sampling point, so successive analyses will indicate variations in the chemical composition. Before sampling large batches of newly mixed photographic  solutions,  allow  sufficient  time  for  all the chemicals to dissolve properly, generally about 30 minutes to an hour. You  should  draw  a  sample  1  inch  below  the surface of the solution with a pipet. In general, a sample bottle should not be shaken, and it should be allowed to stand for 10 minutes after the sample is taken from the processing solution. This wait allows large  particles  to  settle  or  turbidity  (caused  by aeration) to clear. Chemical   Certification The chemical certification of processing solutions seldom   requires   a   complete   chemical   analysis. Ordinarily,  determining  the  pH  and  the  specific gravity  of  the  solution  is  enough,  particularly  when these  tests  are  followed  up  with  valid  sensitometric tests. CERTIFYING  THE  pH  OF  SOLUTIONS.—pH is  one  of  the  first  tests  made  of  a  photographic solution.  The  pH  of  a  solution  is  the  negative logarithm of the hydrogen ion concentration. The electrical potential between the glass electrode of a pH meter and the solution surrounding it indicates the hydrogen ion concentration. The  pH  value  of  a  solution  changes  with temperature;  therefore,  to  obtain  a  reproducible  pH value,   you   must   standardize   the   temperature. Generally, you make pH measurements at the same temperature that the photographic process is operated. A   change   of   10°F   produces   a   pH   change   of approximately 0.10 in a processing solution at a pH of 10.5 and a change of approximately 0.20 at a pH of 13.0. Thus temperature control is more important at higher pH values. To properly use, calibrate, and adjust the meter consult the instruction manual or the particular pH meter you are using. When certifying the pH of a solution, you can take the pH reading of only one sample or you can read several samples. Of course, the more samples you   read,   the   more   reliable   your   certification. Usually, you must determine the pH value of more than one sample. After four samples have been read, the meter should be cross-checked, then standardized. Also, remember that no more than 15 minutes should elapse  between  any  pH  meter  standardization  or cross-check. When using the multiple sample procedure, take the  pH  for  each  sample  and  average  the  values. Check the average value against the standard pH for that solution. When the pH is within limits, enter the value in the appropriate location on the certification sheet.  When  the  pH  is  not  within  limits,  further investigation is necessary. CERTIFYING THE SPECIFIC GRAVITY OF SOLUTIONS.—Many applications of specific gravity are used in certifying a solution; for example, the actual   dilution   of   a   developer   is   obtained   by comparing a fresh developer to a used one. Another application may be to compare a fresh fixing bath to a  used  one.  Since  the  silver  content  of  a  fixer increases  with  use,  it  is  logical  to  assume  that  a change  in  specific  gravity  will  occur.  Through experience, you will be able to establish upper- and lower-control limits for specific gravity of the various solutions needing such a check. Specific  gravity  tolerances  are  provided  by  the manufacturer  of  photographic  processing  solutions. The specific gravity must be taken at the temperature recommended   by   the   manufacturer,   because temperature affects the specific gravity of a solution. Specific   gravity   standards   for   black-and-white processing solutions are set at 70°F, because many of these solutions are used in 70°F surroundings. By  themselves,  the  specific  gravity  readings  you take  are  not  enough  to  tell  what  is  wrong.  They simply  indicate  a  change.  So,  further  testing  is required.  When  a  developer  is  being  replenished properly, specific gravity remains constant. If the flow of replenisher stops, a change of specific gravity will  become  evident.  Replenisher  flowmeters  should indicate  such  a  problem,  but  a  backup  check  of specific gravity and pH, along with control strips, are recommended as well. SENSITOMETRIC  CERTIFICATION  OF SOLUTIONS.— Once the developer has been mixed, a sensitometric strip should be processed to check the developer solution. Assuming the pH and specific gravity measurements of the developer are within tolerance, this test can certainly validate the activity of the developer. 2-22