Developing a potassium test, salifert-style (or alternative)

[Dan_P]

Here's an update on the slow part of this.
Rick and I have been sending data back and forth and sometimes the variability is really low:

(units are hanna checker units, ignore.)

And sometimes, the variability is really high.

(ignore hanna checker units.)

So we dug into the Hach method a bit more, to see if they expect the same variability with their turbidity measure or if they expect better. And they expect +-6% which is a good bit better. Hanna does a similar one and expects +-7%.
So I wanted to look closer at the Hach method and see if there are clues we could copy.

One is that they use much much lower concentration (0-7mg/L K) but our method gives zero turbidity at that low level.
another is that they use a packet of EDTA for chelation of Ca, Mg etc as Randy mentioned


sidenote: They also use a reagent that is formaldehyde + methanol, but I think that is just to remove ammonia interference (known to precipitate with tetraphenylborate) and the methanol seems to be to stabilize the formaldehyde. So I think those mechanics are irrelevant for saltwater.

So let's look at stoichiometry, ad Dan and Randy talked about...
I asked Hach how much sodium tetraphenylborate is in their packets, they say 15 grams in 100 packets so 0.150g = 150mg.
150mg of NaTPB for 25mL sample of max 7mg/L K is a huge stoichiometric excess. I get 0.44 mMoles of TPB, and 0.0044 mMoles of K. So this is 100:1 ratio (I think).

Red Sea default back-titration method is a near 1:1 ratio. What we've been doing with Red Sea reagents to adapt for turbidity is like 4:1 for the Recipe that Rick and I have been using.
We can't go 100:1 like Hach because that would make it cost about what the Hach does, but we can go 10:1 or 20:1 without too much problem.

Unsure if either a scoop of EDTA or doubling or tripling the TPB Reagent B will lower the variability, but those seem possibly significant differences that may explain the higher variability that @Rick Mathew and I have seen sometimes.

So, some tinkering ahead I suppose.

Really nice work. Just wondering, a la Rick, what precision do we need for reefing?

 

[Rick Mathew]

Really nice work. Just wondering, a la Rick, what precision do we need for reefing?

The recommended level is 380-420...a range of 40 ppm so if it was important to stay within that range (which I am not sure that it is) a 10% measurement error +- 4 ppm would give superior results whereas +- 12 ppm would be on the edge of acceptability...This is always the question what is the "real" range...That is beyond my pay grade.....Base on what @taricha has done I don't think it is too much of a stretch for his procedure to hit in the 10% range...The RED SEA reported error is between 10-12% and Fauna says theres is between 3-4%...I just finished a RED SEA Precision and accuracy measurement set at 300,400 and 500 ppm ....Hope to get to the Fauna soon...

 

[Dan_P]

The recommended level is 380-420...a range of 40 ppm so if it was important to stay within that range (which I am not sure that it is) a 10% measurement error +- 4 ppm would give superior results whereas +- 12 ppm would be on the edge of acceptability...This is always the question what is the "real" range...That is beyond my pay grade.....Base on what @taricha has done I don't think it is too much of a stretch for his procedure to hit in the 10% range...The RED SEA reported error is between 10-12% and Fauna says theres is between 3-4%...I just finished a RED SEA Precision and accuracy measurement set at 300,400 and 500 ppm ....Hope to get to the Fauna soon...

Thanks, that clarifies things for me.

 

[taricha]

Really nice work. Just wondering, a la Rick, what precision do we need for reefing?

yeah, I feel like if we can squeeze the variability below +-10% then i'll declare victory. If not, then I don't know how useful it would be.
+-10% - That would mean a measurement of 450 ppm K could have a true value of near ~500 that would be too high for my liking, or 410 which is just fine and lower than usual for my salt mix.

 

[JCOLE]

@taricha Thank you for taking this on!!

I am battling low K levels right now and running low on my Salifert kit. Would be great to have an alternative. Following this one closely!

 

[taricha]

Potassium Test Kit Instructions

Ok, thanks to big help from Rick and Dan - here's the procedure for the test method. (Calculation equation will follow soon, but a chart is provided as a peek to how the checker PO4 to potassium ppm mapping will look)

"Modified Red Sea K - Recipe 2.3"
The test is literally only one reagent (Red Sea Reagent B - tetraphenylborate solution), but the procedure sounds almost comically detailed because the amount of scattering that you measure is very dependent on the way in which the mixing is done. Shaking, splashing, vigorous stirring all affect the way the precipitate forms and - because it’s hydrophobic - how the precipitate grabs onto bubbles and water surface. What follows is the procedure that we found gives tight repeatability and avoids frustrating inexplicable outliers.


