TM    5-6630-215-12 Table 2-2. Sample Sizes for Low Concentrations Measuring  -  Water  sample Multiply indicator Divide   Result Tube size (and buffer, if any) used by: By 1 15 ml 2 30 ml 45 ml 4 60 ml 1 2 3 4 1 2 3 4 3 (1)   Standard   additions   is   performed   by   adding   a   small   amount   of a  standard  solution  to  a  previously  analyzed  sample  and  then  repeating the  analysis  using  the  same  reagent, instrument   and   technique. The amount  of  increase  in  the  test  result  should  exactly  equal  the  amount of  standard  added. (2)   For   example,    if  a  25-ml  water  sample  is  analyzed  for  iron and  is  found  to  contain  1.0  mg/l,    the  result  can  be  checked  by  adding 0.10  ml  of  a  50.0  mg/l  iron  standard  solution  to  another  25-ml  portion of  the  water  sample  and  repeating  the  analysis. The  result  of  the analysis  on  the  second  sample  should  be  1.2  mg/l  iron  since  the standard  added  has  a  value  of  0.2  mg/l;  i.e., 0.1 ml X  50  mg/l  =  0.2  mg/l 25 ml If  0.2  mg/l  is  recovered  from  the  0.2  mg/l  addition,  the  analyst  can conclude  that  the  first  answer  was  correct  and  that  the  reagents, instruments  and  method  used  are  all  working  properly. Table 2-3. Dilutions for High Concentrations Water sample (use pipet) 50.0 ml 25.0 ml 10.0 ml 5.0 ml Demineralized  water  used to bring volume to approximately 50 ml (use graduated cylinder) 0 ml 25 ml 40 ml 45 ml Multiplication factor 1 2 5 10 (3)   Should   the   second   analysis   not   give   the   correct   amount   of increase  in  the  iron  content,    it  must  be  concluded  that  the  first answer  may  also  be  incorrect. The  analyst  must  then  determine  why  the technique  did  not  work. By   using   a   logical   troubleshooting   approach, the  source  of  the  problems  can  be  determined  whether  the  fault  lies  in the  reagent,    the  instrument  and  apparatus,  the  test  procedure  or  an interfering   substance   present   in   the   test   sample. 2-9