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Old 06-26-2014, 08:04 AM   #11 (permalink)
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Default Re: Test Tube Cleaning

I wait to see if Jeff has a comment on this.
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Old 06-26-2014, 10:23 AM   #12 (permalink)
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Default Re: Test Tube Cleaning

From Standard Methods: Phosphates can exist in several possible forms: orthophosphates, condensed phosphates (pryo-, meta- and other polyphosphates) and organically bound phosphates.

Orthophosphates are simply the unpolymerized or simplest phosphate unit, PO4---. That is the phosphate species most colorimetric tests analysis for, including the Hach Test. BTW, Which Hach test are you using? I'm guessing PhosVer 3 (Ascorbic Acid)?

...

According to my analytical textbook, the second pKa of the H3PO4 is ~ pH 7.144. This means that the salt KH2PO4 is actually slightly basic.

...

Here's my personal take on all of this. First of all, i'm not a fan of Hach in general. Most of their test kits are nothing more than classical water chemistry packaged in such a way that you have to use their 'kits'. Since i actually work in a lab, it makes no sense for me to buy their 'kits'. That said, i also have hundreds or thousands of dollars in analytical equipment to support my analysis procedures and to verify that things are working correctly. Not everyone has that luxury. (Also, several of their tests are certified for use by the EPA. If they weren't accurate, do you really think they would be acceptable for environmental compliance?)

Here's a good example of what i mean by that. Just because you have a sensitive balance doesn't mean jack. You can calibrate it? Great, but it's still meaningless. How do you know that your balance is ACCURATE? Do you have your balance certified every year? Do you have verification weights to prove that you're able to measure things accurately? How do you know that your calibration weight is correct? Do you have an internal calibration weight? That's probably more accurate, but still no guarantee that's it's accurate.

Unless you are using ACS or analytical quality reagents, certified volumetric pipettes and glassware, certified standards and secondary solutions, how can you have any confidence in your analysis results at all? Home test kits are great for getting a good estimate of your water chemistry since they are based off of classical water chemistry, but i think it's very naive to say it's all Hach's fault. I would credit you for finding issues with your procedure and questioning your results, but without the backing up of your results from an accredited laboratory (or at least a bunch of other test kits using certified reagents and standards), i think you're going to have a hard time going to the company and complaining.
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Old 06-26-2014, 11:34 AM   #13 (permalink)
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Default Re: Test Tube Cleaning

I am using the Stannaver Method for PO4. The low range is 0 to 4.4 ppm and the high range is 0 to 44 ppm. The two scales I use are in use for 18 months and have calibration weights. I see what your saying but I am not after pinpoint accuracy. I purchase the Hach PO4 and Nitrogen test kits just to get a bit more accuracy then the hobby kits. I been making my own solutions of my fertilizers and I never seen a problem such as PO4 reading going as high as 40 to 44 ppm until recently. Prior to my post, I was running my tank at 1 to 2 ppm using the same solution containing 43 grams of KH2PO4 to 500 ml and I adjusted my tank concentration to 2 ppm after 7 days without no problem. Then while seeing some recommendations to get rid of greenspot algae by increasing the PO4 concentration; is when I decided to raise it to 6 ppm. I used Wet's fertilizer calculator to determine the amount to use. Then when it was time to test, The first day it was reading 16 ppm, the second 13 ppm and now it is probally around 10 ppm. Then when I tested my KH2PO4 by dry dosing 0.05 grams to 1 gallon of tap water which has a PO4 level tested at 0.8 ppm, my reading is 40 to 44 ppm.
The reading should have been 9.22 ppm.
If I do not solve this, I will rely on dry dosing KH2PO4 but at much less then the recommended dose and then test to see where I am at and go from there. Maybe the scales are at fault.
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Old 06-27-2014, 05:26 AM   #14 (permalink)
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Default Re: Test Tube Cleaning

What is the stannaver method? I can't seem to find that method in the Hach catalog or Water analysis handbook. According to their Water Analysis Handbook (Which is free, btw - and details all of their methods), none of the P analysis methods utilize a Sn reagent or anything of the sorts.

