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Bound sulfur dioxide in beer is liberated through the addition of acid and subsequent heating. Under a stream of nitrogen gas, the sulfur dioxide is transferred into a receiver containing hydrogen peroxide, and the sulfuric acid thus created is titrated with sodium hydroxide.

Sulfite (sulfurous acid) is oxidized in the presence of oxygen through sulfite oxidase (SO₂-OD) to sulfate [1].

SO₂^2- + O₂ + H₂O \(^{\underrightarrow {\text{SO}_2 {\text{-OD}}}}\) SO₄^2- + H₂O₂

The hydrogen peroxide produced by this reaction is reduced in the presence of reduced nicotinamide adenine dinucleotide (NADH) by the enzyme NADH peroxidase (NADH-POD).

H₂O₂ + NADH + H⁺ \(^{\underrightarrow {\text{NADH-POD}}}\)2 H₂O + NAD⁺

The amount of NADH consumed in reaction (2) is equivalent to the quantity of sulfites, including the quantity of sulfites chemically bound to aldehydes. The NADH is the substance being measured and is determined photometrically based on its absorption at 340 nm.

SO₂ is primarily present in beer or wine in a bound form (mostly on aldehydes, ketones and sugars). Sulfur dioxide is liberated through the addition of 2 M sulfuric acid and subsequent heating to 95 °C. In an acidic medium, this reaction forms SO₂ ⋅ H₂O from carbonyl sulfite adducts, which can no longer act as a functional nucleophile. Additional heating reduces the solubility of gases, causing SO₂ to diffuse through a Teflon membrane where it is dialyzed in a 1% sulfuric acid solution. It then reacts with formaldehyde to form a carbonyl sulfite adduct (HO-CH₂-SO₃H). This adduct binds at a temperature of 45 °C to para-rosaniline forming a red complex; the absorbance is measured at 560 nm with a UV-VIS detector. When performing the analysis, care should be taken to only use the Teflon membranes listed below; otherwise, other Maillard reaction products can cause incorrect blank values.

In addition to other ions, sulfite can be determined using an ion exchanger with ion chromatography.