There are many methods for measuring sulphites
the most widespread:
1) the so-called Frantz Paul REFERENCE METHOD:
OIV method (International Organization of Vine and Wine)
a stream of nitrogen entrains the so2 in an acid solution which is then assayed by colorimetry
It is a reliable method but long (10 minutes per dosage )and requires laboratory glassware
2) COLORIMETRIC METHODS (one analyzes a change of color):
methods using iodine (example: ripper method):
Iodine is oxidized by SO2 (SO2 + I2 + 2H2O → 2HI + H2SO4), starch color change is used ( becomes violet in the presence of diiod (I2) )
This is a simple but unusable method when the wine (or food) to be dosed is colored because it is very difficult to see the change of color.
There is also a strong interference with ascorbic acid (C vitamin ).
Many other chemical species react with iodine (which can be reductive or oxidative).
Some test strips are based on this principle, therefore unusable or unreliable in 90% of cases.
the method using of ellman's reagent (DTNB):
It is the method used in oenological labs on the automatic analysis equipments
This method shows very little interference with other chemical species, the reagent is very stable (can be kept for several years)
This method requires the use of a spectrophotometer (no visual analysis possible)
sulfitester is based on this method
3) ENZYMATIC METHOD:
An enzyme (sulfite oxidase) converts sulphite ions (s03--) to sulphate ions (SO4--)
SO32- + O2 + H2O -> SO42- + H2O2
An absorbance change is measured using a UV spectrophotometer (340 nm)
Enzymes are fragile, expensive, you need a UV spectrophotometer and interference with ascorbic acid is also present
4) COMBINE ENZYMATIC/ELECTROCHEMICAL METHOD:
You might know glucometers, small portable devices for dosing glucose in blood with a drop of blood on a strip.
This strip contains enzymes which by reaction produce a tiny electric current measured by the device.
Some laboratories work on the same principle to detect active sulphites in wines (target customers are winemakers).
But active sulftites are a minor part of the total sulphites responsible for intolerances or allergies.
Sulphitester uses the best price /quality ratio method with innovative colorimeter design and innovative reagent pre filled dose
NB: For more information on oenological analysis visit http://www.optomesures.fr/index.php/fr-fr/