1. In a Red Sea Vial, add 1.00mL of the sample water and 3.00mL of distilled water to make 4.00mL total. Swirl 15 seconds to mix. This is the Diluted sample.

All liquid additions to the hanna reaction cuvette should be done by turning the cuvette at a 45 degree angle, slowly adding the liquid to the side of the glass and letting it flow smoothly down to the bottom. Add all liquids to the same side of the glass cuvette.


2. To a hanna cuvette, add 0.200mL of Red Sea Potassium Reagent B (Tetraphenylborate solution) using the above procedure.

3. To the hanna cuvette, add 0.500mL of the Diluted sample from step 1.

4. Swirl the cuvette smoothly for 10 seconds at a modest rate - aim for 3 swirls per second.

5. Let it sit for 4 minutes.

6. Add 10.0mL distilled water to the hanna cuvette using the same procedure above.

7. Cap and invert the cuvette slowly and smoothly 10x. One invert and back upright every second.

8. Use a different hanna cuvette with clear water (distilled / tap) as the blank “C1”.

9. After the reacted cuvette has sat for 1-2 minutes, measure in the hanna PO4 checker.

(Optional) 10. The sample turbidity is stable for at least an hour, so it is possible to invert the reacted cuvette a couple more times, let it sit 1-2 minutes and remeasure (can take the average of triplicate measurements). There might be a tiny improvement in lowering variability this way, or can be used as a check against outlier measurement weirdness, but with this procedure it doesn’t seem strictly necessary.

Here’s a preliminary look at how the procedure performs…

My data using the hi774 checker in 35 ppt NaCl-water is in the X’s, and using the more complex Millero recipe (includes all major elements) is the diamonds. Rick’s data using a different PO4 checker hi713 is the stars. (Points plotted are the average of 3 re-measurements.)

It is hoped that the agreement will be better using the same model of hanna checker, but this is still good enough to demonstrate that the recipe is usable.

 

[David S]

Potassium Test Kit Instructions

Ok, thanks to big help from Rick and Dan - here's the procedure for the test method. (Calculation equation will follow soon, but a chart is provided as a peek to how the checker PO4 to potassium ppm mapping will look)

"Modified Red Sea K - Recipe 2.3"
The test is literally only one reagent (Red Sea Reagent B - tetraphenylborate solution), but the procedure sounds almost comically detailed because the amount of scattering that you measure is very dependent on the way in which the mixing is done. Shaking, splashing, vigorous stirring all affect the way the precipitate forms and - because it’s hydrophobic - how the precipitate grabs onto bubbles and water surface. What follows is the procedure that we found gives tight repeatability and avoids frustrating inexplicable outliers.


1. In a Red Sea Vial, add 1.00mL of the sample water and 3.00mL of distilled water to make 4.00mL total. Swirl 15 seconds to mix. This is the Diluted sample.

All liquid additions to the hanna reaction cuvette should be done by turning the cuvette at a 45 degree angle, slowly adding the liquid to the side of the glass and letting it flow smoothly down to the bottom. Add all liquids to the same side of the glass cuvette.


2. To a hanna cuvette, add 0.200mL of Red Sea Potassium Reagent B (Tetraphenylborate solution) using the above procedure.

3. To the hanna cuvette, add 0.500mL of the Diluted sample from step 1.

4. Swirl the cuvette smoothly for 10 seconds at a modest rate - aim for 3 swirls per second.

5. Let it sit for 4 minutes.

6. Add 10.0mL distilled water to the hanna cuvette using the same procedure above.

7. Cap and invert the cuvette slowly and smoothly 10x. One invert and back upright every second.

8. Use a different hanna cuvette with clear water (distilled / tap) as the blank “C1”.

9. After the reacted cuvette has sat for 1-2 minutes, measure in the hanna PO4 checker.

(Optional) 10. The sample turbidity is stable for at least an hour, so it is possible to invert the reacted cuvette a couple more times, let it sit 1-2 minutes and remeasure (can take the average of triplicate measurements). There might be a tiny improvement in lowering variability this way, or can be used as a check against outlier measurement weirdness, but with this procedure it doesn’t seem strictly necessary.

Here’s a preliminary look at how the procedure performs…

My data using the hi774 checker in 35 ppt NaCl-water is in the X’s, and using the more complex Millero recipe (includes all major elements) is the diamonds. Rick’s data using a different PO4 checker hi713 is the stars. (Points plotted are the average of 3 re-measurements.)

