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

[taricha]

The Salifert potassium test is a pretty good test. Here's some regression data I did using known KCl additions.

This kit that I had was giving measured values responding to K additions about 30% lower than the addition amount. This is not too bad, can be corrected by controlling the drop size or just having a regression equation.

In any case, Salifert K kits seem possibly unavailable for the foreseeable near future ("backordered for a year" is the quote attributed to somebody at BRS). And I'm unaware of any other well-functioning test kits.
https://www.reef2reef.com/threads/salifert-potassium-test-availability.988432/

So this thread is here to collect info on how to generate a Potassium test kit that might perform similarly to the salifert. (Or determine that it's really impractical to do so.)

Some basics:
1.00 mL of sample water
0.50 mL of a reagent that makes everything go very cloudy (swirl 20 seconds)
5 drops of a clear reagent that ????
Then the titration reagent:
The titrant may be using some polyelectrolyte chemistry. It's a blue liquid that is immediately neutralized to colorless early in the titration.
At the end of the titration, the last drop causes the sample to go from cloudy white /pale yellow to pale blue. That is, the blue drop is no longer neutralized and the test solution instead of simply being cloudy, aggregates big clumps.


I haven't seen any good references that sound very close to the test method. If you've run across any papers that might help, please share.

 

[taricha]

Thanks to @Rick Mathew and @Dan_P here's a few papers that outline the situation.

this abstract gets in the general direction:
http://pedologica.issas.ac.cn/trxben/article/abstract/19630310

But this one seems to lay out exactly the situation:
Modified titration of potassium

abstract...
A simplified titrimetric method for potassium is proposed. In the usual method, potassium is precipitated by sodium tetraphenylborate and the excess of the reagent is back-titrated with a quaternary ammonium salt, but the precipitate must be removed to prevent the reaction between the precipitate and the titrant. Polyvinylpyrrolidone can be used to deactivate the precipitate so that it need not be removed.

I think this tells what reagents 1, 2, and (part of) 3 are doing.
This covers everything but the blue color indicator in reagent 3.

There are a number of color indicators that would change color in response to whether the tetraphenylborate or quaternary ammonium compound was in excess. Here's one example:
Titration of long-chain quaternary ammonium compounds using tetraphenylboron
...Dichlorofluorescein is added as an indicator. The sample is then titrated with 0.06 N aqueous sodium tetraphenylboron. As long as free QAC is present the mixture will be pink. When the tetraphenylboron has reacted with all of the QAC, the indicator becomes yellow...

Here's another that uses the same principle but may be a better match for the color indicator behavior.
Determination of tetraphenylborate by two-phase titration with tetraphenylphosphonium salt
...A visual indicator method has been developed for the titration of tetraphenylborate (TPB) with a 5 × 10−5, M tetraphenylphosphonium (TPP) solution. A hydrophobic dye, tetrabromophenolphthalein ethyl ester (TBPE), is used as an indicator in the presence of 1,2-dichloroethane. The organic phase changes from yellow to blue at the endpoint...

The above papers probably flesh out enough info to get a test like the salifert K test working.

But Dan pointed me to another method that is far more elegant and may only need one reagent.... more on that later.

 

[taricha]

follow-up on earlier...

There are a number of color indicators that would change color in response to whether the tetraphenylborate or quaternary ammonium compound was in excess. Here's one example:
Titration of long-chain quaternary ammonium compounds using tetraphenylboron
...Dichlorofluorescein is added as an indicator. The sample is then titrated with 0.06 N aqueous sodium tetraphenylboron. As long as free QAC is present the mixture will be pink. When the tetraphenylboron has reacted with all of the QAC, the indicator becomes yellow...

Here's another that uses the same principle but may be a better match for the color indicator behavior.
Determination of tetraphenylborate by two-phase titration with tetraphenylphosphonium salt
...A visual indicator method has been developed for the titration of tetraphenylborate (TPB) with a 5 × 10−5, M tetraphenylphosphonium (TPP) solution. A hydrophobic dye, tetrabromophenolphthalein ethyl ester (TBPE), is used as an indicator in the presence of 1,2-dichloroethane. The organic phase changes from yellow to blue at the endpoint...

here's a quick check confirming that you can indeed move the color indicator back and forth blue-yellow-blue depending on whether you have an excess of reagent 1 or reagent 3.

