Green/pink TA vs Blue/Clear TA indicators?

tomfrh

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Jan 30, 2018
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I have clear choice labs green/pink indicator and a cheap blue/clear indictator.

The green indicator goes pink with 8 drops (80ppm) and the blue indicator goes clear after 10 drops (100pom). Same water, same titration agent.

any ideas why they give a different answer? Do they work differently? What exactly do they measure?
 

JoyfulNoise

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They are just different indicator dyes. The green/red dye (its more red than pink) is an easier transition to see. The blue clear dye is going to have more “slop” in the measurement because not everyone will see the “clear” point the same way.

The dyes are an indicator of pH. The chosen transition point for TA is a pH of 4.5. When you’ve added enough sulfuric acid titrant to lower the pH below 4.5, you have used up all of the alkaline buffering species. That defines the total amount of alkalinity in the water.
 

JoyfulNoise

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In case you’re dying to know, the green/red indicator is a combination of bromocresol green and methyl red pH indicators.
 

tomfrh

Well-known member
Jan 30, 2018
489
Australia
They are just different indicator dyes. The green/red dye (its more red than pink) is an easier transition to see. The blue clear dye is going to have more “slop” in the measurement because not everyone will see the “clear” point the same way.

The dyes are an indicator of pH. The chosen transition point for TA is a pH of 4.5. When you’ve added enough sulfuric acid titrant to lower the pH below 4.5, you have used up all of the alkaline buffering species. That defines the total amount of alkalinity in the water.

Thanks. That’s useful to know. I didn’t realise it was simply a pH indicator.

im still wondering why the green/red turns red before the blue clear. It’s still very blue at the point the green/red turns red?
 

JoyfulNoise

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Are you sure it’s not yellow instead of clear?
 

JoyfulNoise

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im still wondering why the green/red turns red before the blue clear. It’s still very blue at the point the green/red turns red?

pH indicator dyes are not exact, that is, there's not a single pH value at which the color changes but more of a pH range over which the color transitions from one to the other. For some dyes, the transition can be very tight, less than 0.5 pH units but, for other dyes, it can be much broader spanning several pH units. Using two indicators with closely overlapping color transitions and a color mixing that creates a distinct visual difference is typically more accurate than a single color transition.

The green/red indicator is far more accurate than the blue indicator because the green/red transition point is very close to where one defines the TA test to be, approximately 4.0-4.5. The blue indicator (which is most likely bromophenol blue) starts to transition from blue to yellow at around 4.6 but then needs to go all the way to a pH of 3 before becoming light yellow. The concentration (normality) of the sulfuric acid titrant will determine how quickly the transition goes.

So, for the blue indicator, it’s not at all unexpected that the drop count is 2-3 drops higher than than the red/green indicator. In other words, the blue indicator is much less accurate.
 

tomfrh

Well-known member
Jan 30, 2018
489
Australia
pH indicator dyes are not exact, that is, there's not a single pH value at which the color changes but more of a pH range over which the color transitions from one to the other. For some dyes, the transition can be very tight, less than 0.5 pH units but, for other dyes, it can be much broader spanning several pH units. Using two indicators with closely overlapping color transitions and a color mixing that creates a distinct visual difference is typically more accurate than a single color transition.

The green/red indicator is far more accurate than the blue indicator because the green/red transition point is very close to where one defines the TA test to be, approximately 4.0-4.5. The blue indicator (which is most likely bromophenol blue) starts to transition from blue to yellow at around 4.6 but then needs to go all the way to a pH of 3 before becoming light yellow. The concentration (normality) of the sulfuric acid titrant will determine how quickly the transition goes.

So, for the blue indicator, it’s not at all unexpected that the drop count is 2-3 drops higher than than the red/green indicator. In other words, the blue indicator is much less accurate.


Thank you very much. This appears to be what I’m seeing. The point at which the blue is starting to fade matches the green/red transition, with clear a couple of drop past that.

and yes it’s actually a faint yellow rather than clear.

Is the green red indicator a combo indicator like you mention?

Also, my “green red” indicator does actually turns pink, not red. I looked online at the Taylor indicator, which turns red like you mention. But the clear choice turns a pink/magenta colour. Do you know what type this is?
 
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