Salifert Phosphate with Increased Sensitivity

[jpcaram]

Has anybody tried to increase the sensitivity of the kit based on their instructions?

It reads "Should you require an even higher sensitivity then double the water sample and the reagents. The scale should then be divided by 2."

But if we double both water sample and reagents the concentration of reagents in the water would remain exactly the same and there shouldn't be any difference.

This seems like an error to me unless the fact that there is more water + reagent in the vial makes it look like a different tone of blue against the white background of the scale card.

Any thoughts? Thanks!

 

[Randy Holmes-Farley]

Has anybody tried to increase the sensitivity of the kit based on their instructions?

It reads "Should you require an even higher sensitivity then double the water sample and the reagents. The scale should then be divided by 2."

But if we double both water sample and reagents the concentration of reagents in the water would remain exactly the same and there shouldn't be any difference.

This seems like an error to me unless the fact that there is more water + reagent in the vial makes it look like a different tone of blue against the white background of the scale card.

Any thoughts? Thanks!

In this kit version, are you looking down through the fluid (would give 2x because it is twice as deep) or sideways through it (would not give 2x)?

 

[JimWelsh]

Has anybody tried to increase the sensitivity of the kit based on their instructions?

It reads "Should you require an even higher sensitivity then double the water sample and the reagents. The scale should then be divided by 2."

But if we double both water sample and reagents the concentration of reagents in the water would remain exactly the same and there shouldn't be any difference.

This seems like an error to me unless the fact that there is more water + reagent in the vial makes it look like a different tone of blue against the white background of the scale card.

Any thoughts? Thanks!

Google "Beer's law". You are doubling the path length, and, thus, doubling the intensity of the color (doubling the absorbance), making it easier to see, but then you also need to divide the result off the card by 2. Makes perfect sense.

 

[jpcaram]

@Randy Holmes-Farley , @JimWelsh , yes, you are looking down and thus twice the depth. Makes sense.

Before I put this one down, I'm going to dig a little deeper since you guys seem to know what you are talking about. Beer's law states that the attenuation of light is proportional to the path length. However, a large fraction of the light is coming in from the sides of the vial before going up into our eyes. With twice the depth it seems we now have twice as much surface for light to come in from the sides. So perhaps this works only as a crude first order approximation? Thanks!

 

[Dan_P]

In this kit version, are you looking down through the fluid (would give 2x because it is twice as deep) or sideways through it (would not give 2x)?

This kit involves placing the vial of test solution on top of the white portion of the reference card. Doubling the solution volume doubles the path length. I think it is the nitrate test that increases the path length by looking through the side.

 

[Randy Holmes-Farley]

@Randy Holmes-Farley , @JimWelsh , yes, you are looking down and thus twice the depth. Makes sense.

Before I put this one down, I'm going to dig a little deeper since you guys seem to know what you are talking about. Beer's law states that the attenuation of light is proportional to the path length. However, a large fraction of the light is coming in from the sides of the vial before going up into our eyes. With twice the depth it seems we now have twice as much surface for light to come in from the sides. So perhaps this works only as a crude first order approximation? Thanks!

In a clear solution (colored but not cloudy), photons hitting your eyes originated from the card and passed straight up. Twice the depth has twice the absorbance (in absorbance units, not percent absorbed).

There is also absorbance of light before getting to the card. That also contributes color. Photons coming down from the top go through twice the fluid, so again, twice the absorbance.

Analysis of photons coming in at an angle from the side and hitting the card is more complicated. Some pass through the same distance regardless of double volume or not, and some a larger distance, approaching twice as far.

Thus, I agree that the total color intensity is not quite twice, but some value a bit less than twice.

 

[Randy Holmes-Farley]

One way to avoid the above complication is to make sure the vial is not sitting on the card, but rather the card is almost completely lit by light not passing through the vial before it gets to the card.

Of course, then the actual numbers associated with a particular color intensity may not be accurate.