 

[taricha]

But I think for method development, I'm more interested in this... that @Dan_P pointed me to earlier.
Here's Hach potassium.
Documentation [pdf]
It simply uses tetraphenylborate to precipitate the K, then measure the turbidity. There are two other reagents to help deal with the fact that tetraphenylborate also can precipitate Ca and Mg.
But I think the K compound is more insoluble that the one formed with other ions. So I'm wondering if maybe by dilution that we can make the interference either small enough or predictable enough to not care about. Then just measure the cloudiness with a hanna checker.

Here's what happens when you add tetraphenylborate to a sample of tank water.

left, blank
center: 0.200mL tank water, diluted to 10mL with distilled - then tetraphenylborate added
right: same 0.200mL of tank water - tetraphenylborate added, then diluted to 10mL with distilled (5x the turbidity.)

Looks like it could work, but the method will need to be pretty specific to give any sort of repeatable results.

 

[Rick Mathew]

But I think for method development, I'm more interested in this... that @Dan_P pointed me to earlier.
Here's Hach potassium.
Documentation [pdf]
It simply uses tetraphenylborate to precipitate the K, then measure the turbidity. There are two other reagents to help deal with the fact that tetraphenylborate also can precipitate Ca and Mg.
But I think the K compound is more insoluble that the one formed with other ions. So I'm wondering if maybe by dilution that we can make the interference either small enough or predictable enough to not care about. Then just measure the cloudiness with a hanna checker.

Here's what happens when you add tetraphenylborate to a sample of tank water.

left, blank
center: 0.200mL tank water, diluted to 10mL with distilled - then tetraphenylborate added
right: same 0.200mL of tank water - tetraphenylborate added, then diluted to 10mL with distilled (5x the turbidity.)

Looks like it could work, but the method will need to be pretty specific to give any sort of repeatable results.

Just an idea...Would adding color to the sample then measuring with an appropriate checker that might pick up the difference in the absorbance due to the scattering of the precipitate?

 

[taricha]

Just an idea...Would adding color to the sample then measuring with an appropriate checker that might pick up the difference in the absorbance due to the scattering of the precipitate?

While the color might help human eyeballs, the scattering alone is easily quantifiable in any hanna checker.
Just gotta figure out how to make the scattering a good measure of K only, or at least have the Ca and Mg contributions be small.

This paper (attached) suggests that raising pH with NaOH precipitates most other interfereng ions, then the clear part that doesn't settle could be treated with tetraphenylborate and the precipitation would be just the K. So that's an option if simply dilution and mixing ratios doesn't tame it.

 

[darknectar]

Hello, I'm very interested in this method. I was using salifert for a couple years and it was the only economical and quick method to test potassium.

Has anyone been able to successfully and conveniently setup their own test kit?

 

[Rick Mathew]

Hello, I'm very interested in this method. I was using salifert for a couple years and it was the only economical and quick method to test potassium.

Has anyone been able to successfully and conveniently setup their own test kit?

Not as of this moment...I am currently evaluating the Fauna Marin Potassium Test...Hope to be done in the next week or so.

 

[Randy Holmes-Farley]

Interesting that Hach says Cl- interferes at levels present in seawater. Might be a very small correction, however, for K+ near the high end of their ranges.

In addition to OH-, chelators such as EDTA will take out Ca++ and Mg++ and not K+

 

[taricha]

Starting in earnest on this method.
First off, the Hach test has 3 reagents - two of which are cheap, and one of which is well over a hundred dollars. My first look, I misread which was the tetraphenylborate and of course it's actually the super expensive reagent, so I won't use the Hach kit as my source for tetraphenylborate.
So instead I'll be using the Red Sea K test as the source for my tetraphenylborate. That is Reagent B in the Red Sea test, and Reagent A is a NaOH solution. (from the Red Sea MSDS)

My plan is to work out how to measure the turbidity with the hanna checker and thus need only the tetraphenylborate (and maybe the NaOH), and thus not need some of the weirder chemicals involved in the back-titration that might be responsible for the loss of availability of the salifert test. Red Sea said by email there are currently no plans to discontinue their kit. Not very comforting, but at least if they did, we'd still only need those two chemicals to DIY it from scratch.

