4

The Determination of Benzoyl Peroxide in Flour

BY JOHN RALPH NICHOLLS, B.Sc., F.I.C.

BENZOYL peroxide is sometimes used for bleaching flour, and the usual methods for its detection depend upon the oxidising action of the unchanged peroxide. In effecting its bleaching action the peroxide is reduced to benzoic acid and, if the whole of the added peroxide is so changed, none remains to give an indication that the flour has been treated. A method which would determine the residual benzoic acid would serve this purpose.

The proportion of benzoyl peroxide which is used is about 1 part to 50,000 parts of flour. In order to be able to test reasonable quantities of flour, say 50 grms., it is necessary to be able to detect and to determine amounts of benzoic acid of the order of a milligram.

I have described a method (ANALYST, 1928, 53, 19) for determining small quantities of benzoic acid, depending upon its controlled oxidation to salicylic acid and the determination of the latter colorimetrically. The success of the method for quantitative work depends upon obtaining a constant proportion of salicylic acid from the benzoic acid. Recently it has been observed that this constant proportion is not found when the whole of the solution, obtained as described (Zoc. ci t . , p. 27), and containing very small quantities of benzoic acid, is oxidised. For example, 1 mgrm. of benzoic acid extracted from a foodstuff indicated rather less than 0.5 mgrm.; 2 mgrms. indicated slightly over 1 mgrm.; and 5mgrms. indicated 4mgrms. It has been shown that these differences are not due to incomplete extraction of the benzoic acid, but are to be attributed to

Publ

ishe

d on

01

Janu

ary

1933

. Dow

nloa

ded

by C

arne

gie

Mel

lon

Uni

vers

ity o

n 26

/10/

2014

08:

22:1

8.

View Article Online / Journal Homepage / Table of Contents for this issue

NICHOLLS: THE DETERMINATION OF BENZOYL PEROXIDE I N FLOUR 5

the test being carried out on a solution obtained by extraction from an immiscible solvent. In spite of the fact that the solution is boiled to remove the traces of dissolved solvent, something remains which reduces the proportion of benzoic acid converted into salicylic acid. If, instead of the immiscible solvent being extracted with alkali, the solvent is evaporated, the residual benzoic acid can be determined satisfactorily.

With this modification the method described (Zoc. cit.) gives a good colour with 1 mgrm. of benzoic acid, and even one-quarter of this quantity gives a perceptible tint when compared with a blank. It therefore appeared sufficiently delicate for the purpose in view.

CONVERSION OF BENZOYL PEROXIDE INTO BENZOIC AcID.-Flour which has been treated with benzoyl peroxide may contain part of the peroxide unchanged, and this must be converted into benzoic acid before the above method of determination can be applied.

Benzoyl peroxide is a remarkably stable substance, considering its composition. It can be heated with water and acids without decomposition, and it can be distilled unchanged in steam. I t is insoluble in water, and while insoluble it is very resistant to oxidising and reducing agents. It dissolves in most organic solvents, and in these solutions its oxidising character is readily shown. For example, when it is dissolved in alcohol the addition of sodium hydroxide solution causes instant decomposition of the peroxide, the alcohol being oxidised to acetaldehyde and acetic acid in proportions varying with the conditions. That the reduction of the peroxide is practically instantaneous can be shown by diluting the alcoholic solution with water immediately after the addition of the alkali; no turbidity (due to insoluble peroxide) is produced. An interesting reaction is that obtained when acetone is the solvent. According to the conditions the oxidation of the solvent may proceed in one of the following ways:

(1) 2CH3COCH3 + 0, = 2CH3CH0 + 2HCHO

(2) CH3COCH3 + 0, = CH3COOH + HCHO = CH,CHO + HCOOH

(3)

(4)

2CH3COCH3 + 30, = 2CH3COOH + 2HCOOH

CH3COCH3 + 20, = CH3COOH + CO, + H,O.

The principal factor affecting the reaction is the strength of the alkali used. By adjusting the conditions so that the reaction proceeds according to (1) the alkali used in combining with the only acid formed, i.e. the benzoic acid produced from the peroxide, is a measure of the peroxide originally present. This forms a rapid method of determining the strengths of commercial preparations containing benzoyl peroxide. The conditions are obtained by adding one volume of N/10 sodium hydroxide solution to one volume of acetone solution of benzoyl peroxide. Immediately after mixing, the solution should be diluted with four or five volumes of water to prevent reaction between the aldehydes and the alkali. The excess of alkali is determined by titration with N/10 acid, phenolphthalein being used as indicator. Provided the acetone and sodium hydroxide solution does

Publ

ishe

d on

01

Janu

ary

1933

. Dow

nloa

ded

by C

arne

gie

Mel

lon

Uni

vers

ity o

n 26

/10/

2014

08:

22:1

8.

