Bleaching Agents and Methods Questions and Answers

Answer: C

A reducing agent such as bisulphite destroys the last of the active chlorine. Without it the cloth yellows and loses strength over the following weeks, sometimes long after it has left the mill. Tendering in storage is the classic symptom.

Answer: D

Both are true and the reason gives the necessity: the damage develops slowly and shows up as tendering in storage, which makes it a particularly awkward fault to trace back to its cause. Bisulphite is the usual antichlor.

Answer: A

Correct answer: the chlorine dioxide it releases

Sodium chlorite works at acid pH and gives an excellent white with very little fibre damage, but it liberates chlorine dioxide, which is toxic, irritant and corrodes ordinary stainless steel. The equipment must therefore be titanium or a specially resistant alloy and the vessels must be extracted, which is what limits its use to fibres such as polyester-cotton and some synthetics where its gentleness is worth the trouble.

Answer: B

The first two are correct. The third is inverted, since chlorite bleaching is an acidic process and it is precisely the acidity that liberates the chlorine dioxide that does the work. Titanium plant is needed throughout.

Answer: C

Dithionite is a reducing bleach and works by a quite different mechanism. Reductive bleaching does not destroy the coloured compounds permanently, so the colour can return on exposure to air. Its use is confined to fibres that oxidising bleaches damage.

Answer: D

Protein fibres are damaged by oxidising bleaches, so a reducing agent is used instead. The white is less permanent, which is why wool often receives a combined oxidative and reductive treatment. Peroxide alone yellows the keratin.

Answer: A

The reduction is reversible, so atmospheric oxygen can regenerate part of the colour. This is why the combined treatment or an optical brightener is often used alongside it. The effect is reversed by atmospheric oxygen.

Answer: B

Correct answer: 2 - 4 - 1 - 3

The fabric is saturated with the liquor, held hot in the J-box for the dwell time the reaction needs, then washed to remove the spent liquor and the dissolved impurities, and finally neutralised and dried. Washing before the dwell would remove the bleach before it had acted, and drying before washing would fix the impurities into the cloth.

Answer: C

Thirty-five per cent of twenty grams is seven grams of the pure compound. Recipes are quoted sometimes on the commercial product and sometimes on the pure basis, and the two must never be confused. Titration of the bath settles the question directly.

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: a recipe may be written on the commercial 35 per cent solution or on 100 per cent peroxide, and the two differ by a factor of about \( 1/0.35 = 2.9 \). Statement 3 is exactly wrong - the distinction matters greatly, since confusing the two bases means dosing at roughly a third or three times the intended strength, which either fails to bleach or damages the fabric.