Nothing interesting in this post, just asking dumb questions so I can figure out how to ask the interesting questions in a less dumb way. The central interesting question to me is how much is the Ca and Mg interference? If it's small, I may not need to care too much about it and can just test through it and correct it away mathematically (we know what Ca and Mg are in our water). If the interference from Ca or Mg is big, then we might need to work harder chemically to remove the interference (pre-precipitation, or chelation as Randy mentioned).

Q1: Is the turbidity (scattering) linear with the amount of tank water added? Yes.

0.150 mL of reagent B into hanna cuvette, raw tank water added, swirled 10sec, wait 5 min, then dilute to 10mL with distilled. Invert 10x to mix. Invert 3x at measurement. Turbidity stable from measurement at 10min to over 1hr later.

Very little tank water is needed to produce strong measureable turbidity, and the turbidity is proportional to the amount of tank water added (when Reag B is held constant)
The error bars represent +-3 units on checker I'm using. And all points fall within that range of the fit line. So linear enough. The blue is the raw data and the Red is that data mapped (by comparison measurements I've done) to what the PO4 value would be on hanna P / PO4 checkers. From here on, I'll work in those units so any hanna P/PO4 checker could be used for this.

Q2: Does the amount of hold time for tank water and tetraphenylborate mixing matter? No.

0.120 mL of reagent B into hanna cuvette, raw tank water added (0.15 or 0.30mL), swirled 10sec, wait variable min, then dilute to 10mL with distilled. Invert 10x to mix. Invert 3x at measurement.

Whatever the tetraphenylborate and the tank water are doing, they are done early and no changes to precipitation happen if you give them a few more minutes. A wait time of 3 or 4 minutes is fine and as good as any other.

Q3: How much Reag B (tetraphenylborate) is needed to precipitate the stuff that ought to be precipitated from tank water? 1:1 or 1.2:1 ratio of Reag B to tank water ought to be fine for now.

Var. vol of Reag B added, 0.20mL TW, swirl 10 sec, hold 4 min, dilute to 10mL. Invert 10x to mix, invert 3x at measurement.

When you get above 1:1 ratio of Reag B to tank water, the final turbidity is less sharply dependent on how much you added. Some amount of this turbidity is unwanted Ca and Mg. Pre-precipitation ought to reduce the amount of Reagent B needed. The Red Sea directions use a smaller 1:4 ratio and have NaOH added first to pre-precipitate. For now, I'll work at the higher ratio and later I'll see how much of what I'm precipitating is K vs Ca or Mg. Additional observation, at lower amounts of Reag B - there are more prominent larger particles and relatively less fine cloudiness - we want uniform cloudiness for measurement, not discrete chunks.


Q4: Does the dilution of the Reag B and Tank Water matter during the mixing?
You can dilute the mix a good bit before it starts to matter.

0.20mL of Reag B added, Varied vol of distilled added, 0.20mL TW, swirl 10 sec, hold 4 min, dilute to 10mL. Invert 10x to mix, Invert 3x at measurement.

This may come in handy, because needing to accurately add 0.200mL of tank water might be a difficult ask, but you can get away with diluting the tankwater to 3 or 4x volume and then add 0.60 or 0.80mL.

Like I said, dumb questions first and no interesting chemistry yet, but now that I know how to measure the turbidity, next I can work out how much of this turbidity is from the K vs the Ca or Mg in the tank water.

 

[Rick Mathew]

Starting in earnest on this method.
First off, the Hach test has 3 reagents - two of which are cheap, and one of which is well over a hundred dollars. My first look, I misread which was the tetraphenylborate and of course it's actually the super expensive reagent, so I won't use the Hach kit as my source for tetraphenylborate.
So instead I'll be using the Red Sea K test as the source for my tetraphenylborate. That is Reagent B in the Red Sea test, and Reagent A is a NaOH solution. (from the Red Sea MSDS)

My plan is to work out how to measure the turbidity with the hanna checker and thus need only the tetraphenylborate (and maybe the NaOH), and thus not need some of the weirder chemicals involved in the back-titration that might be responsible for the loss of availability of the salifert test. Red Sea said by email there are currently no plans to discontinue their kit. Not very comforting, but at least if they did, we'd still only need those two chemicals to DIY it from scratch.