 

[biom]

lol guys I don't think we humans are any good in counting photons. Even the hobby grade Hanna checker is far far better than human eye is in this.
Salifert test is as good as Hanna is (may be better I would say) but our eyes are not all good in reading it.
I personally don't see clearly the difference between 0.03 and 0.1 on Salifert color chart and with my age it becomes even worse. But still I think it is quite useful because it is fast and simple. All I want to see is this very light blue color that is my goal which means PO4 is between 0.03 and 0.1 if there is no at all blue tint I would continue dosing NWS level phosphate daily.
But back to the topic, instead of doubling the reagents and water I am using not the original strange shaped vial but the cylindrical one from the other Salifert tests (KH for example). They give more height (i think it was about 25% more) and the blue is more visible from the top. In this case you should multiply by 0.75 to receive correct numbers but again I dont think it is that important, if you see this very light blue it is ok. If you don't see and there are problems in your thank then you can consider dosing, if the blue is quite intensive and there are problems in the tank you should consider more accurate tests and lowering phosphates.

 

[gbroadbridge]

lol guys I don't think we humans are any good in counting photons. Even the hobby grade Hanna checker is far far better than human eye is in this.
Salifert test is as good as Hanna is (may be better I would say) but our eyes are not all good in reading it.
I personally don't see clearly the difference between 0.03 and 0.1 on Salifert color chart and with my age it becomes even worse. But still I think it is quite useful because it is fast and simple. All I want to see is this very light blue color that is my goal which means PO4 is between 0.03 and 0.1 if there is no at all blue tint I would continue dosing NWS level phosphate daily.
But back to the topic, instead of doubling the reagents and water I am using not the original strange shaped vial but the cylindrical one from the other Salifert tests (KH for example). They give more height (i think it was about 25% more) and the blue is more visible from the top. In this case you should multiply by 0.75 to receive correct numbers but again I dont think it is that important, if you see this very light blue it is ok. If you don't see and there are problems in your thank then you can consider dosing, if the blue is quite intensive and there are problems in the tank you should consider more accurate tests and lowering phosphates.

I'm one of those Stange people who can easily see the difference between 0.03 and 0.1 with the Salifert test.

I use a daylight ring fluorescent fitting (used for fine electronics work) to illuminate the test vial and card and look through the ring

It correlates quite closely with the Hanna ULR Phosphate results for me and is the backup test.
The faintest blue tint is all I need to see.

 

[Dan_P]

Thus, I agree that the total color intensity is not quite twice, but some value a bit less than twice.

I am close to being interested in testing your prediction

I have a fiber optic cable for my spectrophotometer and should be able to compare the intensity of color vs depth of test solution. Any thoughts on the experimental setup? I was thinking of measuring the light reflected up through the solution above the solution with a light source above the vial, off to the side and at about 45 degrees.

 

[brclark82]

I have both the Hanna ULR and the Salifert Phosphates tests. My Hanna always reads .03-.04.

Before I got the Hanna I would use the higher sensitivity test for Salifert and I would think I didn’t detect any color with my naked eye but then I put a vial of water with no reagent in it next to the test vial and they definitely aren’t the same color meaning it has to be more than 0. I just decided I couldn’t trust my eyes and just got the Hanna.

 

[Randy Holmes-Farley]

I am close to being interested in testing your prediction

I have a fiber optic cable for my spectrophotometer and should be able to compare the intensity of color vs depth of test solution. Any thoughts on the experimental setup? I was thinking of measuring the light reflected up through the solution above the solution with a light source above the vial, off to the side and at about 45 degrees.

That sounds like a nice experiment. Just a few thoughts...

The fiber optic end, is it directional in its uptake of light? That is, in a setting like this were the end will necessarily be lit from multiple directions, if it is pointing down into the solution, will it take light from directly below, or also from an angle off to the side? Direct sideways? Our eyes can focus on the card and only consider the color of the card, but for a light gathering and average device, light that is coming from places other than the card could impact the results.