It is hoped that the agreement will be better using the same model of hanna checker, but this is still good enough to demonstrate that the recipe is usable.

Great work.
Could I use the Phosphorus ULR checker and then convert to Phosphate to figure my Potassium concentration?

 

[taricha]

Great work.
Could I use the Phosphorus ULR checker and then convert to Phosphate to figure my Potassium concentration?

Yes, almost, with a tweak.
Problem is that the checker you are talking about maxes out at 200 ppb P which is ~0.6 ppm PO4. Based on our chart that'll max out around 400ppm Potassium.
So it'll require either a dilution, or using less sample water. We'll have an answer for you, but it's a loose end at the moment.

 

[David S]

Yes, almost, with a tweak.
Problem is that the checker you are talking about maxes out at 200 ppb P which is ~0.6 ppm PO4. Based on our chart that'll max out around 400ppm Potassium.
So it'll require either a dilution, or using less sample water. We'll have an answer for you, but it's a loose end at the moment.

Great
Thanks so much!

 

[CoralFragZ]

Great Work!.. Since Red Sea Potassium Kit is also retired, Is there a way to make your own tetraphenylborate solution? I would love to try this method but cannot find the reagent. TIA

 

[Rick Mathew]

Great Work!.. Since Red Sea Potassium Kit is also retired, Is there a way to make your own tetraphenylborate solution? I would love to try this method but cannot find the reagent. TIA

Fauna Marin Potassium test kit Reagent A works and appears to be available but it is appears to be a lower concentration TPB and making the procedure different in step 2, 3 and 6 ...

This is the updated version using the Fauna Marin Reagent A.

In step 2 use 1.00mL---- Reagent A (old version was .5)
in step 3 use 1.00 mL---Diluted Tank Water
and in step 6 use 8.0 mL--- Distilled water

The rest of the procedure is the same as @taricha posted above (#26)

This procedure was developed using the HI-713 and not the HI-744 but based on the initial analysis this should work on the HI-744 by the above changes, but it is yet to be validated. Still working on it.

Here is the Regression for the Fauna Reagent A...Please note this has not been completely validated. It was only tested using one sample of the reagent and there could be variability between lots of reagent as we suspect for the Red Sea Reagent. There has also not been any replication of this work....So use with caution...Not sure we are going to proceed any further with this approach...We are looking at a different reagent..(see post here https://www.reef2reef.com/threads/d...le-or-alternative.988758/page-2#post-11906536 ) # 35

Here is a link where I got my Fauna Test Kit Refill

Fauna Marin AquaHomeTest Refill K Potassium Test Kit

Refill Reagents for Fauna Marin AquaHomeTest K Potassium Test Kit.

www.marineandreef.com

Rick

 

[theatrus]

Great Work!.. Since Red Sea Potassium Kit is also retired, Is there a way to make your own tetraphenylborate solution? I would love to try this method but cannot find the reagent. TIA

The reagent is attainable, but what is the solution? Looks like it needs a non polar solvent.

(Also this thread is fascinating and thanks for the read and research. I’m very much an electronics person but love getting exposed to practical chemistry)

 

[taricha]

Great Work!.. Since Red Sea Potassium Kit is also retired, Is there a way to make your own tetraphenylborate solution? I would love to try this method but cannot find the reagent. TIA

What's your source of that info?
I know that BRS lists it as discontinued, but emails from Red Sea dispute that. In either case, I was told...
"It is currently out of stock and it does appear to be a stocking issue, however, we do not have a reason as to why. I was able to find a few online vendors that still have it in stock. "
-that's from Red Sea so it's not a very comforting reply.

Other sources include Fauna Marin as Rick mentioned, and also there's a kit from exaqua that is just the TPB reagent powder (probably) and uses turbidity like us.

Before I move on, I'm going to end up working out the method in terms of a straight tetraphenylborate source (?? mL of ??mM solution) and so whatever the hobby kits do in the future, one need only acquire the compound to do the test.

I do not know if the issues with our test kits availability are from the EU regulations and if so which chemical(s) in the kits are the problem.

SDS of the Red Sea kit is attached.
My guesses for chemicals that could be giving problems in the EU are either the Tetraphenylborate or the Cetrimonium bromide (the titrant used in the last step of these kits for determining excess TPB). I don't actually have any hard info on this question.