Nothing interesting in this post, just asking dumb questions so I can figure out how to ask the interesting questions in a less dumb way. The central interesting question to me is how much is the Ca and Mg interference? If it's small, I may not need to care too much about it and can just test through it and correct it away mathematically (we know what Ca and Mg are in our water). If the interference from Ca or Mg is big, then we might need to work harder chemically to remove the interference (pre-precipitation, or chelation as Randy mentioned).

Q1: Is the turbidity (scattering) linear with the amount of tank water added? Yes.

0.150 mL of reagent B into hanna cuvette, raw tank water added, swirled 10sec, wait 5 min, then dilute to 10mL with distilled. Invert 10x to mix. Invert 3x at measurement. Turbidity stable from measurement at 10min to over 1hr later.

Very little tank water is needed to produce strong measureable turbidity, and the turbidity is proportional to the amount of tank water added (when Reag B is held constant)
The error bars represent +-3 units on checker I'm using. And all points fall within that range of the fit line. So linear enough. The blue is the raw data and the Red is that data mapped (by comparison measurements I've done) to what the PO4 value would be on hanna P / PO4 checkers. From here on, I'll work in those units so any hanna P/PO4 checker could be used for this.

Q2: Does the amount of hold time for tank water and tetraphenylborate mixing matter? No.

0.120 mL of reagent B into hanna cuvette, raw tank water added (0.15 or 0.30mL), swirled 10sec, wait variable min, then dilute to 10mL with distilled. Invert 10x to mix. Invert 3x at measurement.

Whatever the tetraphenylborate and the tank water are doing, they are done early and no changes to precipitation happen if you give them a few more minutes. A wait time of 3 or 4 minutes is fine and as good as any other.

Q3: How much Reag B (tetraphenylborate) is needed to precipitate the stuff that ought to be precipitated from tank water? 1:1 or 1.2:1 ratio of Reag B to tank water ought to be fine for now.

Var. vol of Reag B added, 0.20mL TW, swirl 10 sec, hold 4 min, dilute to 10mL. Invert 10x to mix, invert 3x at measurement.

When you get above 1:1 ratio of Reag B to tank water, the final turbidity is less sharply dependent on how much you added. Some amount of this turbidity is unwanted Ca and Mg. Pre-precipitation ought to reduce the amount of Reagent B needed. The Red Sea directions use a smaller 1:4 ratio and have NaOH added first to pre-precipitate. For now, I'll work at the higher ratio and later I'll see how much of what I'm precipitating is K vs Ca or Mg. Additional observation, at lower amounts of Reag B - there are more prominent larger particles and relatively less fine cloudiness - we want uniform cloudiness for measurement, not discrete chunks.


Q4: Does the dilution of the Reag B and Tank Water matter during the mixing?
You can dilute the mix a good bit before it starts to matter.

0.20mL of Reag B added, Varied vol of distilled added, 0.20mL TW, swirl 10 sec, hold 4 min, dilute to 10mL. Invert 10x to mix, Invert 3x at measurement.

This may come in handy, because needing to accurately add 0.200mL of tank water might be a difficult ask, but you can get away with diluting the tankwater to 3 or 4x volume and then add 0.60 or 0.80mL.

Like I said, dumb questions first and no interesting chemistry yet, but now that I know how to measure the turbidity, next I can work out how much of this turbidity is from the K vs the Ca or Mg in the tank water.

EXCELLENT!!!

 

[taricha]

This might end up being much much easier than I thought.

Q5 How does Ca concentration affect the turbidity compared to the K concentration? It looks like within plausible Ca levels, the Ca concentration has no effect on the turbidity, and the result is nicely linear with K concentration.

I took tank water and diluted it to 70% concentration (30% distilled water), then dosed known K spikes (from KCl) going from ~70% up to about 130% of "normal tankwater" K level.

I then took the same 70% tank water, spiked it with Ca from ~70% to about 130% of "normal tankwater" Ca. Then I ran the same K spikes as before on this "high Ca" water.

0.200mL of Reag B in hanna cuvette, 0.200mL Sample water, swirl 10sec, hold 4 min, dilute to 10mL with distilled, invert 10x to mix, invert 3x at measurement.

