TOTAL SULFITES IN FOODS

UIC, Inc.Adaptation of the Modified Monier-Williams Method

PRINCIPLES OF OPERATION

As in the modified Monier-Williams method, samples are treated with acid and heated to release SO₂. A nitrogen carrier gas sweeps the SO₂ through a reflux condenser and into the S Coulometer cell. The S Coulometer automatically titrates the SO₂ and digitally displays the amount.

PROCEDURE

1. Assemble and prepare the components for operation as described in the instruction manuals.
2. Determine the blank and run a standard to confirm proper operation of the system.
3. Charge the sample tube with a known weight of sample and attach the tube to the apparatus. Sample size should be selected to contain 1-3mg of SO₂.

   Note: Solid and viscous liquid samples may either be weighed out directly into the sample tubes or weighed out into porcelain, platinum, alumina or similar sample containers and then placed in the tube. Water and liquid samples handles with a syringe can be directly injected through the septum at the top of the apparatus.

4. Move the sample tube into position over the heater, pump acid into the reaction tube, and reset the coulometer.

   Note: When injecting samples, a sample tube with a stopcock can be used. This eliminates the need to open the system between sample runs.

5. 5. When all of the SO₂ has been evolved and titrated, the CM5017S automatically detects the endpoint, ends the analysis and prints the result to the printer and/or diskette.

(*) – Endpoint determination and result calculations are performed automatically based on user selectable settings entered into the CM5017S Coulometer.

RESULTS

When samples contain over 1 mg SO₂, the coulometer measurement repeatability is +/-0.15%. Reproducibility is typically +/-0.5% at 1000 ppm. When sample availability or volume limits the amount of SO₂ evolved to small amounts, the precision is generally better than 1 ug SO₂. Sample stability can limit both the accuracy and precision of the test.

Analysis times are typically 10 to 15 minutes. Some samples react very slowly, thus extending the analysis time.

A major advantage of the S Coulometer is the analysis completion can be observed. Low results due to incomplete reactions and wasted time can thus be avoided. Other advantages include the easy operation of the system and option modification for using different acids, carrier gases or scrubbers for different applications.

Table I

Comparison of Total Sulfites in Dried Fruit (% SO₂)

Table II

Total Sulfites in Beer (mg SO₂/L)

Table III

Total Sulfites in Wine (mg SO₂/L)