The reagent is attainable, but what is the solution? Looks like it needs a non polar solvent.

If I get what you're asking.... the sodium Tetraphenylborate dissolves easily in water, but when it meets K, it forms very water-insoluble potassium tetraphenylborate - which is the basis for the test. The turbidity of the insoluble particles is measured. You don't want to dissolve K-TPB.

 

[darknectar]

Thank you so much! Your work is a real blessing. I was using the salifert potassium test to measure potassium on a vermiponic farm (organic hydroponic). our levels hover around these...TDS: 1000ppm, Ammonium: 1-4ppm, Potassium: 140-200ppm, Calcium: 200ppm, Magnesium: 100ppm, Sodium: 100ppm, Chloride 150ppm

I would like the potassium test to be have less than 10% error (less than 15ppm). I have the Red Sea Potassium test kit and the hanna checker HI717 for high range phosphate.

Will this hanna checker work?
Will the 1-4ppm ammonium levels be a problem? or does that just add error of 1-4ppm potassium?
Any reason why low NaCl levels will affect the test?
Any suggestion on where to buy Sodium tetraphenylborate in the USA?

Thank you again, Benny

 

[taricha]

I would like the potassium test to be have less than 10% error (less than 15ppm). I have the Red Sea Potassium test kit and the hanna checker HI717 for high range phosphate.

the variability day to day will be less than 10%. There seems to be some lot-to lot issues with Red Sea reagent that is making us look into other sources for tetraphenylborate for the test.

Will this hanna checker work?

Not in the same way as the PO4 checkers we've used. It measures PO4 a different way, so you'll need your own calibration curve....

Method

adaptation of the Standard Methods for the Examination of Water and Wastewater, 18th edition, Amino Acid method

It uses the amino acid method instead of the ascorbic acid method that the other PO4 checkers do, so you can't expect the absorbance scale of that high range checker to be similar to the other PO4 checkers.

Will the 1-4ppm ammonium levels be a problem? or does that just add error of 1-4ppm potassium?

(I think the following is correct) ammonium will bind in place of K to the TPB and precipitate, so 1 mole of ammonium will get counted like 1 mole of K, so 1-4ppm of ammonium will get counted as ~ 2.2x that much K in ppm.

Any reason why low NaCl levels will affect the test?

I have no reason to think that lack of NaCl will change the way the precipitation seems to work. I haven't run the test on K in a freshwater sample.

Any suggestion on where to buy Sodium tetraphenylborate in the USA?

Due to the Red Sea issues I mentioned, (weird lot-to lot performance and squirrelly answers about future availability) We're currently looking at the exaqua reagent (5-10% TPB) because it's intended to be used the way we want to use it (turbidity) and it's cheap.

Hach sells pure Na-TPB and it's more than you'll ever need, but expensive.
https://www.hach.com/p-potassium-3-reagent-powder-pillows-pk100/1432399

 

[darknectar]

the variability day to day will be less than 10%. There seems to be some lot-to lot issues with Red Sea reagent that is making us look into other sources for tetraphenylborate for the test.


Not in the same way as the PO4 checkers we've used. It measures PO4 a different way, so you'll need your own calibration curve....

Method

adaptation of the Standard Methods for the Examination of Water and Wastewater, 18th edition, Amino Acid method

It uses the amino acid method instead of the ascorbic acid method that the other PO4 checkers do, so you can't expect the absorbance scale of that high range checker to be similar to the other PO4 checkers.

(I think the following is correct) ammonium will bind in place of K to the TPB and precipitate, so 1 mole of ammonium will get counted like 1 mole of K, so 1-4ppm of ammonium will get counted as ~ 2.2x that much K in ppm.


I have no reason to think that lack of NaCl will change the way the precipitation seems to work. I haven't run the test on K in a freshwater sample.



Due to the Red Sea issues I mentioned, (weird lot-to lot performance and squirrelly answers about future availability) We're currently looking at the exaqua reagent (5-10% TPB) because it's intended to be used the way we want to use it (turbidity) and it's cheap.

Hach sells pure Na-TPB and it's more than you'll ever need, but expensive.
https://www.hach.com/p-potassium-3-reagent-powder-pillows-pk100/1432399

Thank you!
I also have the below checkers, would any of them be better than the High Range Phosphate HI 717?
Hanna Iron Checker HI 721
Hanna Manganese Low range HI 96748
Hanna Copper Checker HI 747

 

[taricha]

@Rick Mathew helped drag this one to the finish line.
We ran into some lot-to-lot variation with Red Sea, which along with not-confidence-inspiring answers about future availability has led us to Exaqua as our preferred source for the tetraphenylborate reagent.
https://exaqua.com/shop/potassium-k-z450/
MSDS sheet attached

The advantages are: reagent intended use is the same as ours (turbidity measurement), no back-titration so no other weird chemicals (quaternary ammonium used in back-titration), cheap ($15-20), likely more convenient for other hobbyists than adapting a packet of a large amount of pure NaTPB from Hach.

