Here is a very easy oxidation of formaldehyde to formic acid using peracetic acid. I would rather do this than the synth from oxalic acid.
Formic acid.
Yasuda, Kiyoshi; Goto, Yukihisa.
Patent JP52113911 (http://l2.espacenet.com/dips/viewer?PN=JP52113911&CY=gb&LG=en&DB=EPD)
(in Japanese)
Abstract
HCO2H was prepared by liquid-phase oxidation of HCHO with MeCO3H at >100°C. Thus, 150 mL/h 29% MeCO3H in AcOEt was added to 100g 37% aqueous HCHO containing 10% MeOH over 5 h at 40°C with 0.8 mol ratio of MeCO3H/HCHO to give HCO2H with 99.1% conversion and 98.5% selectivity.
Quite nice method for preparation of peracid solutions -
Journal of Organic Chemistry, 1959, Vol 23, N 12, pp. 1823-1826
Read it carefully and you will be enlightened (some right glassware strongly reccomended tho)
Edit: This article has been posted in Post 460155 (https://www.thevespiary.org/talk/index.php?topic=10978.msg46015500#msg46015500)
(Rhodium: "Prep of anhydrous peracetic acid solutions", Newbee Forum) /Rhodium