Question

The ionic strength of samples in which the Na+ ion concentration should be determined can vary between 0.01 and 0.1 M. This large fluctuation in the ionic strength introduces an unacceptable error in the concentration determination of Na+. To minimize the error related to the fluctuations in the ionic strength each sample is diluted in a 1:10 ratio with a 5 M NH4NO3 ionic strength adjustment buffer (10 mL sample + 90 mL ionic strength adjusting NH4NO3). Calculate the ionic strength of the samples with 0.01 and 0.1 M original ionic strength after the addition of the ionic strength adjustment buffer.

Answer 1

For 0.01M Na+

After dilution C (Na+) = 0.001 M, Z ( Na+) = 1,

C ( NH4+) = 5×90/100 = 4.5M, Z (NH4+) = 1

C(NO3-) = 4.5M, Z (NO3-) = -1

I = 4.5005

For 0.1M Na+

C(Na+) = 0.01M, Z(Na+) = 1, C (NH4+) = 4.5M, Z(NH+) = 1,

C (NO3-) = 4.5M, Z ( NO3-) = -1

I = 4.505