Colorant testing purity levels

Specifications and Standards. Typical specifications for phthahc anhydride are given in Table 10. AH specifications are measures of purity. Sohdification point is a sensitive indicator of absolute purity, and is a key specification. Another key specification is molten color stabiUty, which is the color after being held at 250°C for two hours. This test ensures acceptable color after shipment in molten form and detects the presence of impurities that can cause discoloration at elevated temperatures. Phthahc acid level is a monitor of how well moisture has been excluded during storage and shipment. 

While concepts like hue and purity easily lend themselves to verbal description, major difficulties arise in any attempt to discuss color depth or depth of shade. The only way to deal with this concept (as with other aspects of color) is to create specimens having the properties discussed. For this reason, a collection of samples equal in color depth was prepared around 1935 so that the fastnesses of various dyes could be meaningfully tested [7]. Studies by Raabe and Koch [5] and by Schmelzer [6] suggest that equal difference (or distance) from white is an essential criterion in selecting specimens of equal color depth. Some color-depth levels are set apart as standard depths of shade (SD) [6]. These find use particularly in the preparation of specimens for fastness testing they permit a consistent way for the presentation of different colorants and describing their effectiveness under various service conditions. 

Regulation also limits the levels of permitted impurities in color additives. Detailed standards of quality and purity have been incorporated into regulatory requirements, establishing maximum amounts of organic impurities such as subsidiary dyes and residues of starting materials, intermediates, or other contaminants. Each batch of color made must be tested for compliance with chemical specifications in order to be certified (130,142,151). Therefore, there is a need for rapid and reliable techniques to separate the dyes from impurities and to monitor the quality of commercial dyes (154). 

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