The low Ca tank water is in Blue (295ppm on Red Sea Ca test), and the high Ca tankwater is in Red (590ppm on Red Sea Ca test). There's statistically no difference. The response to K concentration was exactly the same within error bars. The needed Ca correction under these conditions looks like zero(!)

Going further, if I take the normal functioning Red Sea K test (according to directions, back-titration method) as a rough estimate of the actual K in my tankwater (K = 449, so 70% = 314ppm), and use that to plot the above data in terms of absolute K concentration vs the turbidity, it gets even more optimistic....

Those intercepts look about as close to zero as you can get - the whole chart looks almost perfectly proportional to K concentration. It suggests that not just Ca, but Mg, and all other components of saltwater may have very little interference effect under these conditions.

This is far from rigorous, but it's a first glance at the question of whether we should expect the rest of the saltwater contents, (Mg, Salinity) to provide a lot of interference or very little. This chart says that the turbidity is due almost exclusively to K concentration and other stuff in the saltwater may have very little effect.

To tighten this up, I'll repeat the K spikes at high and low Mg, and high and low NaCl, but we may already know where this is headed.

 

[Rick Mathew]

This might end up being much much easier than I thought.

Q5 How does Ca concentration affect the turbidity compared to the K concentration? It looks like within plausible Ca levels, the Ca concentration has no effect on the turbidity, and the result is nicely linear with K concentration.

I took tank water and diluted it to 70% concentration (30% distilled water), then dosed known K spikes (from KCl) going from ~70% up to about 130% of "normal tankwater" K level.

I then took the same 70% tank water, spiked it with Ca from ~70% to about 130% of "normal tankwater" Ca. Then I ran the same K spikes as before on this "high Ca" water.

0.200mL of Reag B in hanna cuvette, 0.200mL Sample water, swirl 10sec, hold 4 min, dilute to 10mL with distilled, invert 10x to mix, invert 3x at measurement.

The low Ca tank water is in Blue (295ppm on Red Sea Ca test), and the high Ca tankwater is in Red (590ppm on Red Sea Ca test). There's statistically no difference. The response to K concentration was exactly the same within error bars. The needed Ca correction under these conditions looks like zero(!)

Going further, if I take the normal functioning Red Sea K test (according to directions, back-titration method) as a rough estimate of the actual K in my tankwater (K = 449, so 70% = 314ppm), and use that to plot the above data in terms of absolute K concentration vs the turbidity, it gets even more optimistic....

Those intercepts look about as close to zero as you can get - the whole chart looks almost perfectly proportional to K concentration. It suggests that not just Ca, but Mg, and all other components of saltwater may have very little interference effect under these conditions.

This is far from rigorous, but it's a first glance at the question of whether we should expect the rest of the saltwater contents, (Mg, Salinity) to provide a lot of interference or very little. This chart says that the turbidity is due almost exclusively to K concentration and other stuff in the saltwater may have very little effect.

To tighten this up, I'll repeat the K spikes at high and low Mg, and high and low NaCl, but we may already know where this is headed.

As usual Sir....Awesome work!!

 

[taricha]

Q6 How does the Mg concentration effect the response to K? It doesn't. There's no difference in the response to K whether the water is very low or very high in Mg.

I took the same 70% tankwater and spiked it with Mg to ~130% of normal Mg concentration, then ran it through the same K spikes as with the 70% Tankwater.

0.200mL of Reag B in hanna cuvette, 0.200mL Sample water, swirl 10sec, hold 4 min, dilute to 10mL with distilled, invert 10x to mix, invert 3x at measurement.

Low Mg data in Blue, High Mg data in Red - the trendlines sit entirely on top of each other. As we suspected from the last chart in the previous post 12, it looks like Mg concentration (and the rest of seawater ingredients) have essentially no effect on the response to K under these conditions.

Q7 How does the salinity (as NaCl concentration) affect the response to K? Again, as we suspected from earlier, it doesn't.

I took the same 70% tankwater and added lab grade NaCl to reach a final S.G. about 120% of normal salinity.

0.200mL of Reag B in hanna cuvette, 0.200mL Sample water, swirl 10sec, hold 4 min, dilute to 10mL with distilled, invert 10x to mix, invert 3x at measurement.

Low NaCl in blue, high NaCl in red and again - the trend lines sit entirely on top of each other.


