Measuring PO4: Hanna ULR versus Triton ICP

[Dr. Jim]

First two were much higher, others lower though. Thanks for sharing

Second one had only a 0.01 difference....so just the first one was "significant".

 

[ScottB]

I tried out the 10ml syringe. The cuvette seems just a couple drops short of the full 10ml. That could contribute some error, but not what we are commonly seeing, right?

 

[Dr. Jim]

I tried out the 10ml syringe. The cuvette seems just a couple drops short of the full 10ml. That could contribute some error, but not what we are commonly seeing, right?

I just checked my vial with a pipette. The line on the vial is off by 0.15ml or 3 drops (too little). So actually this is only 1.5% which is probably negligible. Ex. A reading of 0.08 would actually be about 0.0788. Probably not enough to worry about....(but why not just measure properly and reduce whatever error we can.....its so easy with a pipette).

 

[Dan_P]

How are you filling your Hannah cuvettes? Are you filling too the line or filling with a 10ml syringe? I use a graduated pipette too fill my cuvettes and 10ml in my pipette fill’s each cuvette above the 10ml line. I’ve double checked the calibration on my pipette as well as cross checked by filling the cuvettes with other 10ml syringes. This could explain your findings being that if the 10ml line on your cuvette is not actually 10ml, it would give you a more concentrated solution and a higher test result.

Would you please share your investigation on how much the PO4 reading changes in the Hanna Checker for every, say, 0.1 mL under and over charge of exactly 10 mL? Is it outside Hanna’s 95% confidence interval?

@Rick Mathew has definitively demonstrated that the PO4 level changes over time in the plastic tubes used to deliver the sample to the vendor. Moreover, this change can be inhibited by sterilizing the sample. The change in the sample, however, is not unusual nor unexpected. You simply cannot ship a water sample in a plastic tube at room temperature and expect it to be the same as the freshly drawn sample.

it is best to ignore the P measurement from ICP vendors.

 

[Dan_P]

Have you noticed how big the difference is between ICP and your checker? OP noted .13 and .03, change of .1, big! Would you say the difference has been close to .1 or less or more? @Dan_P also

I am going to ask @Rick Mathews to share this information for the reason that it is his.

 

[ScottB]

Would you please share your investigation on how much the PO4 reading changes in the Hanna Checker for every, say, 0.1 mL under and over charge of exactly 10 mL? Is it outside Hanna’s 95% confidence interval?

@Rick Mathew has definitively demonstrated that the PO4 level changes over time in the plastic tubes used to deliver the sample to the vendor. Moreover, this change can be inhibited by sterilizing the sample. The change in the sample, however, is not unusual nor unexpected. You simply cannot ship a water sample in a plastic tube at room temperature and expect it to be the same as the freshly drawn sample.

it is best to ignore the P measurement from ICP vendors.

Well that clears that up then.

Something new every day.

 

[Rick Mathew]

Wasn't sure where this post should go, but I will start here.

Just curious if anyone else is seeing a consistent discrepancy between their Hanna values and those from ICP for PO4. I use the Hanna for phosphorus and convert that value. According to Hanna, my PO4 stays around .13 +-. Each time I send out an ICP, I get .03 to .04.

I've now run 4 ICP on two systems and the same discrepancy appears each time with the Hanna showing an extra .1 above ICP.

I wear nitrile gloves and store cuvettes in RODI. I have replaced the cuvettes. My process is fairly meticulous and consistent and the values I get are fairly smooth. My reagents are always current. Should I just buy a new Hanna?

Scott:

Here is a link to the experiment I conducted this summer with regard to this question that @Dan_P mentioned ...I agree with Dan...the straight up ICP measurements on PO4 questionable at best..

SAMPLE STORAGE AND ITS IMPACT ON PO4 MEASUREMENT AND ICP MEASUREMENT RESULTS (PART 2)

My First Trip To Wonderland!...Down the Rabbit Hole! My first trip to Wonderland started in late 2019 when I started experimenting to find the answer to a question that had been troubling me for over 2 years. Since the very beginning of this project it has been my goal to understand why my...

www.reef2reef.com

 

[Rick Mathew]

How are you filling your Hannah cuvettes? Are you filling too the line or filling with a 10ml syringe? I use a graduated pipette too fill my cuvettes and 10ml in my pipette fill’s each cuvette above the 10ml line. I’ve double checked the calibration on my pipette as well as cross checked by filling the cuvettes with other 10ml syringes. This could explain your findings being that if the 10ml line on your cuvette is not actually 10ml, it would give you a more concentrated solution and a higher test result.

I have also noted variability in the 10 mL marker line on the Hanna cuvettes...I did the same thing you did...fill with a calibrated pipette...

rick

 

[ScottB]

Scott:

Here is a link to the experiment I conducted this summer with regard to this question that @Dan_P mentioned ...I agree with Dan...the straight up ICP measurements on PO4 questionable at best..

SAMPLE STORAGE AND ITS IMPACT ON PO4 MEASUREMENT AND ICP MEASUREMENT RESULTS (PART 2)

My First Trip To Wonderland!...Down the Rabbit Hole! My first trip to Wonderland started in late 2019 when I started experimenting to find the answer to a question that had been troubling me for over 2 years. Since the very beginning of this project it has been my goal to understand why my...

www.reef2reef.com

I am very impressed with this article; it is very well presented. Thank you for sharing.

 

[Drew1900]

Would you please share your investigation on how much the PO4 reading changes in the Hanna Checker for every, say, 0.1 mL under and over charge of exactly 10 mL? Is it outside Hanna’s 95% confidence interval?

