Analytical Method: Sulfite in Shrimps

Instruments and Reagents

Product No. 173016 Spectroquant^® VIS Spectrophotometer Prove 100 or
Product No. 173017 Spectroquant^® UV/VIS Spectrophotometer Prove 300 or
Product No. 173018 Spectroquant^® UV/VIS Spectrophotometer Prove 600 or
Product No. 109748 Spectroquant^® Photometer NOVA 30 or
Product No. 109751 Spectroquant^® Photometer NOVA 60 or
Product No. 173632 Spectroquant^® Colorimeter Move 100
Product No. 114946 Rectangular cells 10 mm and/or
Product No. 114947 Rectangular cells 20 mm and/or
Product No. 114944 Rectangular cells 50 mm
Product No. 101746 Spectroquant^® Sulfite Test
Product No. 109535 MColorpHast™ Universal indicator strips pH 0 - 14
Product No. 137085 Sodium hydroxide solution 0.05 mol/l
Product No. 108418 Titriplex^® III
Product No. 116754 Water for analysis

Reagent 1

Place about 50 mL 0.05 molar sodium hydroxide solution into a 100-mL Erlenmeyer flask. Subsequently dissolve 0.005 g Titriplex^® III in it and make up to the mark with sodium hydroxide solution. Cool this solution with an EDTA concentration of 50 mg/l in the refrigerator or on ice to about 0 - 4 °C.

Reaction

In neutral solution, sulfite ions react with 2.2'-dinitro-5.5'-dithiodibenzoic acid (Ellman's reagent) to form an organic thiosulfate. This reaction results in the release of a thiol that is determined photometrically.

Measurement Range

1.0 - 60.0 mg/l SO₃^2-

Sample Preparation

Homogenize 10 g of ready-to-cook shrimps and 50 mL of cooled sodium hydroxide/EDTA solution in a mixer for about 1 minute. Subsequently transfer the entire sample, with the aid of another 50 mL of sodium hydroxide/EDTA solution and 100 mL of ice-cooled water for analysis, to a 250-mL volumetric flask and stir on a magnetic stirrer for about 20 min. Any foam present should disappear. Subsequently make up to the mark with water for analysis and swirl the flask to homogenize. Then filter through a fluted filter, discarding the first few milliliters.

Note: The shrimps must be processed rapidly, as sulfite is quickly oxidized in air. Opened packages have only about ¼ of the original content after 24 hours.

Analysis

The measurement is carried out with the above mentioned test set.

Calculation

Measured value [mg/l] x 25 = sulfite content [mg/kg] (kg and l equalized)

Results

Different shrimp samples were tested.

Sample	Measured Value [mg/l] Spectroquant®	Recovery Rate [%]
Shrimp 1	40.2	-
Shrimp 1 spiked with 2 mg/l sulfite	42.7	101.2
Shrimp 1 spiked with 4 mg/l sulfite	44.3	100.2
Shrimp 1 spiked with 6 mg/l sulfite	46.1	99.8
Shrimp 1 spiked with 8 mg/l sulfite	48.0	99.6
Shrimp 1 spiked with 10 mg/l sulfite	49.6	98.8

Sample	Measured Value [mg/l] Spectroquant®	Recovery Rate [%]
Shrimp 2	37.8	-
Shrimp 2 spiked with 2 mg/l sulfite	39.6	99.5
Shrimp 2 spiked with 4 mg/l sulfite	40.3	96.4
Shrimp 2 spiked with 6 mg/l sulfite	41.2	94.1
Shrimp 2 spiked with 8 mg/l sulfite	45.2	98.7
Shrimp 2 spiked with 10 mg/l sulfite	48.6	101.7

Sample	Measured Value [mg/L] Spectroquant®	Comparative Measurement Value [mg/L] MColortest® Product No. 111148
Shrimp 3 measurement 1	41.8	41.3
Shrimp 3 measurement 2	42.6	40.6
Shrimp 3 measurement 3	41.7	41.3