Question

Chloride in a water sample can be determined indirectly by atomic absorption spectrometry by precipitating chloride as AgCl(s) with a measured amount of AgNO3 that is excess of the chloride content of the sample. The AgNO3 is added and the sample is centrifuged. The remaining Ag+ in the solution (supernatant) is quantified and the chloride is measured by assuming that chloride was quantitatively precipitated.

Chloride in a water sample can be determined indirectly by atomic absorption spectrometry by precipitating chloride as AgCl(s) with a measured amount of AgNO3 that is excess of the chloride content of the sample. The AgNO3 is added and the sample is centrifuged. The remaining Ag in the solution (supernatant) is quantified and the chloride is measured by assuming that chloride was quantitatively precipitated. A 10.00mL aliquot of the sample and a 10.00 mL aliquot of a 100.00 ppm chloride standard is placed into two separate 100.00 mL volumetric flasks. A blank (containing no chloride) was prepared with 10.00 mL of deionized water instead of a sample in a third 100.00 mL volumetric. Twenty five (25.00 mL) of a silver nitrate solution are added each volumetric with a pipette. The samples are then diluted to volume and after allowing time for precipitation the mixtures are centrifuged and each supernatant is measured by AA. The absorbance signals are as follows in arbitrary units: blank 12.8, standard 5.7 sample 6.8. What is the concentration of chloride in the water sample in ppm units?

Answer 1

In the exercise we must consider the following factors:

• The relation between the absorbance and the concentration is directly proportional.
• The stoichiometric ratio in the AgCl is 1:1.
• The chloride was quantitatively precipitated (all the chloride are precipitated adding a know concentration of the precipitating agent).

Adding Ag(NO3)(ac) to a recipient with Cl- (ac), will form AgCl(s).

Measuring the three samples supernatant, will give us the absorbance in response of the Ag+ excess, the measure of the blank will give us the maximum absorbance value (the Ag+ didn't reaccionate there).

Doing a difference between the blank and the standard & sample, will give us the value of the Ag+ that reaccionate to form AgCl(s).

For the standard: 12,8-5,7 = 7,1.

For the sample: 12,8-6,8 = 6.

With that, we must expect that sample concentration in Cl- is smaller than 100.00ppm.

Doing a relationship knowing the Cl- concentration of the standard:

7,1 - 100.00ppm Cl-

6 -> 84.5 ppm Cl- approximately.