@Rick Mathew has definitively demonstrated that the PO4 level changes over time in the plastic tubes used to deliver the sample to the vendor. Moreover, this change can be inhibited by sterilizing the sample. The change in the sample, however, is not unusual nor unexpected. You simply cannot ship a water sample in a plastic tube at room temperature and expect it to be the same as the freshly drawn sample.

it is best to ignore the P measurement from ICP vendors.

I haven’t gathered any data on the amount that this effects the test results. I’m just going off my observations and creating a theory. I’ll try to find some time this week and test that theory using different sample amounts.

 

[Rick Mathew]

I am very impressed with this article; it is very well presented. Thank you for sharing.

Thank you...very kind of you

Rick

 

[Dan_P]

I haven’t gathered any data on the amount that this effects the test results. I’m just going off my observations and creating a theory. I’ll try to find some time this week and test that theory using different sample amounts.

That would be fantastic, thanks!

 

[Randy Holmes-Farley]

Your sample would be more concentrated. If you’re sample size is say 9ml, and you add the reagent too that, it will create a more concentrated sample than a sample size of 10ml.
Hannah checkers work off of light readings. If you put the reagent into 5ml of water, it’s going to make a darker sample than if you put the reagent in 10ml of water. Kinda like when you mix Kool-Aid. The more water you add, the more the color of the Kool-aid is diluted.

That is not true.

The level of the liquid may impact the light passing through it and the subsequent reading, but adding more or less reagent (by adding less tank water with the same amount of reagent) does not concentrate the phosphate and will not necessarily lead to a higher value. The uncolored reagents are already present in excess. It is only (or at least mostly only) the concentration of phosphate that leads to color.

 

[Randy Holmes-Farley]

So the reagent is more concentrated, makes sense.

Yes, but that does not impact the values all that much.

 

[Randy Holmes-Farley]

Correct

Incorrect.

 

[Drew1900]

That is not true.

The level of the liquid may impact the light passing through it and the subsequent reading, but adding more or less reagent (by adding less tank water with the same amount of reagent) does not concentrate the phosphate and will not necessarily lead to a higher value. The uncolored reagents are already present in excess. It is only (or at least mostly only) the concentration of phosphate that leads to color.

Oh, very interesting. So the phosphate will only react too the amount of reagent that it needs to change color. So weather you use say, 8ml or 10ml, that shouldn’t affect the reading?

How does this transfer over to the colored reagents like the ones in the calcium tester?

Thank you for the correction!

 

[Randy Holmes-Farley]

Oh, very interesting. So the phosphate will only react too the amount of reagent that it needs to change color. So weather you use say, 8ml or 10ml, that shouldn’t affect the reading?

How does this transfer over to the colored reagents like the ones in the calcium tester?

Thank you for the correction!

The amount of reagent in a phosphate test is a minor consideration. That is why the reagents for a phosphate test (and some others) are often measured with spoons that lack high precision and reproducibility. There needs to be enough present to react with even the highest amount of phosphate, but unless you are at the top end of the phosphate concentration, most of it goes unused.

Down at the very low end of phosphate measurement, stray absorption from the reagents may be a limiting factor (or not) depending on how pure it is. In that case, if you do not measure a blank and subtract it, you might get a little boost in absorption (and hence a higher than real value) from excessive reagent.

The amount of liquid in relation to the light path may not be a minor consideration. The top of the liquid must be above the top of the light beam.

In a calcium test, the colored reagent is very noncritical. The critical measure is the amount of acid titrant added. As long as you can see the color, you could add much less or much more of the color. Same for an alk test.

 

[Drew1900]

The amount of reagent in a phosphate test is a minor consideration. That is why the reagents for a phosphate test (and some others) are often measured with spoons that lack high precision and reproducibility. There needs to be enough present to react with even the highest amount of phosphate, but unless you are at the top end of the phosphate concentration, most of it goes unused.

Down at the very low end of phosphate measurement, stray absorption from the reagents may be a limiting factor (or not) depending on how pure it is. In that case, if you do not measure a blank and subtract it, you might get a little boost in absorption (and hence a higher than real value) from excessive reagent.

The amount of liquid in relation to the light path may not be a minor consideration. The top of the liquid must be above the top of the light beam.

In a calcium test, the colored reagent is very noncritical. The critical measure is the amount of acid titrant added. As long as you can see the color, you could add much less or much more of the color. Same for an alk test.

I always wondered how the accuracy of those spoons affected the test results. Thanks for the quick chemistry lesson!

 

[Randy Holmes-Farley]

I always wondered how the accuracy of those spoons affected the test results. Thanks for the quick chemistry lesson!

I'm sure it is beyond most folks interest level, but here's a similar (not identical) assay for phosphate, where the author tested the sensitivity of the assay for various aspects of the test. note that in Figure 2 on page 8, for example, if you have enough molybdate to react with the phosphate, more does not lead to more absorbance:

https://inis.iaea.org/collection/NCLCollectionStore/_Public/04/042/4042323.pdf

 

[Dan_P]

That is not true.

The level of the liquid may impact the light passing through it and the subsequent reading, but adding more or less reagent (by adding less tank water with the same amount of reagent) does not concentrate the phosphate and will not necessarily lead to a higher value. The uncolored reagents are already present in excess. It is only (or at least mostly only) the concentration of phosphate that leads to color.

There is also some leeway in the amount of reagent you can spill and get the same results. @Rick Mathew talked with Hanna technical support about the variable amount of reagent in each packet and found out that there is excess reagent.

I have run the PO4 test with 5 mL of sample, split the powder and obtained the same results without being very exact about splitting the powder (5-10 % high or low).

When the amount of reagent deviates substantially, say on the high side, the zero point might be effected. Zero PO4 gives a reading more than zero. Going the other, shorting the reagent substantially might start to effect the color intensity because of the incomplete conversion of PO4 to the colored adduct. But, we are talking about major goofs, the kind of goof that you can’t miss.