Which of the following are you using? They list 3 methods:

Amino Acid Method: In a highly acid solution, ammonium mobybdate reacts with orthophosphate to form molybdophosphoric acid. This complex is then reduced by the amino acid reagent to yield an intensely colored molybdenum BLUE compound. Test results are measured @ 530 nm.

Molybdovanadate Method: Orthophosphate reacts with molybdate in an acid medium to produce a mixed phosphate/molybdate complex. In the presence of Vanadium, a YELLOW molybdovanadophosphoric acid complex is formed. Test results are measured @ 430 nm.

PhosVer 3 (Ascorbic Acid) Method: Orthophosphate reacts with molybdate (and Sb) in an acid medium to produce a mixed phosphate/molybdate (and Sb) complex. Ascorbic acid then reduces the complex to produce an intense BLUE compound. Test results are measured @ 880 nm.

My experience has been with method 3, the Ascorbic Acid Method, since this is the method listed in Standard Methods.

I agree w/ your overall assessment of the problem that things were fine and analyzing okay until recently, with no change in your dosing regime. I would suspect the analysis results as well. Contamination is always on my mind @ work and i think you're on the correct path.

I would doubt the scales would suddenly go bad, though. Unless it's some sort of electronic failure, though, in which case you can usually tell right away that something is amiss. And i would think that a bad scale would result in a systematic error, where all the results were off, not just one test and not just suddenly.
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Old 06-27-2014, 08:00 AM   #15 (permalink)
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Default Re: Test Tube Cleaning

MY mistake. It is called Stannous method. The Hach catolog number is 147500 for the kit. If you imput that number in their search box, the product comes up and then click on parameter/reagent. You will see that it says Stannous method. Here are the chemicals that came with the kit.
Phosphate 2 reagent, catolog # 1061-99 contains Potassium sulfate & Stannous Tartrate.
Ammonium Molybdate reagent, Catolog 110-32H
Deionized Water, Catolog 272-42

Yesterday, I think I may have found two problems with my scales.
The jewelers scale can read to 3 decimals points (0.000). When I start to add a tiny amount to it, it does not indicate anything on the scale until I add a bit more. This explains why 0.05 grams to 1 gallon of water test out as 40 ppm. The real scale reading was higher then what was indicated. I'll have to check it further to see if there's some residue effecting the reading. The capacity is 20 grams, the resolution is 0.001 gram and the tolerance is + or - 0.003 grams.

The larger scale, I believe may have a simular problem. Its capacity is 300 grams X 0.1 gram.
So one of the things I will do first is to vacumn both scales and then change the batteries even though there is no indication of low battery power.
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Old 06-27-2014, 08:28 AM   #16 (permalink)
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Default Re: Test Tube Cleaning

I don't suppose you have anything that can accurately hold 20Gs? Extra tank perhaps. Then you could fill it and add 1g. The larger the standards, the more accurate it should be. It would be easier than fighting with hundredths of a gram.
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Old 06-27-2014, 08:54 AM   #17 (permalink)
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Default Re: Test Tube Cleaning

Your right Tugg.
I was trying to do too much with my small jewelers scale to determine accuracy. I intend to do something simular to what you suggest. On the next water change, I will test right after. Then determine my parameter adjustments. Then about 30 minutes after dosing, I will test again to see how much my dose has effected the tank. It will take time, but if my scales are off that bad, then I'll go back to using teaspoons and dry dose.
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Old 06-30-2014, 09:13 AM   #18 (permalink)
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Default Re: Test Tube Cleaning

Okay, it took a little bit of looking, but i found it. The Stannous method is not listed in the Hach Water analysis handbook but a similar test is listed in Standard methods, SM 4500-P D (Stannous Chloride Method). Several things that stood out in my mind (1) Chloride interferes @ 75 ppm and (2) Rate of color development and intensity of color depend on the temperature of the final solution where each 1oC increase producing about 1% increase in color. Hence, hold samples, standards, and reagents within 2oC of one another and in the temperature range of 20-25oC. Also, the color fades over time so be sure to not wait too long to measure the concentrations.

Tugg, the idea of using an aquarium to make up a standard solution with the premise that 'larger volumes have less error' is crap. No offense directed at you, but i see this stupid argument made all over the internet (esp by Tom Barr) and i just don't understand it.

