How to oxidise iron in spinach to iron(III)
Use ceric ammonium sulfate.[1,2]  Quoting from reference 2:

2 Reagents
(i) 0.25 per cent ceric ammonium sulphate in 8.8 N sulphuric acid.
(ii) 40 per cent potassium thiocyanate solution.
(iii) (a) Stock standard ferrous iron solution - 100 μg iron per ml.
(b) Working standard ferrous iron solution - 0.3 to 10 μg iron per ml
2.2 Procedure
To five ml of clear water sample in a test tube 0.5 ml of ceric ammonium sulphate solution is added and mixed thoroughly by shaking. Finally, 0.5 ml of potassium thiocyanate solution is added and again mixed properly. Exactly 5 minutes after addition of thiocyanate solution, the absorbance of ferric thiocyanate complex is measured at 480 nm in a spectrophotometer (Spekol, Carl-Zeiss, Jena).
A series of freshly prepared working standard ferrous iron solutions (upto 10 ppm iron) were treated similarly and the absorbances measured at 480 nm. From the calibration curve, plotted with concentrations of different standard iron solutions against absorbances, the iron content of the unknown sample is evaluated.

References

Willard, H. H.; Young, P. Ceric Sulfate as a Volumetric Oxidizing Agent. Ii. Determination of Iron. J. Am. Chem. Soc. 1928, 50 (5), 1334–1338. DOI: 10.1021/ja01392a013.

Goswami, D. C.; Kalita, H. Rapid Determination of Iron in Water by Modified Thiocyanate Method. DSJ 1988, 38 (2), 177–182. DOI: 10.14429/dsj.38.4835.
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