Assuming it is fairly directional, I'd rig up some sort of jig that can hold it just above the fluid surface to minimize light coming from slight angles sideways that may not be passing through the fluid at all. But you want to minimize blocking light coming down from above into the fluid and then onto the card.

 

[Dan_P]

That sounds like a nice experiment. Just a few thoughts...

The fiber optic end, is it directional in its uptake of light? That is, in a setting like this were the end will necessarily be lit from multiple directions, if it is pointing down into the solution, will it take light from directly below, or also from an angle off to the side? Direct sideways? Our eyes can focus on the card and only consider the color of the card, but for a light gathering and average device, light that is coming from places other than the card could impact the results.

Assuming it is fairly directional, I'd rig up some sort of jig that can hold it just above the fluid surface to minimize light coming from slight angles sideways that may not be passing through the fluid at all. But you want to minimize blocking light coming down from above into the fluid and then onto the card.

The fiber optic probe is directional. The experimental set up you describe should be fine.

I thought of something even simpler that would be available to anyone with a digital camera. I will cue up this work in a few days week.

 

[Snoopy 67]

I went digital because of "the eyes" trying to determine what I see".
I would like to hear more on camera use please.

 

[Reef.]

I'm one of those Stange people who can easily see the difference between 0.03 and 0.1 with the Salifert test.

I use a daylight ring fluorescent fitting (used for fine electronics work) to illuminate the test vial and card and look through the ring

It correlates quite closely with the Hanna ULR Phosphate results for me and is the backup test.
The faintest blue tint is all I need to see.

Would you say that the ring is better at seeing the blue tint over just reading the vial outside?

 

[gbroadbridge]

Would you say that the ring is better at seeing the blue tint over just reading the vial outside?

Yes. It provides a consistent 6000K color temperature with no shadowing.

 

[Reef.]

Yes. It provides a consistent 6000K color temperature with no shadowing.

might try that then…just did a quick Amazon search, seem to be only around £25?

 

[gbroadbridge]

might try that then…just did a quick Amazon search, seem to be only around £25?

The CRI of the light source is important.
You may need to upgrade the bulb on a cheaper fixture.

 

[Dan_P]

Summary

Does increasing the light path length in a Salifert vial for the phosphate test increase sensitivity? Yes.

Does doubling the volume of the phosphate test solution double the light path length? No. The path length for 10 mL is 22 mm, 20 mL 38 mm.

Does doubling the test solution volume look like doubling the concentration? Yes.

Experiment

I used a methylene blue solution for my model phosphate test solution, making up three solutions, RO/DI, the full strength solution of methylene blue in RO/DI arbitrarily designated “1.0”, and a 50% dilution designated “0.5”. I measured color intensity with an iPad camera and extracted the average red pixel value frim the photograph with the free software RGB Colormeter. Here is the photograph that was analyzed.

As you can see, the 20 mL solution of 0.5 appears to be the same as the 10 mL 1.0 solution.

The average red pixel value (absorbance or color intensity) was plotted against concentration to give a slightly curved calibration curve (blue line). Based on the path length, I calculated the expected absorbance for the 20 mL solution of 0.5 (grey dot). The orange dot is the observed absorbance of the 20 mL 0.5 solution. As @Randy Holmes-Farley predicted, the color intensity is less then expected due to a shorter path length alone, though visually you could not discern this difference nor would you likely notice that the color corresponded to 0.86 rather than 1.0.

In practical terms, doubling the sample volume in the Salifert phosphate test doubles the sensitivity.

 

[JimWelsh]

Does doubling the volume of the phosphate test solution double the light path length? No. The path length for 10 mL is 22 mm, 20 mL 38 mm.

Interesting point about the irregular shape of the vials. For vials with a consistent horizontal cross-section area, this would be a "yes" answer.