Standardization

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Analytical Sciences Digital Library

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Working with a partner, complete the following problems.

Here are mass spectrometric signals for methane in H₂:

CH₄ (vol %) 0 0.062 0.122 0.245 0.486 0.971 1.921

Signal (mV) 9.1 47.5 95.6 193.8 387.5 812.5 1671.9

Subtract blank value from all other values. Then use the method of least squares to find the slope and intercept and their uncertainties
Replicate measurements of an unknown gave 152.1, 154.9, 153.9, and 155.1 mV and a blank gave 8.2, 9.4, 10.6 and 7.8 mV. Subtract average blank from average unknown to find average corrected signal for the unknown
Find concentration of the unknown and uncertainty

The chromium in an aqueous sample was determined by pipetting 10.0 mL of the unknown into each of the five 50.0 mL volumetric flasks. Various volumes of a standard containing 12.2 ppm Cr were added to the flasks, following which the solutions were diluted to volume

Unknown, mL	Standard, mL	Absorbance
10.0	0.0	0.201
10.0	10.0	0.292
10.0	20.0	0.378
10.0	30.0	0.467
10.0	40.0	0.554

Plot data using a spreadsheet
Determine an equation for the relationship between absorbance and volume of standard
Calculate the statistics for the least square relationship in (b)
Determine the concentration of Cr in ppm in the sample
Find the standard deviation of the result in (d)

The following data were obtained in calibrating a calcium ion selective electrode for the determination of pCa. A linear relationship between the potential and pCa is known to exist.

pCa= - log [Ca²⁺]	E, mV
5.00	-53.8
4.00	-27.7
3.00	+2.7
2.00	+31.9
1.00	+65.1

Plot data
Find least square expression for the best straight line through the points
Find standard deviation for slope, intercept
Calculate the pCa of a serum solution in which the electrode potential was 10.7mV. Find its error.

A shipboard flow injection analytical procedure for measuring Fe²⁺ in sea water is based on chemiluminescence from the dye brilliant sulfoflavin in the presence of Fe²⁺ and H₂O₂. Data from a series of standard additions is shown below. Use all 12 points to construct a standard addition graph and find [Fe²⁺] in the unknown and its error.

Sample	Detector signal
Unknown	12.0	12.0	11.0
Unknown + 5.25 nM Fe²⁺	27.2	26.5	26.5
Unknown + 7.88 nM Fe²⁺	39.9	41.7	39.1
Unknown + 10.5 nM Fe²⁺	53.9	56.2	55.3

Plot data
Find least square expression for the best straight line through the points
Find standard deviation for slope, and intercept

The following data were obtained by adding the same amount of internal standard to a fixed volume of solutions containing known amounts of analyte and to samples of unknown analyte concentration. Plot data using a spreadsheet. Use Excel to determine the percentage of analyte in the unknown and its uncertainty.

Percent analyte	Analyte signal (arbitrary units)	Internal standard signal (arbitrary units)
0.05	18.3	50.5
0.10	48.1	64.1
0.15	63.4	56.2
0.20	63.2	42.7
0.25	98.7	53.8
unknown	58.9	49.4

A series of Ca and Cu samples was run to determine the atomic absorbance of each element.

Ca(μg/mL)	A422.7	Cu (μg/mL)	A324.7
1.00	0.086	1.00	0.142
2.00	0.177	2.00	0.292
3.00	0.259	3.00	0.438
4.00	0.350	4.00	0.579

Find the average relative absorbance (A422.7/A324.7) produced by equal concentrations (μg/mL) of Ca and Cu.
Copper was used as an internal standard in a Ca determination. A sample known to contain 2.47 μg Cu/mL gave A422.7 = 0.218 and A324.7 = 0.269.
Calculate the concentration of Ca in micrograms per milliliter.

Contributors and Attributions

Rosalynn Quinones-Fernandez, Marshall University (quinonesr@marshall.edu)
Sourced from the Analytical Sciences Digital Library