Salifert KH/Alk testing help

[|sCRIBe|]

I've read the instructions and been using these kits for a couple years but want to know people's thoughts regarding the testing process.

The instructions state when the color changes from the original blue to a pink, take the reading. Do you take the reading when there is still hints of blue or do you continue to add reagent until the blue is fully gone and only the pink is visible?

This step of when to take the reading can vary the results greatly so I don't know if I have 8.3 or 6.0 because literally a few extra drops of reagent can be this difference

 

[Brew12]

I've read the instructions and been using these kits for a couple years but want to know people's thoughts regarding the testing process.

The instructions state when the color changes from the original blue to a pink, take the reading. Do you take the reading when there is still hints of blue or do you continue to add reagent until the blue is fully gone and only the pink is visible?

This step of when to take the reading can vary the results greatly so I don't know if I have 8.3 or 6.0 because literally a few extra drops of reagent can be this difference

The color change needs to be complete. I find it only takes a drop or two to make the complete change. I do slow down to adding a drop every 20 or 30 seconds and mixing it well in between.

 

[Scott.h]

I know exactly what you mean. There is a 3 drop difference between slightly pink to pink pink. Which translates to a difference to .1 to .2 dkh in potential error. The more important thing is stability. Testing it the same way, reading the same way each time. I've also found the way that I test with Salifert to test 10% low compared with triton. Salifert error or my testing error, I'm not sure, but the more important thing is that I realize this when correcting things. That said, triton doesn't test alk, but I'm keeping it right on line 56 of the syringe every day.

 

[JimWelsh]

I am of the opinion that the correct equivalence point color is more of a "lavender" color, and am quite certain that if you reach a completely pink color you have overshot the endpoint.

 

[Randy Holmes-Farley]

If you have a pH meter you can determine the endpoint color exactly. Endpoint is about pH 4.2-4.3 or so. pH moves fast per drop at then endpoint.

 

[JimWelsh]

I have found the endpoint for ASW to be closer to pH 4.6. Hach says for samples with an alkalinity of appx 150 mg/L (dKH = 8.4), use pH 4.6. Your own DIY Alk test cites an endpoint of pH 4.5. Admittedly, the difference in the amount of titrant it takes to get from pH 4.6 to pH 4.2 is small, but if people are asking about how to not overshoot the endpoint, I'm concerned that suggesting too low a pH value for the endpoint may exacerbate the problem.

FWIW, Hach sells pH 4.5 buffer powder pillows expressly for the purpose of creating a solution for demonstrating the correct endpoint color; simply dissolve the powder pillow in 50 mL of water, and add a couple of drops of the bromocresol green / methyl red indicator to get the color you should be matching at the endpoint. In my experience, the Hach alkalinity indicator dye behaves exactly the same as the Salifert alkalinity indicator dye.

 

[Randy Holmes-Farley]

How did you determine the exact endpoint?

It varies with the alkalinity and is higher at lower alkalinity.

I believe I got the 4.2 value from Millero, and the 4.5 from Pankow for freshwater. I'm not at home now to check.

 

[JimWelsh]

How did you determine the exact endpoint?

Various ways. Among them, I recorded the data from titration curves and plugged them into this calculator: https://or.water.usgs.gov/alk/
I also have titrated alkalinity standards I have carefully made using nitrogen-sparged 18 Megohm water and a primary standard Na2CO3 solution I have made using baked Na2CO3 cooled overnight in a vacuum dessicator, and found the endpoint to be around pH 4.6 for typical reef tank alkalinity levels (7 to 10 dKH or so).

 

[Randy Holmes-Farley]

But those are not seawater, right?

 

[Randy Holmes-Farley]

He does say pH 4.5 here, but I'm pretty sure I saw 4.2 elsehwere:

https://www.researchgate.net/profil...f_seawater/links/0046353bc001b5ee83000000.pdf

 

[|sCRIBe|]

I appreciate all the comments but my question doesn't seem to get answered which is when do you stop adding reagent? When the blue is completely gone and only the pink is visible or when the color change begins and you can still see the blue and pink together?

 

[Randy Holmes-Farley]

I appreciate all the comments but my question doesn't seem to get answered which is when do you stop adding reagent? When the blue is completely gone and only the pink is visible or when the color change begins and you can still see the blue and pink together?

We are trying to answer it. Jim said "I am of the opinion that the correct equivalence point color is more of a "lavender" color, and am quite certain that if you reach a completely pink color you have overshot the endpoint."

 

[Randy Holmes-Farley]

CO2 in Seawater: Equilibrium, Kinetics, Isotopes
By R.E. Zeebe, D. Wolf-Gladrow
Elsevier, Oct 15, 2001 - Science - 360 pages


footnote 9 page 32 says the carbonate endpoint is pH 4.3


https://books.google.com/books?id=V...UAhWBLSYKHXXlCkkQ6AEIMDAC#v=onepage&q&f=false

 

[JimWelsh]

But those are not seawater, right?

The calculator still analyses the titration curve, and calculates the pH of the endpoint, matrix notwithstanding. My standards were in an ASW matrix according to the popularly quoted recipe given by Millero.

 

[JimWelsh]

footnote 9 page 32 says the carbonate endpoint is pH 4.3

But his matrix is "simplified seawater", which notably lacks borate, among other things. Perhaps this causes a shift in the pH of the endpoint?

 

[|sCRIBe|]

Do you take your reading right when there is a chemical change and you see the pink lavender color but can also still see the blue color too or add more reagent until the blue is all gone?

 

[Randy Holmes-Farley]

The calculator may (or may not) be accurate in seawater. It askes for conductivity to:

"The specific conductance is used to estimate the ionic strength of the sample. Ionic strength and sample temperature are used to calculate the equilibrium dissociation constants of water and carbonic acid, which are then used in the calculation of hydroxide, carbonate, and bicarbonate concentrations."

But the pKa of carbonate and bicarbonate in NaCl of similar conductivty is not the same as it is in actual seawater.

 

[JimWelsh]

Do you take your reading right when there is a chemical change and you see the pink lavender color but can also still see the blue color too or add more reagent until the blue is all gone?

I take the reading when there is still some blue in the lavender, then add one more drop to make sure it goes too pink. The last reading when there was still some bluish tinge is what I consider the endpoint.

 

[JimWelsh]

The calculator may (or may not) be accurate in seawater.

This goes to the calculation of alkalinity from the titration curve data, but for the purposes of this thread, I'm only talking about the calculation of the endpoint's pH from the raw data using the inflection point method. That's going to be independent of matrix considerations.

 

[Randy Holmes-Farley]

There seem to be differences of opinion on the endpoint. Not sure why.

https://www.google.com/url?sa=t&rct...=UTF-8&usg=AFQjCNHGk4wAalmiq-TRHtfSdReMZOBjww

International Ocean Discovery Program
Autotitrator pH/Alkalinity User Guide 1 3 January 2014
User Guide: Autotitrator pH/Alkalinity

"Method Theory To measure alkalinity, a pore water sample is titrated with an acid to an endpoint at which carbonate is converted to bicarbonate and bicarbonate is converted to carbonic acid. In seawater, this endpoint occurs at about pH = 4.2."