EXAMPLE 1-1 Ten-millimeter aliquots of a natural water sample were pipet- ted into 50.00-mL volumetric flasks. Exactly 0.00, 5.00, 10.00, 15.00, and 20.00 mL of a standard solution containing 11.1 ppm of Fe3+ were added to each, followed by an excess of thiocya- nate ion to give the red complex Fe(SCN)2+. After dilution to volume, the instrument response S for each of the five solu- tions, measured with a colorimeter, was found to be 0.240, 0.437, 0.621, 0.809, and 1.009, respectively. (a) What was the concentration of Fe3+ in the water sample? (b) Calculate the standard deviation in the concentration of Fe3+.