Methods for the Examination of Water and Wastewater.

Test Preparation

Instrument-Specific Information

Table 1 and Table 2 provide sample cell and orientation requirements for reagent addition tests, as well as sample cell and adapter requirements for AccuVac Ampul tests. These tables also list the instruments compatible with this test.

Before Starting

• Install the instrument cap on the DR 900 cell holder before ZERO or READ is pushed.
• For best results, measure the reagent blank value for each new lot of reagent. Replace the sample with deionized water in the test procedure to determine the reagent blank value. Subtract the reagent blank value from the sample results automatically with the reagent blank adjust option.
• Review the Safety Data Sheets (MSDS/SDS) for chemicals used and use recommended personal protective equipment.
• Dispose of reacted solutions according to local, state, and federal regulations. Refer to Safety Data Sheets for disposal information for unused reagents and consult environmental, health, and safety staff for further disposal information.

Items to Collect

Powder Pillows

• PhosVer® 3 Phosphate Reagent powder pillow, 10-mL (1)
• Sample cells (2)

AccuVac Ampuls

• PhosVer® 3 Phosphate Reagent AccuVac® Ampul (1)
• Beaker, 50-mL (1)
• Sample cells (1)
• Stopper for 18-mm tubes and AccuVac Ampuls (1)

Sample Collection and Storage

• Collect samples in clean glass or plastic bottles cleaned with 6 N (1:1) hydrochloric acid and rinsed with deionized water.
• Do not use a detergent containing phosphate to clean sample bottles, as it can contaminate the sample.
• Analyze samples as soon as possible for best results.
• If immediate analysis is not possible, filter and keep samples at or below 6 °C (43 °F) for a maximum of 48 hours.
• Allow sample temperature to increase to room temperature before analysis.

Powder Pillow Procedure

1. Start program 490 P React. PP. Refer to Instrument-specific information on page 1 for details on sample cells, adapters, or light shields.
2. Prepare the sample: Fill a sample cell with 10 mL of sample.
3. Add the contents of one PhosVer 3 Phosphate Reagent Powder Pillow to the cell. A blue color indicates the presence of phosphorus.
4. Immediately close the sample cell and shake vigorously for 20-30 seconds.
5. Start the instrument timer. A 2-minute reaction time begins. (If the sample was digested using Acid Persulfate digestion, a 10-minute reaction period is necessary.)
6. Prepare the blank: Fill a second sample cell with 10 mL of sample.
7. When the timer expires, clean the blank sample cell.
8. Insert the blank into the cell holder.
9. Push ZERO. The display should show 0.00 mg/L PO₄³⁻.
10. Clean the prepared sample cell.
11. Insert the prepared sample into the cell holder.
12. Push READ. Results show in mg/L PO₄³⁻.

AccuVac Ampul Procedure

1. Start program 492 P React. PV AV. Refer to Instrument-specific information on page 1 for details on sample cells, adapters, or light shields.
2. Prepare the blank: Fill the sample cell with 10 mL of sample.
3. Prepare the sample: Collect at least 40 mL of sample in a 50-mL beaker. Fill the AccuVac Ampul with sample, keeping the tip immersed until the ampul fills completely.
4. Close the AccuVac Ampul and shake for approximately 30 seconds. Accuracy is not affected by undissolved powder.
5. Start the instrument timer. A 2-minute reaction time begins. (If the sample was digested using Acid Persulfate digestion, a 10-minute reaction period is necessary.)
6. When the timer expires, clean the blank sample cell.
7. Insert the blank into the cell holder.
8. Push ZERO. The display shows 0.00 mg/L PO₄³⁻.
9. Clean the AccuVac Ampul.
10. Insert the prepared sample AccuVac Ampul into the cell holder.
11. Push READ. Results show in mg/L PO₄³⁻.

Interferences

Accuracy Check

Standard Additions Method (Sample Spike)

Use the standard additions method to validate the test procedure, reagents, and instrument, and to identify interferences.

Items to Collect:

• Phosphate standard solution, 50 mg/L PO₄³⁻ ampule
• Ampule breaker
• Pipet, TenSette®, 0.1–1.0 mL and tips
• Mixing cylinders, 25-mL (3)

1. Use the test procedure to measure the sample concentration and keep the (unspiked) sample in the instrument.
2. Navigate to the Standard Additions option in the instrument menu.
3. Select values for standard concentration, sample volume, and spike volumes.
4. Open the standard solution.
5. Prepare three spiked samples: add 0.1 mL, 0.2 mL, and 0.3 mL of the standard solution to three 10-mL portions of fresh sample using the TenSette pipet. Mix well. (For AccuVac® Ampuls, add 0.2 mL, 0.4 mL, and 0.6 mL of standard solution to three 50-mL portions of fresh sample.)
6. Use the test procedure to measure the concentration of each spiked sample, starting with the smallest spike.
7. Select Graph to compare expected results with actual results.

Note: If actual results significantly differ from expected results, verify sample volumes and spike measurements. Ensure they match the standard additions menu selections. If results are outside acceptable limits, the sample may contain an interference.

Standard Solution Method

Use the standard solution method to validate the test procedure, reagents, and instrument.

Items to Collect:

• 50 mg/L phosphate standard solution
• 100-mL volumetric flask, Class A
• 4-mL volumetric pipet, Class A and pipet filler safety bulb
• Deionized water

1. Prepare a 2.00-mg/L phosphate standard solution: Add 4.00 mL of a 50-mg/L phosphate standard solution to a volumetric flask (or use available mixed parameter standards containing 2.0 mg/L phosphate). Dilute to the mark with deionized water. Mix well. Prepare this solution daily.
2. Use the test procedure to measure the concentration of the prepared standard solution.
3. Compare the expected result to the actual result.

Note: Factory calibration can be adjusted using the standard adjust option to match the expected standard solution value. This adjustment can improve test accuracy with minor variations in reagents or instruments.

Method Performance

Method performance data is derived from laboratory tests under ideal conditions. Actual results may vary.

Summary of Method

Orthophosphate reacts with molybdate in an acid medium to form a mixed phosphate/molybdate complex. Ascorbic acid reduces this complex, producing an intense molybdenum blue color. Measurement is performed at 880 nm for spectrophotometers (DR 1900: 710 nm) or 610 nm for colorimeters.

Consumables and Replacement Items

Required Reagents

Required Apparatus

Recommended Standards

Optional Reagents and Apparatus