The method of standard additions was used to determine nitrite in a soil sample. A 1.00-mL portions of the soil extract sample was transferred to four 25-mL volumetric flasks. To each flask, increment volumes of a 1.00 x10-3 M nitrite was added according to the table below. A colorimetric reagent was added to each flask to convert the nitrite into a colored product, and finally diluted to volume. What is the concentration of the nitrite in the soil extract based on the absorbance data below (with blank correction included)?

Question

The method of standard additions was used to determine nitrite in a soil sample. A 1.00-mL portions of the soil extract sample was transferred to four 25-mL volumetric flasks. To each flask, increment volumes of a 1.00 x10-3 M nitrite was added according to the table below. A colorimetric reagent was added to each flask to convert the nitrite into a colored product, and finally diluted to volume. What is the concentration of the nitrite in the soil extract based on the absorbance data below (with blank correction included)?

vol. of standard nitrite added, mL
5.00
10.00
15.00
20.00
25.00

Absorbance
0.079
0.161
0.236
0.318
0.403

Answer 1

Plot the data, absorbance on the y axis and concentration on the x axis.

Plot the data and determine the slope of the line, which would be, "m" from the equation: y=mx+b.

After solving for the equation of the line, use the equation of the line to determine the x intercept. Let y=0 and solve for x from the equation y=mx+b. Your value will be negative.

***note b is the y-intercept and is where the line crosses the y axis, which is when x=0.

The absolute value of the x intercept times the concentration of the standard should give you the concentration of your sample.