So here's the overall situation - this method is nearly done.
In principle, the literature indicated that Ca or Mg could also be precipitated by tetraphenylborate along with the K, causing some interference in the form of turbidity that isn't from K. And the Hach method expects interference from full seawater Chloride concentrations, but in practice these things turn out not to occur to any detectable extent under these selected mixing conditions for the entire low to high range of plausible tankwater Ca, Mg, Cl (and Na) concentrations. The slopes of response to K concentrations for all these have been tightly similar, and all seem to intercept very near zero. The below summary chart illustrates how identically the method behaves under all these conditions.

Although I could just use these regressions to work out the formula and be done, you can see from the above that the dilution ratio I've been using would put everything over ~350ppm K out-of-range-high on the hanna ULR phosphate checker (up to 0.90 ppm PO4) which is the most common checker that people might have available for this - so I'll refigure the dilution with distilled water at the end probably aiming to set 600ppm K as max on the checker = 0.90 ppm PO4, and adjust the mixing directions for more convenient sample water volumes than trying to get people to precisely do <0.200mL of sample water.

I'll then use that new recipe - run some regressions with a couple of different saltwater backgrounds - tank water, new salt mix, maybe use the recipe Randy posted for seawater from Millero to make a near-zero-K seawater, then declare victory - and see if it is repeatable for others.

This turbidity method will not be as precisely repeatable as the back-titrations that Salifert and Red Sea standard method do. But it has other advantages: It'll be efficient on chemicals - only 1 reagent, fast to execute - can probably run the whole thing with the hanna 3:00 timer, and simple method. I think the uncertainty in the K value it gives will probably still be fine for tank water purposes.

 

[Rick Mathew]

Well done!...

Excellent work Sir!!

 

[Randy Holmes-Farley]

Nice!

 

[Dan_P]

Q3: How much Reag B (tetraphenylborate) is needed to precipitate the stuff that ought to be precipitated from tank water? 1:1 or 1.2:1 ratio of Reag B to tank water ought to be fine for now.

Var. vol of Reag B added, 0.20mL TW, swirl 10 sec, hold 4 min, dilute to 10mL. Invert 10x to mix, invert 3x at measurement.

When you get above 1:1 ratio of Reag B to tank water, the final turbidity is less sharply dependent on how much you added. Some amount of this turbidity is unwanted Ca and Mg. Pre-precipitation ought to reduce the amount of Reagent B needed. The Red Sea directions use a smaller 1:4 ratio and have NaOH added first to pre-precipitate. For now, I'll work at the higher ratio and later I'll see how much of what I'm precipitating is K vs Ca or Mg. Additional observation, at lower amounts of Reag B - there are more prominent larger particles and relatively less fine cloudiness - we want uniform cloudiness for measurement, not discrete chunks.

As always, awesome! Here are some additional thoughts.

In this process, potassium tetraphenylborate is precipitated. It has a low solubility of 0.18 mg/L. Potassium accounts for about 10% of this mass which means not much potassium is left in solution when a stoichiometric amount of reagent is added.

When not using a stoichiometric amount of the borate (Not saying you are. I don’t know the molarity of the solution you are using), I am not exactly sure why there should be a difference in turbidity because as the amount of potassium increases, I would expect the same amount of precipitate to form above a certain level. For example, a sub-stoichiometric ratio of K to borate of 4 :1 will give the same amount of precipitate as a 3:1 ratio. The borate is the limiting reagent. So unless the borate is in excess of the highest amount of potassium expected, the amount of precipitate will stop correlating with potassium concentration at some point.

As for consistently creating uniform turbidity, you are likely to obtain a more uniform particle size distribution if the tetraphenylborate is added to diluted aquarium water. Fast precipitations can produce messy agglomerated particles with wide particle size ranges, but with low concentrations or ultra high mixing rates a more uniform and smaller particle size distribution is possible.

 

[Randy Holmes-Farley]

As always, awesome! Here are some additional thoughts.

In this process, potassium tetraphenylborate is precipitated. It has a low solubility of 0.18 mg/L. Potassium accounts for about 10% of this mass which means not much potassium is left in solution when a stoichiometric amount of reagent is added.

