A Chelation Competition: DTPA vs. EDTA

One of the recurring needs of the Cantor Arts Center’s outdoor art collection is the removal of mineral deposits from metal sculptures. The water from the sprinklers and the fertilizer from the lawn contain minerals that slowly build up on the sculptures’ surfaces and eventually detract from the artwork’s aesthetic function.

Detail of mineral deposits built up on the surface of a steel sculpture

So what’s the solution to this continuing need?  We can’t turn off the sprinklers or remove all the fertilizers because the landscaping – the artwork’s visual context – depends on them.  While we can prevent future mineral deposits by adjusting the irrigation, removing previous ones can be done with chelation.

The etymology of chelation is Greek χηλή, chelè, meaning claw. The term relates to chemical binding processes where multiple extending parts of a molecule surround to enclose an ion.  In the illustration below you can see how the shape of a chelator is similar to the claws of a crab.

EDTA (black) bonded with a metal ion (red)

Chelators come in many different shapes and sizes and chemical companies develop them for specific purposes. The above illustrated EDTA (ethylene diamine tetraacetic acid) is one of the most common chelation agents; it can be found in many consumer products from face wash to food. At Stanford we use EDTA and another chelation chemical DTPA (diethylene triamine pentaacetic acid) for various specialized cleaning needs – including removing the above mentioned mineral deposits. By adjusting the pH of the different chelation solutions we can further refine and optimize their effectiveness. We start by referring to published research on the different chelators. Then we mix and test several solutions on the sculptures. The solution that produces the best results gets the job. At Stanford, the continuing needs of the outdoor art collection make the competition between chelating agents as fierce as the admissions process.

Detail of the same area after a single chelation treatment

 

One response to “A Chelation Competition: DTPA vs. EDTA

  1. Would you be willing to post or pass on a short bibliography of the references you’ve found most useful to your work on these two chelators?
    Best,
    Arlen Heginbotham
    Associate Conservator, J. Paul Getty Museum
    (… and Stanford Alum)

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