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Old 06-24-2014, 05:58 PM   #1 (permalink)
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Default Test Tube Cleaning

I have a suspicion that my glass test tube for PO4 testing have been contaminated by the PO4 reagent and may not be salvageable. The test tube was rinse well and scrub with a small bottle brush after every test. But it may not be enough. Many years ago, a chemist recommended using muriactic acid to clean test tubes but I failed to follow up on that advice. Yesterday, I tested and I got a 2 ppm reading. So today, I added enough KH2PO4 that should have brought it up to 6 ppm. When I tested again, the test read 16 ppm. I am using a "Hach" test kit and do not doubt the quality. So I added some acid to the tube and shaked it a bit and let it sit for about 3 minutes. I noticed it took on a slight blue tint. Afterwhich, I dumped it and rinsed it well and used the small bottle brush on it. I tested again and got the same result (16ppm). My conclusion is that the reagent residue does not come off easily if the tube is not acid washed immediantly after testing.
I also checked my dose with the Fertilator and Wet's calculator and they were the same. I am also looking into the possibility that my KH2PO4 is coming out of solution. It contained 36 grams to 500 ml. This would increased the strength as the contents get lower in the bottle.
Tommorow, I will test again using a brand new test tube to see if this confirms my conclusion.
Your thoughts?

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Old 06-25-2014, 05:57 AM   #2 (permalink)
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Default Re: Test Tube Cleaning

Are you mixing with other ferts, or is this just a single dosing bottle with KH2PO4? Make sure you're using distilled water. With a solubility of 22.6g/100mL, you shouldn't have any precipitation.

Our typical macro NPK mixes all have K as the cation (KNO3, KH2PO4, K2SO4). So a Macro mix is fine. If you add Ca, Mg, or other traces, you'll precipitate.

If Phosphate is mixed with pretty much any other cation other than ammonium (NH4) or the group 1A metals (Li, Na, K, Rb, Cs), it will form precipitates. So with tap water, it will react with the calcium and magnesium.
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Old 06-25-2014, 07:59 AM   #3 (permalink)
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Default Re: Test Tube Cleaning

The phosphate analysis procedure is based off of the reaction of phosphates with an antimony and molybdenum complex, which is reduced with ascorbic acid to produce the characteristic blue color. Because the color reagents are metals, it does make sense that the containers should be washed with a weak acid solution (like 1:9 HCl or HNO3) since metals are most soluble in acidic solutions. (Hot dilute HCl is even better for rinsing...)

Looking up the analysis procedure in the Standard Methods book, positive interferences include arsenates (AsO4---) and silica. That said, a 100 ppm Si interference will cause the phosphate analysis to be biased high by only 0.025 ppm PO4-P. (That's in units of P - i don't have the conversion of P to PO4 off the top of my head). Maybe you need to use a plastic test tube for the Phosphate analysis?

You might also want to soak your cleaning brush in an acid cleaning solution as well ... How do you know the brush isn't the source of the contamination? Also, check your cleaning solution (soap) - some soaps use phosphates in them, which could be another source of contamination.
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Old 06-25-2014, 08:36 AM   #4 (permalink)
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Default Re: Test Tube Cleaning

I use distilled water for my individual fertilizers. Another PO4 test using a new test tube indicated I have 13 ppm PO4 in my 75 gallon tank. I had about 2 ounces left in the KH2PO4 container and I dumped it out and rinse the container with tap water. This time I added 43 grams of KH2PO4 to 500 ml of distilled water using labware to measure out 500 ml. I checked the calibration of my scale and it was off by 0.8 grams or 0.0186 per gram error. I do not think that is enough to cause the high error I am getting.
So to check the accuracy of Wet's calculator, I made up a new batch of KH2PO4 in my 500 ml container using 43 grams. According to Wet's calculator, 1.5 ml of it, dosed in 1 gallon of water should give me 23.78 ppm. But my high range test procedure using the HACH orthophosphate kit indicates 40 ppm after multiplying by 10 as indicated in the instructions.
I wonder if its possible that my source water has a high level of PO4, The average on my county water report is 0.15 ppm.
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Old 06-25-2014, 09:09 AM   #5 (permalink)
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Default Re: Test Tube Cleaning

It's certainly possible to have high phosphates in your tap water - i know that i do. And if you really want to have the accuracy of your results checked, you could always send them to me and i can check them in the lab. Phosphate analysis is one of the easier tests to do.
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Old 06-25-2014, 12:22 PM   #6 (permalink)
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Default Re: Test Tube Cleaning

I'll test the source water. My kit was testing PO4 normally for at least 2 tests on the low range scale at 1 to 2 ppm on both tests. Now all of a sudden when still using the remainder from the old KH2PO4, the PO4 went to 16 ppm and today down at 13 ppm in the tank. On my next water change, I think I will have a better idea of whats going on and I will does only 25% of Wet's recommended dose in ml for my tank to see where this leads me.
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Old 06-26-2014, 06:24 AM   #7 (permalink)
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Default Re: Test Tube Cleaning

The source tapwater is 0.8 ppm PO4. So now I am looking at the chemcial itself. So using 1 gallon of tapwater and I dry dosed 0.05 grams (using a jewelers scale and calibrated) to it and according to the fertilator and Wet's calculator the result in my PO4 test should be 9.22 ppm. But the test read 44 ppm. This is simular to a result I got when I tested my 500 ml solution in 1 gallon of water using 1.5 ml. It appears something happened to the KH2PO4 which is stored in a ziplock plastic bag and placed amongst other fertilizers in a heavy duty vinyl bag.
So the question is: How can a fertilizer gain concentration? Or can the fertilator and Wet's calculator be wrong? I am perplexed.
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Old 06-26-2014, 06:50 AM   #8 (permalink)
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Default Re: Test Tube Cleaning

I really don't like the idea of using tap for a baseline. I would get a gallon of distilled. It's only like $1.

If you repeated the results with distilled, then I would look at the test kit being no good. I highly doubt the simple salt can go bad. It's hygroscopic so perhaps it could hydrate some, but that would give a lower reading, not higher.

If it's caked up or moist, you could bake it in the oven on a low setting for a short while. But like I sad, I doubt that it's the salt. I'm guessing the Tap or the Test Kit.
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Old 06-26-2014, 07:07 AM   #9 (permalink)
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Default Re: Test Tube Cleaning

Thanks Tugg.
You know, I should have mentioned that the test kit measures orthophosphate and I know that all aquarium test kits measure phosphate. How much of a differance does this make. I should kick myself if it does.
If the test kit is suspect, then the Hach company has a lot of explaining to do.
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Old 06-26-2014, 07:46 AM   #10 (permalink)
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Default Re: Test Tube Cleaning

This is getting past my chem skills comfort zone, but here is some speculation that hopefully JeffyFunk can comment on.

When the salt dissolves, the two H+ could be dissociating and increasing the acidity. Perhaps the addition of the KH2PO4 is allowing additional orthophosphate to be released from the tap with the increased acidity.

You could test the theory by taking a phosphate test of your tap, then adding some vinegar (acetic acid, CH3COOH has no P in it) to acidify it, and then take another reading of the phosphate.

Again... speculation and shooting in the dark on my part.
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