82 CHEMICAL EXAMINATION 



chloric acid, containing 0.2 grm. of ferric chloride per litre, 

 are added to 5 c.cms. of milk in a porcelain basin and the mix- 

 ture heated to boiling with constant stirring. The presence of 

 formaldehyde is indicated by a violet colouration. 



Shrewsbury and Knapp Test. 4 The reagent for this 

 test consists of a freshly prepared mixture of pure concentrated 

 hydrochloric acid with 0.1 per cent of pure nitric acid. 5 c.cms. 

 of the sample are placed in a test tube and vigourously shaken 

 with 10 c.cms. of the reagent, the mixture is heated in a water 

 bath to 50 C. for ten minutes and finally rapidly cooled to 

 about 15 C. A violet colouration denotes the presence of 

 formaldehyde, and a rose pink colouration, its absence. The 

 depth of the colouration, between 0.2 and 6 parts per million, 

 is approximately proportional to the amount of formaldehyde 

 present, so that this method may also be used for the estimation 

 of the preservative. When the amount exceeds six parts per 

 million, the milk should be suitably diluted. 



Estimation of Formaldehyde. In addition to the method 

 previously mentioned, various others have been devised for the 

 estimation of formaldehyde, but not one as yet can be relied 

 upon to give accurate results. Most of these are based upon the 

 volatilisation of the aldehyde by distillation of an acid solution, 

 and subsequent volumetric estimation. Probably the most 

 useful is the following. To 100 c.cms. of sample contained in a 

 500 c.cm. Kjeldahl flask add 1 c.cm. of 1 : 3 sulphuric acid and 

 distil over 20 c.cms. (care is necessary if frothing is to be 

 avoided). The formaldehyde in the distillate, amounting to 

 approximately one-third of the total, is estimated iodometrically. 



N 

 25 c.cms. of iodine solution are added to the distillate and 



normal caustic soda is added, drop by drop, until the liquid 

 becomes a clear yellow. After standing for fifteen minutes, 

 dilute sulphuric acid is added in excess to liberate the uncom- 



N 

 bined iodine. The solution is then titrated with sodium 



thiosulphate, using a starch solution as the indicator in the end 



