TESTS FOR MIXTURES

                                            MIXTURES TESTS

       Of further tests the tirst should be to determine if the dye stuff is a mixture .A large number of commercial dye stuffs are mixtures but a competent dye house will find few of them worth buying ; the value test previously given will eliminate most of them .It is a good general rule never to buy a mixture  though dye manufacturer has after much experiment found a blend of two or which exhausts on tone in jigger or warp dyeing and such blends if tinctorially of good value are useful.In most cases however a skilled dyer will produce results as good as those given by the mixture and more cheaply with individual dye stuffs whose properties he knows precisely and which are normally kept in the dye stores.

       Dye stuff mixtures are usually prepared by blending powders and only very occasionally by salting out a mixed solution .Therefore if a small amount is picked up on the end of a spatula and blown on to a piece of wet filter paper or in some cases on to cons . Sulphuic acid in a watch glass the individual specks become separated in space and dissolved separately according to their color . It is often possible to make a rough estimate of the number of dye stuffs their proportions and colors . A small percentage of one color additional to the main dye stuff may be permitted as a legitimate addition for shade standardization whilst many dye stuffs connote be made without the presence of homologues etc.

        The average size of the molecular aggregates in a dye stuff solution varies very considerably from one dye stuff to another and as the aggregate size determines the speed at which the dye stuff passes through the pores of filter paper it is useful to put a drop of solution on to some thick filter paper and observe the way the frop spreads. A mixture usually shows rings of color the more soluble and less aggregated dye stuff travelling farther and more rapidly than the rest.

        Sharper separation may sometimes be obtained by putting the drop adjacent to a previously applied drop of strong salt solution .

       This test may be modified by hanging an inch wide strip of filter paper then (10) ins. long vertically over a dish of dye stuff solution with the lower end just dipping into the liquid. After perhaps ten minutes there will be distinct separation into zones . A mixture in which the colored zones are close together is in general likely to be better balanced as regards rate of exhaustion on dyeing than a mixture in which the zones are widely separated.Dye stuffs that contain such diluent as dextrin will climb hardly at all nor will very aggregated dye stuffs of the quick exhausting bad leveling type . Roughly speaking dye stuffs that climb to (80%)or more of the height of their solution water are good leveling acid colors a climb of (70%)indicates a neutral wool color and less than (40%)indicates a direct cotton color . There is however some overlap and some colors greatly aggregated in cold solution are well dispersed in a boiling dye bath. The capillary strip method may be used to examine the effect of salt and acid additions. Often in a mixture the addition of a suitable amount of salt will repress the climb of one component completely.

       The method may be extended into a chromatographic analysis but this is not suitable for routine examination and moreover there is little published work on dye stuffs . It is however now clear that complete and clean separations of the component dye stuffs in a mixture and also of impurities present may be accomplished in many cases be choice of an adsorbent of suitable particle size and chemical affinity coupled with a suitable elutriant.

      Other methods of examination for mixtures are 1 dyeing six hanks with zero one two four eight and (16%)of salt 2 preparing a boiling dye bath and dyeing six hanks successively for one minute each 3 dyeing one hank at fifty six to sixty deg. C. for a quarter of an hour and then exhausting the bath by dyeing with a second hank followed by dyeing at seventy deg. C.with a cotton hank . In all these cases changes in hue indicate mixtures.