Simply, you can ‘google’ ...

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Simply, you can ‘google’ (“citric acid” “test strip”) your request and you will find a lot of solutions, like this one: https://www.indigoinstruments.com/test_strips/biology/miscellaneous/vitamin-c-citric-acid-color-chart.html

or:

https://www.admeo.us/products/citric-acid-reagent-kit