FIXING, WASHING, AND DRYING 405 



To make a test solution add 1 cc. (J-'4 dram) of the above solution to 250 cc. (8 oz.) 

 of water in a clear glass container. Allow the wash water from several negatives or 

 prints to drip into the test solution. If the color of the solution remains unchanged 

 the photographic materials may be considered to be completely washed. The violet 

 color of the test solution will turn to orange in about 30 sec. if a small amount of hypo 

 from the drippings of the wash water is added. With larger amounts of hypo added 

 to the test solution, the color will turn to a greenish yellow. Change of color of the 

 test solution indicates incomplete washing. 



The success of this test depends upon the water being free from oxidizable organic 

 matter, for, if it is not, the organic matter will react the same as the hypo. To ascer- 

 tain that the water is free from organic matter, it may be tested in the following man- 

 ner. Prepare two samples of the permanganate-soda test solution using distilled 

 water. To one of these samples, add a known volume of drippings from the wash 

 water, then add an equal amount of tap water to the second test solution which is to 

 serve as a control. If the sample to which the tap water has been added remains 

 violet, the absence of organic matter is indicated. If the color of the solution is 

 changed slightly by the addition of tap water, the presence of organic matter is indi- 

 cated. In this case, the amount of hypo present in in the first solution will be indi- 

 cated by the relative color change of the two samples. If both samples turn the same 

 color, no appreciable amount of hypo is indicated as being present in the first solution. 

 If the sample to which tap water was added turns pink, whereas the sample to which 

 hjrpo was added turned yellow, the presence of hypo would be indicated in the water 

 in which the prints were being washed. 



Electrical methods of measuring conductivity of solutions may be used to deter- 

 mine the presence of hypo. The simplest of these methods, which is not so sensitive 

 as some of the chemical tests, makes use of an ohmmeter such as is used in radio-set 

 test equipment. The ohmmeter consists of a d'Arsonval or similar type of d-c 

 measuring instrument, a small dry battery, and a variable resistance for adjusting the 

 meter to its initial or zero setting, all connected in series with two terminals which are 

 insulated from one another. In use, the terminals of the instrument are placed in 

 clear water, and the rheostat is adjusted to the zero reading of the meter. When it is 

 desired to test a solution for the presence of hypo, the terminals are immersed in the 

 M^ash water, and the meter reading is observed. If the meter reads zero, the absence 

 of hypo is indicated. If the meter reads some value greater than zero, the amount of 

 hypo present is roughly proportional to the meter reading. 



In order that such a device may be independent of the extent to which its electrodes 

 are immersed, it is essential that the same area of the electrodes always be immersed 

 when making measurements. Usually, in most commercial instnmients, the elec- 

 trodes are exposed a given amount, the remaining area being protected by insulation. 

 The device will then give significant readings so long as the exposed portions of the 

 electrodes are completely immersed in the liquid. The success of such a device 

 depends to a large extent upon the sensitivity of the meter, upon the area of 

 the exposed electrodes, and upon the separation between the electrodes. 



]Vashing of Prints. — Hickman and Spencer in the Photographic Journal, 1925, 

 vol. 63, page 443, showed that prints required a greater washing time than plates or 

 film. While the larger part of the hypo is removed from the emulsion in a compara- 

 tively short time, a certain amount is tenaciously retained by the fibers of the paper 

 support and is difficult to remove. For this reason much longer times of washing are 

 required for prints than for plates or film. Prints on good paper should be washed at 

 least 30 min. in a running stream of water while the simple double-weight paper should 

 receive a 1- to ll2-hr. washing. Increasing the velocity of the water or the flow of 

 water over the print does not decrease the time of washing correspondingly as is the 