The repeatability is good, and replication between myself and Rick has been tight, whether using a plain NaCl-water matrix or a more complete Millero recipe SW mixture, and across different test kits, and whether using lab grade KCl or NIST traceable purchased standard.

Later I'll post the full procedure for this.

 

[taricha]

Here's the full recipe.
First time you open the reagent, check the Exaqua reagent and stir it with the included stick to ensure uniformity. You can take the closed bottle and tap it upside down to check that all powder is liberated from the bottom corners. Then you can shake it around and thoroughly stir it. (In some cases this appeared to matter - in others it didn't.)

Recipe 2.4 E ("E" for exaqua reagent)
Video of the procedure below can be seen here. (I didn't have gloves available, but the powder inevitably gets on you so I'd recommend gloves or at least thorough handwashing right after.)

Diluted sample
1. In a Red Sea Vial, add 1.00mL of the sample water and 9.0mL of distilled water to make 10.0mL total. Swirl 15 seconds to mix. This is the Diluted sample.

Dissolving the TPB reagent
2. To a hanna cuvette, add 1 level exaqua scoop of the TPB reagent powder. Gently press against the inside of the bottle to level and finish leveling with a flat surface - I use the inside of the reagent bottle cap. Then tap the scoop to lose any extra powder not in the spoon portion.
Add the scoop carefully to the hanna cuvette. You'll have to tap more times than you think to get it all out into the cuvette.
(see 1:30 to 2:30 in the video for this tricky step)

3. Add 0.500mL distilled water to the hanna cuvette. Cap and shake vigorously for 15 sec to completely dissolve the powder reagent. Let the closed cuvette sit for ~1min to let all the TPB solution run back down to the bottom.

Reaction
NOTE: Additions to the TPB solution in the hanna reaction cuvette should be done by turning the cuvette at a 45 degree angle, slowly adding the liquid to the side of the glass and letting it flow smoothly down to the bottom. Add all liquids to the same side of the glass cuvette.



4. To the hanna cuvette, gently add 0.500mL of the Diluted sample from step 1 as described above.

5. Swirl the cuvette smoothly for 10 seconds at a modest rate - aim for 3 swirls per second.

6. Let it sit for 4 minutes.

Dilution and measurement
7. Add 10.0mL distilled water to the hanna cuvette using the same procedure above. Cap and invert the cuvette slowly and smoothly 10x. One invert and back upright every second.

8. Use a different hanna cuvette with clear water (distilled / tap) as the blank “C1”.

9. After the reacted cuvette has sat for 1-2 minutes, measure in the hanna checker.

(Optional) The sample turbidity is stable for at least half an hour, so it is possible to invert the reacted cuvette a couple more times, let it sit 1-2 minutes, re-measure and take the average. The variation is small - usually +-1 checker units in line with normal checker noise.

Calculation
for the hi764 checker - ULR nitrite (see graph of performance in previous post)
[ppm K] = ( [hi764]+12) / 0.129
In the video, my tank water at 1.026 s.g. read 45 and 43 on the checker, so 44 avg.
[ppm K] = ( [44]+12) / 0.129 = 434ppm K - this is consistent with other methods that also measured my K to be 420-450 ppm.

for the hi774 checker ULR Phosphate
[ppm K] = ( [hi774]+.103) / 0.00172

Note that this checker has a maximum of 0.90 units so as you go above 550 ppm K the meter will be at or near maxing out.

@Rick Mathew also has calibration data for the hi713 checker.

 

[David S]

Thanks so much, for your diligence.
Can you tell me if I can use either the Hi707 (freshwater nitrite) or the Hi736 (phosphorus ulr) checkers, and if so how much would I have to dilute?
Thanks

 

[taricha]

Thanks so much, for your diligence.
Can you tell me if I can use either the Hi707 (freshwater nitrite) or the Hi736 (phosphorus ulr) checkers, and if so how much would I have to dilute?
Thanks

I have the hi707, which should be fine for this. I should be able to get you enough info to do a calibration curve on that one.