When not using a stoichiometric amount of the borate (Not saying you are. I don’t know the molarity of the solution you are using), I am not exactly sure why there should be a difference in turbidity because as the amount of potassium increases, I would expect the same amount of precipitate to form above a certain level. For example, a sub-stoichiometric ratio of K to borate of 4 :1 will give the same amount of precipitate as a 3:1 ratio. The borate is the limiting reagent. So unless the borate is in excess of the highest amount of potassium expected, the amount of precipitate will stop correlating with potassium concentration at some point.

As for consistently creating uniform turbidity, you are likely to obtain a more uniform particle size distribution if the tetraphenylborate is added to diluted aquarium water. Fast precipitations can produce messy agglomerated particles with wide particle size ranges, but with low concentrations or ultra high mixing rates a more uniform and smaller particle size distribution is possible.

One complication is how much of the tetraphenylborate added is remaining in solution, complexed to other ions. That's is a complexity that may require higher than stoichiometeric amounts of the reagent to precipitate all or most of the potassium.

That is why, for example, seawater can contain a lot more of a number of insoluble materials than would dissolve in fresh water:

Chemistry and the Aquarium: Calcium

Randy discusses calcium and its role in helping maintain a healthy marine aquarium.

reefs.com

In seawater, the situation is slightly more complicated. While the majority of calcium ions are still free, some (about 10-15%) are present as an ion pair with sulfate, forming the neutral ion pair CaSO4 (Figure 2). These types of soluble ion pairs are short lived, forming and breaking apart quite rapidly. Nevertheless, they can have significant impact on the properties of seawater. This ion pair is in turn hydrated with water molecules, as shown in Figure 2.

Calcium similarly forms ion pairs with carbonate and bicarbonate. While these comprise a small fraction of the total calcium, the calcium carbonate ion pair comprises a fairly large portion of the total carbonate (together with magnesium, about 2/3 of the carbonate). These ion pairs consequently tend to lower the free concentration of carbonate, and thereby help to inhibit precipitation of calcium carbonate, and consequently increase its
solubility.

Finally, calcium forms ion pairs with fluoride, hydroxide, borate, the various forms of phosphate, and other ions to smaller extents that are unimportant to the free calcium concentration, but may impact the free concentrations of these other ions (especially phosphate, where calcium binds to more than 70% of the PO4—). In almost all cases, however,
the effect of calcium is smaller than the effect of magnesium on these ions, both because the concentration of magnesium is higher, and because in some cases it actually interacts more strongly (MgF+ compared to CaF+, for example).

 

[taricha]

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

In addition to OH-, chelators such as EDTA will take out Ca++ and Mg++ and not K+

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.

 

[Dan_P]

One complication is how much of the tetraphenylborate added is remaining in solution, complexed to other ions. That's is a complexity that may require higher than stoichiometeric amounts of the reagent to precipitate all or most of the potassium.

That is why, for example, seawater can contain a lot more of a number of insoluble materials than would dissolve in fresh water:

Chemistry and the Aquarium: Calcium

Randy discusses calcium and its role in helping maintain a healthy marine aquarium.

reefs.com

In seawater, the situation is slightly more complicated. While the majority of calcium ions are still free, some (about 10-15%) are present as an ion pair with sulfate, forming the neutral ion pair CaSO4 (Figure 2). These types of soluble ion pairs are short lived, forming and breaking apart quite rapidly. Nevertheless, they can have significant impact on the properties of seawater. This ion pair is in turn hydrated with water molecules, as shown in Figure 2.

Calcium similarly forms ion pairs with carbonate and bicarbonate. While these comprise a small fraction of the total calcium, the calcium carbonate ion pair comprises a fairly large portion of the total carbonate (together with magnesium, about 2/3 of the carbonate). These ion pairs consequently tend to lower the free concentration of carbonate, and thereby help to inhibit precipitation of calcium carbonate, and consequently increase its
solubility.

Finally, calcium forms ion pairs with fluoride, hydroxide, borate, the various forms of phosphate, and other ions to smaller extents that are unimportant to the free calcium concentration, but may impact the free concentrations of these other ions (especially phosphate, where calcium binds to more than 70% of the PO4—). In almost all cases, however,
the effect of calcium is smaller than the effect of magnesium on these ions, both because the concentration of magnesium is higher, and because in some cases it actually interacts more strongly (MgF+ compared to CaF+, for example).

Great clarification. I will eventually catch on