Exercise

Practical 13 - Estimation of Sulphur Dioxide

Aim: To estimate the level of sulphur dioxide contents in fruit and vegetable products

Theory and principle:
Sulphur dioxide in food product is added as class II preservative and may exists as undissociated sulphurous acid, as free bisulphate or sulphite ions or as combined SO2 in the form of hydroxy sulphonates. Sulphur dioxide is measured as free or total SO2. Estimation of SO2 in squashes, jam, RTS drinks is important as its presence beyond the prescribed FPO limits make the product not confirming to Indian Food Laws. Generally 2 parts of KMS (Potassium metabisulphite) releases one part of SO2 explained as under:
SO2 is added in the form of potassium meta-bisulphite (KMS)

K2S2O5=2(SO2) +K2O

(39x2)+(32x2)+(16x5) = 2(32+32)
78+64+80 = 2(64)
222 = 128

128 x 100
% SO2 in KMS = ------------------- = 57.70%
222
Thus, 1g of KMS releases 0.57g of SO2.

Free SO2 is measured by direct titration with iodine. In the estimation of total SO2 the combined SO2 is liberated by treatment with excess alkali at room temperature, subsequent acidification to prevent recombination and titration with iodine. The method used is known as Modified Ripper Titration Method.

Reagents, glassware and apparatus required
  1. Diluted H2SO4 (1+3): 1 part H2SO4 in 3 parts of water
  2. 0.02 N Iodine solutions: Dissolve 2.53g of iodine in a solution of 24g potassium iodide in 200ml water and dilute to 1 litre (Standard Iodine solution in ampoule can also be used).
  3. 1% Starch indicator: Mix 1 g soluble starch with 15 ml of cold water and pour into 100 ml of hot water. Boil for 1-2 min.
  4. Sodium carbonate
  5. 5 N NaOH: Dissolve 200 g NaOH pellets in water and dilute to 1 litre in a volumetric flask. Standardize by titrating against standard H2SO4 solution.
  6. Formaldehyde (36-40%)
  7. Conical flask, volumetric flask, beaker, pipettes and burette.
Procedure: For estimation of Free SO2
  1. Take known weight (10-20 g) of sample (juice, squash, jam, pulp etc.) in two separate flasks.
  2. Add 5 ml of diluted H2SO4 (1+3) to each to acidify.
  3. Add 0.5 g sodium carbonate in the first flask to expel the air present in the sample
  4. Add 5-7 drops of starch solution as an indicator.
  5. Titrate immediately with 0.02 N iodine solution to blue colour end point, record titre value as ‘a’.
  6. To determine the titre due to iodine reducing substances other than SO2, acidify a similar aliquot of substance with 5ml of diluted H2SO4 (1+3).
  7. Add 10 ml of formaldehyde in the second flask and allow to stand for 10 minutes.
  8. Add starch solution as an indicator and titrate against iodine to a faint permanent blue colour and record titre value as ‘b’
  9. Record volume of iodine used by free SO2 present in the sample by subtracting titre b from titre a, (‘a¬¬-b’) and calculate free SO2 as per given expression.
Procedure for estimation of Total sulphur dioxide
  1. From the same sample taken for free sulphur dioxide, take a known volume of the sample in two flasks and add 5ml of 5 N NaOH to each.
  2. Stir gently and allow standing for 20 minutes.
  3. To one of the sample add 7 ml of 5 N HCl.
  4. Add 1 ml of 1% starch solution as an indicator and titrate immediately against 0.02 N iodine to a definite dark blue colour.
  5. Record the titre value as ‘c’. This titration indicates the total iodine reducing value of the sample.
  6. To determine the reducing substances other than sulphites like ascorbic acid acidify the second sample in the same way with 5 ml of diluted H2SO4.
  7. Add 10 ml of formaldehyde (36-40%) to bind the sulphite and allow the sample to stand for 10 minutes.
  8. Add starch solution as indicator and titrate rapidly with constant stirring (or use mechanical stirrer) against 0.02 N iodine solution until a dark blue colour appears which persists for atleast 15 seconds.
  9. Record the titre value as ‘d’
  10. Calculate the volume of iodine used by the total SO2 present in the sample by subtracting titre d from titre c (‘c-d)’
Observations

Sample No.

Weight of sample

Titre ‘a’

ml

Titre ‘b’

ml

Titre Free SO2

 (a-b)

Titre ‘c’

ml

Titre ‘d’

ml

Titre

Total SO2

(c-d)

Free SO2 ppm

Combined

Total SO2 ppm

1

2

3


Calculations:
1 ml of 0.02 N Iodine = 0.64 mg of SO2

Titre × 0.64 × 1000
Free SO2 (ppm) = --------------------------------
Weight of sample

Titre × 0.64 × 1000
Total SO2 (ppm) = ------------------------------
Weight of sample

Combined SO2 (ppm) = Total SO2 - Free SO2


Problem:
25 g of pineapple squash was analysed for estimation of free, total and bound SO2. The prepared sample was titrated against 0.02N Iodine solution for free SO2 to give a titre value of bound SO2 (a) as 10 ml. The titre for iodine reducing substances other than SO2 (b) was 2.0 ml. For estimation of total sulphur dioxide similar 25 g pineapple squash sample was prepared and titrated against 0.02N NaOH iodine solution. The titre for total iodine reducing substances (c) was recorded as 16 ml while titre for total iodine reducing substances other than sulphites was 4 ml. calculate Free SO2, Total SO2 and Combined SO2 in the pineapple squash.

Solution:
Free SO2
Weight of sample taken for estimation of free SO2= 25 g
Titre value of bound SO2 (a)= 10 ml

Titre value of iodine reducing substances other than SO2 (b) = 2 ml

a-b= 10-2= 8 ml
Titrex0.64x1000
Free SO2 ppm= ------------------------------
wt. of sample

8x 0.64x1000
= ---------------------
25

= 204.8 ppm

Total SO2
Weight of sample taken for estimation of total SO2 = 25 g
Titre value for total iodine reducing substances (c) = 16 ml
Titre value for total iodine reducing substances other than sulphites (d) = 4 ml

c-d= 16-4= 12 ml
Titrex0.64x1000
Total SO2= --------------------------
Wt. of sample

12x0.64x1000
= ---------------------
25

= 307.2 ppm


Bound SO2 = Total SO2- Free SO2
= 307.2-204.8
= 102.4 ppm

Inference: The given pineapple squash contained 307.2 ppm Total SO2, 204.8 ppm Free SO2 and 102.4 ppm Bound SO2.
Last modified: Wednesday, 7 March 2012, 10:51 AM