Congo red test

As already indicated, the measurement of dehydrochlorination rates is not a practical way of assessing the effect of a stabiliser. Thus the congo red test sometimes specified in standards, in which a piece of congo red paper is held in a test tube above a quantity of heated PVC and the time taken for the paper to turn blue due to the evolution of a certain amount of hydrogen chloride, cannot be considered as being of much value. 

Excess nitrous acid causes an immediate blue color at the point of contact with starch-iodide test paper. At all times there must be an excess of mineral acid (Congo red test paper). 

The mixture is filtered hot and the insoluble residue is washed with distilled water. The total filtrate is now partially neutralized with 300 cc. of 40 per cent sodium hydroxide solution, while the mixture is well stirred and cooled, and then a saturated solution of 3750 g. of crystallized sodium acetate is added. The Congo Red test for mineral acid should then be entirely negative. Care must be taken not to make the solution alkaline with sodium hydroxide (Note 7). After standing for three days at room temperature, the precipitated cystine is filtered on a suction iunnel. This crude material, containing in addition to the... 

After testing the suspension for complete acidification (with Congo red test paper), it is transferred by compressed air, while still hot (70°C), to a cloth-lined vacuum filter. The dinitrophenol is washed on the filter 3-4 times with water until the acid content in the washings is reduced to 0.01%. Then it is centrifuged in copper ... 

Assay Preparation Transfer about 4 g of sample, accurately weighed, into a 250-mL Erlenmeyer flask, and add 80 mL of 0.5 N potassium hydroxide and 0.5 mL of phenolphtha-lein TS. Connect an air condenser at least 65 cm long to the flask, and heat the mixture on a hot plate for about 2.5 h. Remove the air condenser and add approximately 10% phosphoric acid to the hot mixture until it is definitely acid to Congo red test paper. Reconnect the air condenser, and heat until the fatty acids are liquified and clear. Cool, and transfer the mixture into a 250-mL separator with the aid of small portions of water and hexane. Extract the liberated fatty acids with three successive 25-mL portions of hexane, and collect the extracts in a second separator. Wash the combined hexane extracts with two 25-mL portions of water, and add the washings to the separator containing the water layer. Retain the combined hexane extracts for the determination of total fatty acids. Transfer the contents of the first separator to a 250-mL beaker, heat on a steam bath to remove traces of hexane, filter through acid-washed, fine-texture filter paper into a 500-mL volumetric flask, and finally dilute to volume with water (Solution I). Pipet 25.0 mL of this solution into a 100-mL volumetric flask, and dilute to volume with water (Solution II). Retain the rest of Solution I for the determination of Glycerin (below). 

Congo Red Paper. Detection by means of this paper depends on the formation of hydrochloric acid by the decomposition of dichloroethyl sulphide with sulphuric acid. The gas to be examined is passed through a wash-bottle containing concentrated sulphuric acid at 55° C., and then over the Congo red test-paper. 

Because practically no PVC is processed and used without the addition of stabilizers, one should know the residual stability of a used PVC product. For this the best way may be the determination of the hydrogen chloride elimination at 180°C under air or nitrogen [50]. The conversion-time curves so obtained provide indication of the residual stability from the induction period and also enable calculation of the rate of HCl split-olf after consumption of the stabilizers. In some cases, however, it maybe sufficient to use a simple Congo Red test (e.g., according to DIN 53 418) instead of the apparatus for measuring the HCl elimination. 

Because of its invasive character and thickness of the tube, gastroscopy is not suitable for monitoring motor function of the oesophagus, stomach, and duodenum, or for pressure recordings. However parietal cell function can be evaluated endoscopi-cally by means of the Congo red test, a dye-spraying method (see below). 

---