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Fig. 2. —Expected gadolinium titration curve using methyl thymol blue indicator. Graph shows that in region 1 of curve, added gadolinium ions would react with unbound diethylenetriamine pentaacetic acid and ethylenediamine tetraacetic acid complexing agents in solution. When all these ligands are combined in a solution, methyl thymol blue—gadolinium will start to form, and absorption at 605 nm will increase. In region 3, all of methyl thymol blue in solution has been consumed and no further increase in absorption will be seen. Transition from region 1 to region 2 (arrowhead) represents endpoint of titration.