Tutorial: Standard Additions

Quantitative Analysis with Multiple Standard Additions

The general procedure for standard additions is to obtain measurements after adding increasing amount of analyte to portions of the sample. There are actually two related ways of doing this: one in which the sample portions are all made up to the same volume after the standard has been added, and one in which the volumes of the sample solutions are different.

Example: Multiple Standard Addition with dilution to constant volume

A 3.12g stick of chewing gum is ashed at high temperature, dissolved in 5mL of concentrated nitric acid and then diluted to the mark in a 100mL volumetric flask. To 25 mL volumetric flasks are added a 5.00 mL aliquot of the dissolved chewing gum sample and an addition of 300.0 ppm aluminum. After dilution to volume, the solution is analyzed for aluminm by graphite furnace atomic absorption at 309 nm. The data are shown in the following table. Report the aluminum content of the gum as a confidence interval.

Answer: 48.1 +/- 2.8 ppm (95% CI)

Sample dilution is not always desirable, since it degrades analytical sensitivity. The following example shows how the standard addition method may be used without dilution.

Example: Multiple Standard Addition without additional dilution

Anodic stripping voltammetry can be used to measure the leachable lead content of pottery and crystalware. A sample of pottery being considered for import is leached with 4% acetic acid for 24 hr. A 50.00 mL aliquot is transferred to an electrolysis cell, and after controlled-potential electrolysis at -1.0V (vs SCE) for 2 minutes, an anodic scan is recorded with the differential pulse method. The procedure is repeated with new 50.00 mL aliquots to which 25, 50, and 75 microliter spikes of standard containing 1000 ppm lead have been added.

What was the concentration of lead in the original acetic acid leach? Report your answer as a confidence interval.

Answer: 1.07 +/- 0.16 ppm (95% CI)