SOP: Quantitative Estimation of Coal-Tar Dyes by the Titanium Trichloride Method

GIFSQ — Global Institute for Food Safety & Quality

Principle

The quantitative estimation of a single food colour is carried out by reduction titration with titanium trichloride. The titanium trichloride is standardized against ferrous ammonium sulphate and potassium dichromate, and the dye content is calculated from the titre and the tabulated dye equivalent (Table 12-4). The dye acts as its own indicator, except tartrazine.

Apparatus required

• 500-ml conical flask
• Gas inlet for a continuous stream of CO₂
• Microburette or burette

Reagents

a. 0.1 N Titanium trichloride (TiCl₃): dissolve 30 g of titanium trichloride in water and dilute to 200 ml; add 200 ml of concentrated HCl (sp. gr. 1.16), boil the mixture for 1 to 2 min, pour into cold water, and dilute to 2000 ml. Preserve the solution under a layer of liquid paraffin in an atmosphere of CO₂.

b. Ferrous ammonium sulphate [FeSO₄·(NH₄)₂SO₄·6H₂O].

c. 40% w/v H₂SO₄.

d. Ammonium thiocyanate (NH₄CNS).

e. 0.1 N Potassium dichromate (K₂Cr₂O₇).

f. Recently boiled distilled water.

g. The dye and the appropriate buffer salt (Table 12-4).

Procedure

Standardization of titanium trichloride

Weigh 3 g of ferrous ammonium sulphate into a 500-ml conical flask

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Pass a stream of CO₂ continuously until the end of the titration

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Without disturbing the stream, add rapidly 50 ml of recently boiled distilled water, 25 ml of 40% H₂SO₄, and 40 ml of 0.1 N potassium dichromate solution

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Titrate with titanium trichloride solution until the calculated end point is nearly reached

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Add 5 g of ammonium thiocyanate and complete the titration; note the volume of titanium trichloride used (A)

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Carry out a blank determination on 3 g of ferrous ammonium sulphate using the same quantities of water, H₂SO₄ and ammonium thiocyanate, with the continuous CO₂ stream; note the titre (B)

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Subtract B from A to get the actual volume of titanium trichloride required

Determination of dye content of sample

Weigh the dye and the appropriate buffer (Table 12-4) salt into a 500-ml conical flask

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Add 150 ml of water; pass a stream of CO₂ into the flask, heat the solution to boiling, and titrate with titanium trichloride solution, maintaining a steady stream of CO₂

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Add titanium trichloride dropwise towards the end of the titration

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The dye acts as its own indicator — except with tartrazine, for which light green SF yellowish is added as indicator

Calculation

Normality of TiCl₃ = (ml of K₂Cr₂O₇ × Normality of K₂Cr₂O₇) ÷ ml of TiCl₃

Dye content in the sample (%) = [ml of TiCl₃ required × Normality of TiCl₃ × Wt of dye equivalent to 1 ml of TiCl₃ × 100] ÷ [Wt of sample × 0.1]

Where,

• ml of TiCl₃ required = the titre of titanium trichloride solution used (ml)
• Normality of TiCl₃ = the standardized normality of the titanium trichloride solution
• Wt of dye equivalent to 1 ml of TiCl₃ = the tabulated value for the dye from Table 12-4 (g)
• Wt of sample = the weight of the dye sample taken (g)

Table 12-4 data (dye, weight of dye taken, buffer salts, dye equivalent of 1 ml of 0.1 N TiCl₃, dye content not less than, absorption maximum, extinction coefficient):

• Amaranth (C₂₀H₁₁N₂O₁₀S₃Na₃): 0.85–0.90 g; 10.0 g trisodium citrate, 15.0 g sodium hydrogen tartrate; 0.01511 g; 85%; 525 nm; 417.0
• Fast red E (C₂₀H₁₂N₂O₇S₂Na₂): 0.24–0.30 g; 15.0 g sodium hydrogen tartrate; 0.01256 g; 85%; 515 nm; 457.0
• Ponceau 4R (C₂₀H₁₁N₂O₁₀S₃Na₃): 0.70–0.80 g; 10.0 g trisodium citrate; 0.01511 g; 70% (printed “7o”); 515 nm (printed “5TS”); 444.0
• Sunset yellow FCF (C₁₆H₁₀N₂O₇S₂Na₂): 0.60–0.70 g; 10.0 g trisodium citrate; 0.01131 g; 85%; 485 nm; 497.0
• Tartrazine (C₁₆H₉N₄O₉S₂Na₃): 0.75–0.80 g; 15.0 g sodium hydrogen tartrate; 0.01336 g; 85%; 430 nm; 462.0 — add 1.0 ml of 1% aqueous light green SF yellowish as indicator near the end point; run a blank omitting only the tartrazine sample
• Brilliant black BN (C₂₈H₁₇N₅O₉S₄Na₄): 0.60–0.70 g; 15.0 g sodium hydrogen tartrate; 0.01086 g; 70% (printed “jo”); 580 nm; 533.0
• Indigotine, indigo carmine (C₁₆H₈N₂O₂(SO₃)₂Na₂): 0.50–0.60 g; 15.0 g sodium hydrogen tartrate; 0.02332 g; 85%; 610 nm; 495.0
• Carmoisine, azorubine (C₂₀H₁₂N₂O₇S₂Na₂): 0.75–0.85 g; 15.0 g sodium hydrogen tartrate; 0.01256 g; 85%; 525 nm; 520.0
• Erythrosine: 85% (determined separately by the gravimetric procedure)

Note: the italicised values in parentheses are flagged — several figures in the available text are OCR-garbled (“7o”, “5TS”, “jo”), and the molecular formulas are carried as interpreted from the garbled readings (e.g. “CyoHy,NzO19S3Na” for amaranth); consult the printed book for Table 12-4.

Note: the book states that the method above applies to all coal-tar dyes permitted under the Fruit Products Order or the Prevention of Food Adulteration Act except Erythrosine, which is determined gravimetrically.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.