Why do alkalinity titration tests expire

[Dennis Cartier]

I am thinking of trying to develop a colourmetric alkalinity tester and am wondering about the shelf life of the reagents. The closest example would be the Salifert alkalinity tests, though I plan to use a combined indicator dye and acid reagent. Which made me start to wonder, why do the Salifert alkalinity test expire? They keep the indicator dye and acid titirant separate, and the acid should not expire if tightly sealed, so is it the indicator dye that shifts it's pH range of colour change?

I am planning on trying to use bromocresol green as my indicator, combined into the acid titrant (if it matters). I am hoping it is as simple as diluting the 0.1N acid standard to 0.02N with water containing 4% bromocresol green dye. I am planning on targeting the same volumes of sample and strengths of titrant as the Alkatronic, which I currently use and like. I would expect to need 2-3 months of stability from the mixed reagent. More than that (e.g. 6 months stable) would be even better.

So back to the original question, what is it about the commercial tests that cause them to expire?

 

[Dan_P]

I am thinking of trying to develop a colourmetric alkalinity tester and am wondering about the shelf life of the reagents. The closest example would be the Salifert alkalinity tests, though I plan to use a combined indicator dye and acid reagent. Which made me start to wonder, why do the Salifert alkalinity test expire? They keep the indicator dye and acid titirant separate, and the acid should not expire if tightly sealed, so is it the indicator dye that shifts it's pH range of colour change?

I am planning on trying to use bromocresol green as my indicator, combined into the acid titrant (if it matters). I am hoping it is as simple as diluting the 0.1N acid standard to 0.02N with water containing 4% bromocresol green dye. I am planning on targeting the same volumes of sample and strengths of titrant as the Alkatronic, which I currently use and like. I would expect to need 2-3 months of stability from the mixed reagent. More than that (e.g. 6 months stable) would be even better.

So back to the original question, what is it about the commercial tests that cause them to expire?

Possibly acid concentration increasing because of solvent evaporation.

 

[Tavero]

I am thinking of trying to develop a colourmetric alkalinity tester and am wondering about the shelf life of the reagents. The closest example would be the Salifert alkalinity tests, though I plan to use a combined indicator dye and acid reagent. Which made me start to wonder, why do the Salifert alkalinity test expire? They keep the indicator dye and acid titirant separate, and the acid should not expire if tightly sealed, so is it the indicator dye that shifts it's pH range of colour change?

I am planning on trying to use bromocresol green as my indicator, combined into the acid titrant (if it matters). I am hoping it is as simple as diluting the 0.1N acid standard to 0.02N with water containing 4% bromocresol green dye. I am planning on targeting the same volumes of sample and strengths of titrant as the Alkatronic, which I currently use and like. I would expect to need 2-3 months of stability from the mixed reagent. More than that (e.g. 6 months stable) would be even better.

So back to the original question, what is it about the commercial tests that cause them to expire?

Here in europa everything has an expiration date. Even sugar, honey, vinegar and water.
No it wont expire as long as you don't leave the bottles open.

 

[Randy Holmes-Farley Verified Community Expert]

Dyes can oxidize to a different form and that may alter the results. I agree that evaporation is the only other way.