tugas !!!! COLORIMETRIC ANALYSIS Substance in the two solutions is identical. For the convenient balancing of the two liquids several mechanical appliances have been devised for the purpose of increasing the speed of the determination or of increasing its accuracy. The Campbell-hurley type of colorimeter illustrated is the common form of such an instrument. This instrument is a modification of the kennicott-sargent type of apparatus and is sometimes known as the kennicott colorimeter. The following is the method of operation as illustrated by the diagram. The unknown solution to be tested is placed in the to be A and, since the volume can be readily governed so as to come to some even graduation, these are only placed at five c.c. intervals. The standard solution is placed in the right-hand to be B which, because of the fact that the accuracy of the test depends upon the careful reading of the volume in this tube, is graduated to single c.c. the readings of tubes A and B have been given as in c.c. but, for such readings to be accurate, tubes A and B must be of the same identical bore. The same effect is obtained in lass accurately made instuments by having the graduations to cm. rather than c.c. the proportion may then be obtained between the heights of the two columns, the sameas tho the units used were c.c. , since the colummns will have colors proportional to the depths regardless of their amplitude. The method used for graduation must be taken into account in figuring the results of the test and will be discussed under that head. Tube B is permanently connected by a glass tube with the reservoir C in which the glass plunger D works, so that the level of the liquid in B may be readily controlled by raising or lowering the plunger. As the tube B and the reservoir. C are made in one piece the standard comes in contact with glass only, thus preventing the possibility of chem. Ical change due to coming into contact with the container. The plunger D is sometimes provided with a rubber collar E to prevent it from coming into contact with the. COLORIMETRIC ANALYSIS Bottom of the reservoir and the resultant breakage possible. The tubes A and B and reservoir c rest on wooden support with holes under A and B for the passage of light. All glass parts are held in place by spring clips which allow for the easy removal of the parts for cleaning. For operation the colorimeter is turned with the back toward a window, preferably a north one, and the mirror G is so adjusted as to reflect skylight upward thru tubes and B. by this arrangement the back of the colorimeter serves as a sceen to cut off all right except that reflected upward from G. The light, passing upward thru the tubes A and B, impinges on the two mirror H and I cemented to brass plates sliding in grooves cut at. An angle of 45 in the sides of the wooden box J. this box has a loosely fitted cover so that it may be removed for the cleaning of the mirror. The mirror H is cut vertically and cemented in such a position as to reflect one half of the circular field of light coming thru the tube A. the light, passing upward thru B, is re-flected horizontally by the mirror I thru a hole in the brass plate supporting the mirror H. one half of the circular field of light from the tube B is cut off by the mirror H , the vertical edge of which acts as a dividing line between the two halves af the circular field. The image of one half of the tube B is then observed in juxtaposition to the opposite half if the image of the tube A. The juxtaposed images are observed thru a tube K, 2.5 cm. in diameter and 16 cm. long, lined with black felt and providedwith an eyepiece having a hole 1.5 mm. in diameter. At the H in the tube K is placed a diaphragm having an aperture 8mm. in diameter. All parts inside the box J except the mirrors are painted black so that no light except that coming thru the tubes A and B passes thruthe tube K. by having the aper. APPARATUS USED Tures in the eyepiece and the diaphragm properly proportioned only the images of the bottoms of tubes A and B can be seen, thus preventing the interference of side ligt, from the vertical sides of the tubes. A person looking thru the eyepiece observes a single circular field divided vertically by an almost imperceptible line when the two solutions are of the same intensity. By manipulating the plunger D the the level of the liquid in B can be easily raised or lowered, thus causing the right half to assume a darker or lighter shade at will. In matching colors with an ascending column in B, that is, gradually deepening the color of the right half of the field, the usual tendency is to stop a little below the true reading, while in comparison with a descending column the opposite is the case. At firs the operator should take a reading in each direction until after a little practice this tendency to error has been overcome. In tests on a large number of titanium solutions by oxidation with hydrogen peroxide, using all concentrations from a very dilute (light yellow) to a fairly concentrated (deep orange) solution, the average percent of error was found be less than one oercent and the median error less than one half percent. In some cases the depth of the column liquid observed cannot be used as a measure of the test substance present as the concentration changes not only the depth of the color but the color itself. An cases where that is so, it will be mentioned in connection with the derections for the particular test. A modified form of balancing instrument has a mirror directly over the tubes A and B reflecting forward the two circles of light which come to it. The mirror is encased in a light proof box painted black on the inside.
Posted on: Mon, 31 Mar 2014 10:14:57 +0000
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