Here's how i believe this idea came about. If you are dealing with VOLUMETRIC glassware, then as the volume of the flask increases, the absolute error increases, but the relative % error decreases. Basically, this absolute error can be attributed to the error associated with reading the MENISCUS, which increases with volume as the diameter of the volumetric flask increases. So when you are doing your calculations for concentration, which is a multiplication function, the error analysis calculation is to use relative % error. So what has the lowest relative % error? The largest VOLUMETRIC flask.

Now, what is the error associated with an aquarium? Answer? YOU HAVE NO IDEA! There is no meniscus to fill to, you have no idea what the actual volume is! How do you know you're reached exactly X gallons? And if you're dumb enough to think for a second that the actual volume is what the manufacturer states (i.e. 20 gallons = 20.00000... gallons)... well, i think we get the idea (and if you don't, then you probably wouldn't understand anything i've written in this post anyway...).

Now that's the problem with the volume issue (which is believe is the largest source of error), but let's look at the mass issue. Generally speaking, you are correct in that if X g > Y g, then X should have a lower relative % error. But, you have to take into consideration what the error is of X & Y, which is related to your scale's sensitivity.

rjordan393, i believe you are correct to look at your scale and consider the possibility that it is 'sticky', which would present itself as a problem more at the low end your analytical range. I have to 'tap' my balance several times in order to be sure that it's stabilized on a value and won't jump around anymore. That said, the only way to verify the low end of your analytical range would be to weigh a low level standard and see how accurate your scale is. (In our laboratory, we have to verify our analytical balance (with a sensitivity of 0.0001 g) with a 20 mg weight (0.0020 g) and a 100 g weight (100.0000 g).
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Old 06-30-2014, 12:20 PM   #19 (permalink)
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Default Re: Test Tube Cleaning

Don't worry about me. I'm pretty tuff to offend. I love a good discussion, and people telling me I'm wrong is the best way to make me learn. Also, by no means would I EVER trust the listed volume of an aquarium manufacturer. However, I would trust my ability to measure the internal dimensions of a rectangular box and calculate the volume with a moderate degree of accuracy.

You also have to remember, most home hobbyists' tools are crude to begin with. Most of us don't have graduated cylinders, we're using a milk jug. There is no scale to measure a meniscus against, we're just filling it back to about an inch from the top and calling that a gallon. Not very accurate. For the aquarium, assuming I could only accurately measure to within just half a cm, I'm still not likely to be more than 1/2 to 1 gallon off. That's about 5% on a 20 gallon. If I'm using a 1 gallon milk jug to begin with, I'm probably dealing with the same relative error measuring that single gallon if not more. So I would call the error in the solvent to be a wash. In either case, crude hobbyist tools probably leave us at around 5%. If I'm using the milk jug to fill the tank, then I may also get a small sense increased accuracy as it should take 20 jugs to fill to my 20 gallon mark.

However, with the larger standard, the error of my scale measuring the solute becomes less of an issue. If the scale measures .001g +/- .003g and I need .05g, I'm dealing with a much higher relative error (6%) than if I measured 1.00g on the same scale (0.3%). In this case rjordan393 has a nicer scale. On a common scale that only does 0.1g resolution you can't even make the smaller standard. You would need to make a dosing solution, and then add a sample of that to the gallon.

When I made my 4dKh solution for my drop checker I used the method of dosing high, and then repetitively diluting the solution by dumping half and adding distilled. Crrrrrrrrrude But we work with what we have.
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Old 06-30-2014, 12:45 PM   #20 (permalink)
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Default Re: Test Tube Cleaning

Quote:
Originally Posted by Tugg View Post
However, with the larger standard, the error of my scale measuring the solute becomes less of an issue. If the scale measures .001g +/- .003g and I need .05g, I'm dealing with a much higher relative error (6%) than if I measured 1.00g on the same scale (0.3%).
This i agree with. However, a solution is only as precise as it's least accurate factor - in this case, the volume measurement. My point is that people don't take the volume measurement into consideration and that's equally as important when making a solution. In this case, it's even more important because the volume measurement becomes the limiting factor because it's the least accurate value in the concentration calculation. Comparing volumetric glassware to an aquarium is silly.
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