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I have a Hanna Phosphorus ULR Checker for measuring phosphates. I stopped using it a long time ago because I got frustrated with it always reading zero. I was pretty sure there was some phosphate in my tank, but it never registered anything. I started testing phosphate again recently and had a similar experience. So, I decided to make some samples and see how the Checker read them. The results were way off what I expected. Before getting in touch with Hanna, I want to make sure I'm not overlooking anything that would explain these results. Here's the procedure I followed:
1. Create a phosphate solution with Na3PO4. I added 7.12 grams to one gallon of deionized water. Since Na3PO4 is 58% phosphate by weight, this produces a solution with a concentration of roughly 1,090 ppm phosphate.
2. Fill a 5 gallon bucket with fresh artificial seawater. Add a small pump for circulation
3. Test the seawater in the bucket and record the result as a baseline.
4. Dose 0.87 mL of the PO4 solution to the seawater. This should increase measurable phosphate to 0.05 ppm over baseline. Test and record result.
5. Dose 0.87 mL of the PO4 solution to the seawater. This should increase measurable phosphate to 0.10 ppm over baseline. Test and record result.
6. Dose 2.6 mL of the PO4 solution to the seawater. This should increase measurable phosphate to 0.25 ppm over baseline. Test and record result
7. Dose 4.34 mL of the PO4 solution to the seawater. This should increase measurable phosphate to 0.50 ppm over baseline. Test and record result.
Notes:
- I would have liked to scale the test up more to avoid measurement error, but I didn't have enough equipment to scale the test any further. This scale should have been adequate to answer the question at hand.
- The Checker's battery was replaced before the tests
- The Checker reagents are new and were ordered 2 weeks ago (lot H100)
- Procedure from the Checker instructions were followed exactly with all tests.
- The same cuvette was used for all tests.
- The same cuvette was used for both the C1 and the C2 test.
- The cuvette was inspected for any scratches or blemishes before the tests began. There were none.
- Before the tests, the cuvette was cleaned with vinegar then deionized water.
- Between each test, the cuvette was rinsed with tap water, then deionized water.
- The cuvette was wiped with a clean, dry cloth before it was placed in the Checker.
- During step C2, the vial was mixed for exactly two minutes as measured by a timer.
- The reagent packets were opened and prepared before the test began to ensure that there was ample time to allow two full minutes of mixing
- The Na3PO4 used was a 99.9% pure food-grade supplement
- The scale used to measure the Na3PO4 has a maximum capacity of 500g and reads in increments of 0.01g
- The container used for the Na3PO4 solution in step 1 was a clean 1 gallon plastic jug that previously contained distilled water. It was rinsed with tap water, then deionized water
- The Na3PO4 solution in step 1 was mixed for several minutes and allowed to settle overnight. It was mixed again before the tests.
- The seawater used in this test was Instant Ocean mixed to 52.5 mS (about 35 ppt). It had been mixing for just over 2 days prior to the tests.
- The 5 gallon bucket from step 2 was rinsed thoroughly with deionized water
- The circulation pump from step 2 was soaked in deionized water overnight
- The doses from steps 4 through 7 were dosed with a clean, new 1 mL syringe
- The samples for the each test were drawn about 6 - 7 minutes after the last dose of PO4 was added.
- To determine the dose of Na3PO4 solution to add, I used the following equation. I refactored a common dosing calculation to allow me to solve for the amount of solution to add:
Where:
Tv = tank volume
Tc = tank concentration
Sv = volume of solution to dose
Sc = solution concentration
Dc = desired concentration
Dc - Sc ≠ 0
Results
Is there something wrong with my testing procedure? Is there an error in one of my calculations, either for the strength of the Na3PO4 solution or the amount I should have dosed? Any input would be appreciated.
1. Create a phosphate solution with Na3PO4. I added 7.12 grams to one gallon of deionized water. Since Na3PO4 is 58% phosphate by weight, this produces a solution with a concentration of roughly 1,090 ppm phosphate.
2. Fill a 5 gallon bucket with fresh artificial seawater. Add a small pump for circulation
3. Test the seawater in the bucket and record the result as a baseline.
4. Dose 0.87 mL of the PO4 solution to the seawater. This should increase measurable phosphate to 0.05 ppm over baseline. Test and record result.
5. Dose 0.87 mL of the PO4 solution to the seawater. This should increase measurable phosphate to 0.10 ppm over baseline. Test and record result.
6. Dose 2.6 mL of the PO4 solution to the seawater. This should increase measurable phosphate to 0.25 ppm over baseline. Test and record result
7. Dose 4.34 mL of the PO4 solution to the seawater. This should increase measurable phosphate to 0.50 ppm over baseline. Test and record result.
Notes:
- I would have liked to scale the test up more to avoid measurement error, but I didn't have enough equipment to scale the test any further. This scale should have been adequate to answer the question at hand.
- The Checker's battery was replaced before the tests
- The Checker reagents are new and were ordered 2 weeks ago (lot H100)
- Procedure from the Checker instructions were followed exactly with all tests.
- The same cuvette was used for all tests.
- The same cuvette was used for both the C1 and the C2 test.
- The cuvette was inspected for any scratches or blemishes before the tests began. There were none.
- Before the tests, the cuvette was cleaned with vinegar then deionized water.
- Between each test, the cuvette was rinsed with tap water, then deionized water.
- The cuvette was wiped with a clean, dry cloth before it was placed in the Checker.
- During step C2, the vial was mixed for exactly two minutes as measured by a timer.
- The reagent packets were opened and prepared before the test began to ensure that there was ample time to allow two full minutes of mixing
- The Na3PO4 used was a 99.9% pure food-grade supplement
- The scale used to measure the Na3PO4 has a maximum capacity of 500g and reads in increments of 0.01g
- The container used for the Na3PO4 solution in step 1 was a clean 1 gallon plastic jug that previously contained distilled water. It was rinsed with tap water, then deionized water
- The Na3PO4 solution in step 1 was mixed for several minutes and allowed to settle overnight. It was mixed again before the tests.
- The seawater used in this test was Instant Ocean mixed to 52.5 mS (about 35 ppt). It had been mixing for just over 2 days prior to the tests.
- The 5 gallon bucket from step 2 was rinsed thoroughly with deionized water
- The circulation pump from step 2 was soaked in deionized water overnight
- The doses from steps 4 through 7 were dosed with a clean, new 1 mL syringe
- The samples for the each test were drawn about 6 - 7 minutes after the last dose of PO4 was added.
- To determine the dose of Na3PO4 solution to add, I used the following equation. I refactored a common dosing calculation to allow me to solve for the amount of solution to add:
Where:
Tv = tank volume
Tc = tank concentration
Sv = volume of solution to dose
Sc = solution concentration
Dc = desired concentration
Dc - Sc ≠ 0
Results
Test | Expected Reading (PPM) | Actual Reading (PPM) |
1 | 0 | 0 |
2 | 0.05 | 0.006 |
3 | 0.1 | 0 |
4 | 0.25 | 0.077 |
5 | 0.5 | 0.15 |
Is there something wrong with my testing procedure? Is there an error in one of my calculations, either for the strength of the Na3PO4 solution or the amount I should have dosed? Any input would be appreciated.