[vejaminemboom]

I was wondering if there was a way to get the absorption of ascorbic acid in a solution with citric acid and disodium hydrogen phosphate?

I’m thinking of measuring the absorption of disodium hydrogen phosphate and citric acid solution and then subtracting the solution with the all three molecules to get the absorption of ascorbic acid by itself. Would this be fine? If not what can I do? I'm doing this to find the concentration of ascorbic acid in a solution so if there are any other methods other than UV-spectrophotometer, those would also be appreciated.

[Hunter2]

Use iodometric titration. Only ascorbic acid is reducer.

[Corribus]

Would this be fine?

In a perfect world, the sum of the spectra for the individual components of a mixture at their respective concentrations is equal to the spectrum for the mixture, but this is only true when the components are non-interacting over the relevant concentration ranges. You can investigate that experimentally.