Ketones are also weak bases, undergoing protonation on the carbonyl oxygen in the presence of Brønsted acids. Ketonium ions (i.e., protonated ketones) are strong acids, with p"K"a values estimated to be somewhere between –5 and –7. Although acids encountered in organic chemistry are seldom strong enough to fully protonate ketones, the formation of equilibrium concentrations of protonated ketones is nevertheless an important step in the mechanisms of many common organic reactions, like the formation of an acetal, for example. Acids as weak as pyridinium cation (as found in pyridinium tosylate) with a p"K"a of 5.2 are able to serve as catalysts in this context, despite the highly unfavorable equilibrium constant for protonation ("K"eq < 10−10). An aldehyde differs from a ketone in that it has a hydrogen atom attached to its carbonyl group, making aldehydes easier to oxidize. Ketones do not have a hydrogen atom bonded to the carbonyl group, and are therefore more resistant to oxidation. They are oxidized only by powerful oxidizing agents which have the ability to cleave carbon–carbon bonds. Ketones and aldehydes absorb strongly in the infra-red spectrum near 1700 cm−1. The exact position of the peak depends on the substituents. Whereas 1H NMR spectroscopy is generally not useful for establishing the presence of a ketone, 13C NMR spectra exhibit signals somewhat downfield of 200 ppm depending on structure. Such signals are typically weak due to the absence of nuclear Overhauser effects. Since aldehydes resonate at similar chemical shifts, multiple resonance experiments are employed to definitively distinguish aldehydes and ketones. Ketones give positive results in Brady's test, the reaction with 2,4-dinitrophenylhydrazine to give the corresponding hydrazone. Ketones may be distinguished from aldehydes by giving a negative result with Tollens' reagent or with Fehling's solution. Methyl ketones give positive results for the iodoform test. Ketones also give positive results when treated with "m"-dinitrobenzene in presence of dilute sodium hydroxide to give violet coloration. Many methods exist for the preparation of ketones in industrial scale and academic laboratories.