Salifert Alkalinity test kit

[mook1178]

After I make a measurement for my alkalinity using the Salifert kit, the solution will turn back to a blue/green color after some time. I go to the reddish color and swirl for 5-10 seconds to make sure all the CO2 evolves from the solution. However, I come back sometime later, maybe 10 minutes, to clean up and the color is back to the blue/green.

Does anyone know the actual pH indicator used?

I know this color change is pH going back up. I can only think that the pH change is due to CO2 evolving from the solution and alkalinity left behind that was not neutralized, resulting in a low/incorrect kH reading.

has anyone else seen this? Is it possible that the pH indicator is not as accurate to stay the red color around the endpoint?

 

[Randy Holmes-Farley]

An alk titration converts all of the bicarbonate and carbonate in the water into CO2. After finishing, some or all of that extra CO2 will come out of the water and enter the air, raising pH and potentially shifting the indicator dye colors.

The release of CO2 to the air is one of the complexities of measuring alkalinity. Theoretically on can determine what the endpoint should be if none or if all of the excess CO2 leaves, but not for some unknown fraction leaving. Most theoretical treatments assume none is leaving, since it can be a slow process.

I do not know what indicators they use, but it may well be more than one. Hach uses a mix of bromcresol green and methyl red.

 

[mook1178]

What are your thoughts on the pH indicator after ~10 mins turning back to the green color?

To get the pH above the endpoint, alkalinity is needed. If it is truly completely neutralized, I would think that rising above the endpoint is not possible, even with the rest of the excess CO2 evolved.

So my questions are:
1. Is the indicator not accurate enough around the endpoint? Changing color back to green even though the pH is still below the endpoint.

2 . Is the alkalinity truly completely neutralized resulting in an incorrect measurement?

Which would you think is the case?

 

[Randy Holmes-Farley]

I’d stick to the normal directions and timing (not waiting).

Jim welsh shows a good color for the endpoint here:

Alkalinity titration endpoint pH & Salifert endpoint color

TL;DR: The endpoint pH is 4.41, and the endpoint color is that "lavender" color I've been telling people about. Over the years, there have been many threads on R2R and other forums debating a couple of basic details relevant to testing for alkalinity in reef aquaria. One of the most...

www.reef2reef.com

 

[Lasse]

I decide to run a test with Salifert and found out that the difference between "Jim Welsh purple" - first semistable weak red (stable < 5 min) and a stable red (stable > 5 min) and the difference was 0.01 ml titrant in each step (0,15 kH)

I can live with that - but it is important that you read at the same colour each reading .

If my KH is 7 or 7.3 is not so interesting for me - its more if it decrease or increase during time

Sincerely Lasse