View Article Online

6 NICHOLLS : THE DETERMINATION OF BENZOYL PEROXIDE IN FLOUR

not stand for more than a minute or two before dilution, the reaction does not proceed measurably beyond equation (1) and is completely represented by

2 C6H,CO-O

I + 4NaOH + 2CH,COCH, = 4C6H5COONa + 2CH,CHO + C6H5CO-O 2HCHO + 2H,O

One ml. of NIIO sodium hydroxide solution = 0.0121 grm. of benzoyl peroxide. For the purposes in view any extracted benzoyl peroxide was converted into

benzoic acid by dissolving it in acetone and adding sodium hydroxide.

both these substances are volatile in steam, a distillation process appeared most suitable, and it was found that in an acid calcium chloride solution 50 or even 100 grms. of flour could conveniently be distilled, yielding the whole of the peroxide or acid in about 300 ml. of distillate. From the distillate the peroxide and acid were extracted by an immiscible solvent; the peroxide was converted into acid as indicated above ; oxidisable substances were removed by treatment with potassium permanganate, and the purified benzoic acid was extracted. I t is at this stage that the immiscible solvent should be evaporated and the test for benzoic acid carried out on the residue. This applies to all determinations of benzoic acid by the method described (Zoc. c i t . ) , irrespective of the solvent used or of the process of extraction.

Details of the Method-About 40 grms. of calcium chloride and 100 ml. of water are placed in a litre flask; 60 grms. of the flour to be tested and 10 ml. of concentrated hydrochloric acid are added, and the flask is connected with a steam- distillation apparatus. Steam is passed, very cautiously at first, until the mass ceases to froth and becomes fluid, and then rapidly, the flask being heated to prevent increase in the bulk of the liquid. About 300 mi. of distillate are collected, saturated with common salt, and extracted twice with 50 ml. of ether. The ether is evaporated at a temperature of about 30" C., a rapid current of air being drawn over the surface. When the residue is practically dry 5 ml. of acetone are used to wash down the sides of the vessel and to dissolve the residue, and 5 ml. of 2 N sodium hydroxide solution are added. The mixture is diluted with an equal volume of water, and the acetone is boiled off. After the liquid has been cooled to 50 to 60" C. it is treated with permanganate solution until an excess is evident by the colour, when 10 ml. of 2 N sulphuric acid are added, and the solution is just decolorised by the addition of oxalic acid. Any insoluble fatty acids are filtered off, and the solution is extracted twice with about 20 ml. of a mixture of equal parts of ether and petroleum spirit. The extracts are evaporated to dryness, conveniently in a boiling-tube, at about 30" C., a current of air being drawn over the surface. When the last trace of solvent has disappeared any benzoic acid will be visible as a crystalline deposit. The residue is dissolved in 15 ml. of warm water, the solution is cooled, 1 ml. of iron solution (2.7 grms. FeC1,+13 ml. of N sulphuric acid to 100 ml.), and 1 ml. of 0.1 per cent. hydrogen peroxide solution (1 ml. of 20 vol. hydrogen peroxide diluted to 60 ml. = 0.1 per cent.) are added, and the mixture is heated just to the boiling-point. After the addition of 0.5 ml. of N sodium hydroxide solution, the mixture is filtered while hot, and the precipitate is

ISOLATION OF BENZOYL PEROXIDE AND BENZOIC ACID FROM FLOuR.-Since

Publ

ishe

d on

01

Janu

ary

1933

. Dow

nloa

ded

by C

arne

gie

Mel

lon

Uni

vers

ity o

n 26

/10/

2014

08:

22:1

8.

View Article Online

ELSDON AND STUBBS FREEZING-POINT OF PASTEURISED AND STERILISED MILKS 7

washed with hot water until the filtrate measures 50 ml. To the cold filtrate one drop of iron solution is added, and the colour produced is matched by adding 0.01 per cent. salicylic acid solution to a solution obtained by adding the reagents to 16 ml. of water and completing the test as described above. The salicylic acid solution is standardised against the colour produced when the test is carried out on a known quantity of benzoic acid.

EXPERIMENTAL REsuLTs.-Tests carried out on a number of untreated flours gave solutions indistinguishable from blanks. Flours to which either 1 mgrm. of benzoyl peroxide or 1 mgrm. of benzoic acid per 50 grms. had been added, gave colours equivalent to that obtained from 1 mgrm. of benzoic acid. A flour to which had been added 8 ounce of 25 per cent. benzoyl peroxide to 2801bs. (= 1 part in 35,840) was tested, and duplicate determinations showed 1 part in 40,000 and 1 part in 33,000.

I wish to thank the Government Chemist for permission to publish this work. GOVERNMENT LABORATORY,

LONDON, W.C.2

Publ

ishe

d on

01

Janu

ary

1933

. Dow

nloa

ded

by C

arne

gie

Mel

lon

Uni

vers

ity o

n 26

/10/

2014

08:

22:1

8.

View Article Online