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Chapter 5: Q32P (page 117)
Internal standard calibration curve. Figure 5-10 is a graph of in Reaction 5-13.
(d) From the uncertaintyof the intercept, find theconfidence interval for the intercept. Does this interval include the theoretical value of zero?
Theconfidence interval for the intercept
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Internal standard. A solution was prepared by mixing of unknown elementwithof solution containingrole="math" localid="1654777035083" of standard elementper millilitre, and diluting to. The signal ratio in atomic absorption spectrometry was (signal from)/ (signal from. In a separate experiment, with equal concentrations of, (signal fromsignal from. Find the concentration ofin the unknown.
Standard addition. An unknown sample of gave a current of in an electrochemical analysis. When of solution containing role="math" localid="1654761474124" was added to of unknown, the current increased to .
(a) Denoting the initial, unknown concentration as , write an expression for the final concentration, , after role="math" of unknown were mixed with of standard. Use the dilution factor for this calculation.
(b) In a similar manner, write the final concentration of added standard , designated as .
(c) Findin the unknown.
How is a control chart used? State six indications that a process is going out of control.
Detection limit. In spectrophotometry, we measure the concentration of an analyte by its absorbance of light. A low-concentration sample was prepared and nine replicate measurements gave absorbances of 0.0047,0.0054,0.0062,0.0060,0.0046,0.0056,0.0052,0.0044, and 0.0058. Nine reagent blanks gave values of 0.0006,0.0012, 0.0022,0.0005,0.0016,0.0008,0.0017,0.0010, and 0.0011.
a) Find the absorbance detection limit with equation 5-3.
b) The calibration curve is a graph of absorbance versus concentration. Absorbance is a dimensionless quantity. The slope of the calibration curve is Find the concentration detection limit with Equation 5-5.
(c) Find the lower limit of quantitation with Equation 5-6.
Europium is a lanthanide element found at parts per billion levels in natural waters. It can be measured from the intensity of orange light emitted when a solution is illuminated with ultraviolet radiation. Certain organic compounds that bindEu(III)are required to enhance the emission. The figure shows standard addition experiments in which10.00mLof sample and20.00mLcontaining a large excess of organic additive were placed in 50-mL volumetric flasks. Then Eu(III) standards (0,5.00,10.00,or15.00mL) were added and the flasks were diluted to50.0mLwithH2O. Standards added to tap water contained0.152ng/mL(ppb) of Eu(III), but those added to pond water were 100 times more concentrated (15.2 ng/mL).
(a) Calculate the concentration of Eu(III)(ng/mL) in pond water and tap water.
(b) For tap water, emission peak area increases by.4.61units when 10.00mL of 0.152ng/mL standard are added. This response is4.61 units/1.52ng = 3.03units per ng ofEu(III). For pond water, the response is12.5units when10.00mLof15.2ng/mLstandard are added, or0.0822units per ng. How would you explain these observations? Why was standard addition necessary for this analysis?
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