WillpoleReefers
Active Member
Just posting an update on my evaluation of this mixed pH 4.5 indicator for endpoint determination in alkalinity titrations in case it is of use to anyone. Further conversation with the supplier lead to a changed view on its composition, they now seem to be admitting to it being a bromocresol green/methyl red combo. Colour doesn’t quite match methyl red in acid solution so I reserve final judgement on that, but it does appear to be an excellent indicator for alkalinity titrations.
A 2.5 meq/L sodium carbonate standard solution was prepared from 99.5% pure sodium carbonate that was heated in the oven at 200C for over 1 hour prior to use. 13.25g of the oven treated material was dissolved in 1 litre of RODI water, this 0.125M stock solution was diluted 100x using a pipette and volumetric flask to give the standard, a 0.00125M/2.5 meq/L sodium carbonate solution.
Titrations used 100 ml samples of standard or tank water, 8 drops of the TA4 solution were added. A 50ml burette was used filled with 0.01M hydrochloric acid( 0.1% tolerance). Alkalinity in meq/L was therefore equal to ml acid used/10.
Approximate titrations with the standard were first performed to develop technique and establish an approximate acid volume required of 25ml to turn the TA4 indicator clearly yellow from blue/green. One final careful titration was performed with final additions made dropwise without reference to the burette scale. The result for the standard was 2.48 meq/L , implying an overall accuracy for standard preparation and indicator titration of approx 1%.
Testing old tankwater revealed a low alkalinity at 19.5 meq/L which I think explains a very unhappy hammer coral in there, will now work on that!
I think for practical purposes a pH meter is not required for typical aquarist titrations based on experience using TA4, looks possible that 1% accuracy is achieveable. Ideally more results would be averaged to get a more precise handle on the method. Hope the write up is of use to someone,
Steve
Here is a grab of the Lovibond instruction page relating to use of the indicator with their test reagents:
A 2.5 meq/L sodium carbonate standard solution was prepared from 99.5% pure sodium carbonate that was heated in the oven at 200C for over 1 hour prior to use. 13.25g of the oven treated material was dissolved in 1 litre of RODI water, this 0.125M stock solution was diluted 100x using a pipette and volumetric flask to give the standard, a 0.00125M/2.5 meq/L sodium carbonate solution.
Titrations used 100 ml samples of standard or tank water, 8 drops of the TA4 solution were added. A 50ml burette was used filled with 0.01M hydrochloric acid( 0.1% tolerance). Alkalinity in meq/L was therefore equal to ml acid used/10.
Approximate titrations with the standard were first performed to develop technique and establish an approximate acid volume required of 25ml to turn the TA4 indicator clearly yellow from blue/green. One final careful titration was performed with final additions made dropwise without reference to the burette scale. The result for the standard was 2.48 meq/L , implying an overall accuracy for standard preparation and indicator titration of approx 1%.
Testing old tankwater revealed a low alkalinity at 19.5 meq/L which I think explains a very unhappy hammer coral in there, will now work on that!
I think for practical purposes a pH meter is not required for typical aquarist titrations based on experience using TA4, looks possible that 1% accuracy is achieveable. Ideally more results would be averaged to get a more precise handle on the method. Hope the write up is of use to someone,
Steve
Here is a grab of the Lovibond instruction page relating to use of the indicator with their test reagents:
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