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Questions and Answers
What is the advantage of measuring a decrease in reagent absorbance at a particular wavelength?
What is the advantage of measuring a decrease in reagent absorbance at a particular wavelength?
- It allows for more accurate measurements of the ion to be determined.
- It simplifies the calibration curve construction.
- It increases the sensitivity of the determination. (correct)
- It eliminates the need for a reagent blank.
How is the normal measurement process of spectrophotometry reversed in certain cases?
How is the normal measurement process of spectrophotometry reversed in certain cases?
- The blank solution is measured against zero absorbance set with the test solution. (correct)
- The test solution is measured against zero absorbance set with the blank solution.
- The reagent blank is measured against the test solution.
- The test solution is measured against the reagent blank.
What is the purpose of the preliminary qualitative examination of the color reaction?
What is the purpose of the preliminary qualitative examination of the color reaction?
- To construct the calibration curve.
- To determine the exact pH range for color development.
- To provide a guide to the optimum pH range for color development. (correct)
- To measure the absorbance of the solution.
Why is it necessary to measure the absorbance of each solution against a reagent blank at the same pH?
Why is it necessary to measure the absorbance of each solution against a reagent blank at the same pH?
What is the range of wavelengths over which the absorbance of each solution should be measured?
What is the range of wavelengths over which the absorbance of each solution should be measured?
What is the purpose of allowing a definite time interval for color development?
What is the purpose of allowing a definite time interval for color development?
Why is it necessary to periodically check the photometric accuracy of the spectrophotometer absorbance scale?
Why is it necessary to periodically check the photometric accuracy of the spectrophotometer absorbance scale?
What is the purpose of using standard copper sulphate, ammonium cobalt sulphate, or potassium chromate solutions?
What is the purpose of using standard copper sulphate, ammonium cobalt sulphate, or potassium chromate solutions?
What is Sandell's expression or sensitivity index?
What is Sandell's expression or sensitivity index?
Why should the absorbance of the reagent and product be measured separately against a solvent blank?
Why should the absorbance of the reagent and product be measured separately against a solvent blank?
What is the molar absorptivity of the colored compound?
What is the molar absorptivity of the colored compound?
What is the recommended temperature range for studying the effect of temperature on the absorbance?
What is the recommended temperature range for studying the effect of temperature on the absorbance?
What is the minimum amount of a colored substance dependent on?
What is the minimum amount of a colored substance dependent on?
What is the limiting factor in detecting the minimum amount of a colored substance?
What is the limiting factor in detecting the minimum amount of a colored substance?
What is the purpose of constructing a calibration curve for the constituent?
What is the purpose of constructing a calibration curve for the constituent?
Why is it necessary to specify the temperature when publishing the sensitivity of the procedure?
Why is it necessary to specify the temperature when publishing the sensitivity of the procedure?
What is the purpose of studying the effect of temperature on the absorbance?
What is the purpose of studying the effect of temperature on the absorbance?
What should be plotted to construct a calibration curve for the constituent?
What should be plotted to construct a calibration curve for the constituent?
What is the usual method to calculate the sensitivity of an instrument?
What is the usual method to calculate the sensitivity of an instrument?
What is the assumption behind the first method of stating sensitivity?
What is the assumption behind the first method of stating sensitivity?
What is the unit of measurement for the sensitivity of an instrument?
What is the unit of measurement for the sensitivity of an instrument?
What is the value of absorbance difference that represents a more attainable practical value?
What is the value of absorbance difference that represents a more attainable practical value?
What is the equation used to calculate the molar absorptivity of the colored compound?
What is the equation used to calculate the molar absorptivity of the colored compound?
Why is the second method of designation of sensitivity based on the molar absorptivity of the colored compound?
Why is the second method of designation of sensitivity based on the molar absorptivity of the colored compound?
What is the optimal range of absorbance values for measurements on a spectrophotometer to minimize photometric error?
What is the optimal range of absorbance values for measurements on a spectrophotometer to minimize photometric error?
What is the purpose of stating the range of linearity in a calibration curve?
What is the purpose of stating the range of linearity in a calibration curve?
What is the significance of recording the slit width at which the measurements were made?
What is the significance of recording the slit width at which the measurements were made?
What is the consequence of projecting a calibration curve beyond the tested range without experimental verification?
What is the consequence of projecting a calibration curve beyond the tested range without experimental verification?
What is the primary advantage of a system that follows Beer's Law?
What is the primary advantage of a system that follows Beer's Law?
What is the significance of the analytical concentration of the species determined in relation to the absorbance values?
What is the significance of the analytical concentration of the species determined in relation to the absorbance values?
Study Notes
Beer’s Law and Absorbance
- A straight line graph indicates adherence to Beer’s Law, free from light polychromaticity or species interaction.
- While straight line data is ideal, non-linear results can still provide valuable information.
- Optimum absorbance measurement range is 0.248 units for minimal photometric error, relating to specific analytical concentrations.
Measurement and Calibration
- Record the slit width used for absorbance measurements to ensure precision.
- Calibration curves should not extend beyond the tested concentration range without verification of Beer’s Law applicability.
- Most analytical chemistry journals now focus on stating the linearity range or providing calibration equations rather than publishing straight line curves.
Photometric Accuracy
- Regularly verify spectrophotometer absorbance accuracy across wavelengths using reliable standards (e.g., copper sulfate).
- Correct absorbance values enhance comparability for other researchers.
Sensitivity in Spectrophotometry
- Sensitivity can be quantified via Sandell's index or molar absorptivity of the colored compound.
- The minimum detectable amount of a colored substance is influenced by the reproducibility of absorbance readings.
- Detecting absorbance changes of 0.001 is often uncertain; techniques may adjust to measure decreases in reagent absorbance instead.
Effect of pH on Measurements
- Initial qualitative analysis guides the optimum pH for color development; control must be tight within a 2-3 pH unit range.
- Measure absorbance against a reagent blank, allowing time for color development and testing across a range of wavelengths.
- A reagent blank should remain stable over time, shielding readings from fluctuations.
Temperature Dependencies
- Temperature significantly affects reactions, particularly oxidation-reduction and organic reagent interactions.
- Investigate absorbance variation with temperature, ideally between 15°C to 35°C.
- It’s essential to note the temperature during sensitivity reporting for accurate procedures.
Calibration Curves and Practical Sensitivity
- Construct calibration curves by plotting absorbance data against known concentrations under optimal conditions.
- An absorbance difference of 0.005 is a practical expectation for sensitivity.
- Sensitivity can be expressed as picograms of the element per square centimeter via absorbance relationships at considered concentrations.
Molar Absorptivity
- Molar absorptivity (ε) calculation derives from the relation: log I0/I = A = ε l c.
- Absorbance (A), concentration (c), and light path length (l) are critical parameters in determining ε within the least photometric error range.
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Description
Understanding the stability of reagent blanks and its effects on analytical reactions. Learn how to study the causes of instability and the impact of temperature, atmosphere, and light on reactions.
