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Hydrometers
Mixers

Photography (Basic) - Introduction to photography and other graphic techniques
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Figure  9-2.–Hydrometer. reading is taken at the point where the top of the meniscus intersects the stem of the hydrometer (fig. 9-2). pH  METERS The acid or alkali state of a solution is measured in pH values. The pH value of developers and fixers influences their activity and proper strength. pH is basically a measure of the degree of acidity or alkalinity of  a  solution.  It  provides  an  invaluable  aid  in determining  the  degree  of  accuracy  with  which  the processing  solutions  have  been  prepared.  Photographic developers usually have a pH of 8 to 12, while fixers range between pH 3.1 and 5. The following scale indicates the location of acids and alkalis by their pH value (strength): pH  VALUES ACIDS NEUTRAL ALKALIS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 A pH of 7 is neutral. Working down from this point, the figures indicate weak acids with a pH of 6 on to strong acids with a pH of 1. Working up from a pH of 7, the figures indicate weak alkalis with a pH of 8 to strong alkalis with a pH of 14. The pH values are numbered on a logarithmic scale. From 0 through 6, each number indicates a degree of acidity 1/10 as strong as the preceding number, but 10 times stronger than the next succeeding or higher number. A solution with a pH value of 4, for example, has a degree of acidity 10 times stronger than a solution with a pH value of 5, but only 1/100 the strength of a solution having a pH value of 2. When determining the degree of alkalinity of a solution, figure it in an opposite manner. From 8 through 14, each number represents a degree of alkalinity 10 times as strong as the last preceding number, but 1/10 the strength of the next higher number; for example, a solution having a pH value of 11 indicates that the solution has an alkalinity 1,000 times stronger than a solution having a pH value of 8, but it would be only 1/100 as alkaline as a solution having a pH value of 13. Litmus paper is used to indicate whether a solution is acid, alkaline, or neutral, but it does not indicate the actual pH value. For this purpose a pH meter should be used. A pH meter is an amplifier meter with a scale that reads from 0 to 14 and an electrode apparatus (Eg. 9-3). A  pH  meter  has  a  reference  electrode  and  a  pH measuring electrode, or these two can be combined into one combination electrode. The pH electrode actually measures the pH, while the reference electrode that contains an electrolyte solution is used only to complete the electrical circuit. The first step in measuring pH is to establish a point of reference by a standardization procedure. To standardize the pH meter, you must place the  electrodes  in  a  calibrated  buffer  solution. Buffer solutions are available at exact pH values for this precise standardization. Always select a buffer with a pH value as close as possible to the pH of the sample to be tested; for example, use a buffer at a pH of 4.00 to test a fixer solution or a pH of 10.00 to test a developer solution.  The  instrument  should  be  standardized  at regular intervals during a long series of measurements or  before  each  use. 9-4







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