1.why it is necessary to select only one wavelength to pass through the sample (

Question

1.why it is necessary to select only one wavelength to pass through the sample (i.e. why don’t we just shine white light into the sample and measure the amount of light that passes through?

2.Do you think that the light that reaches the detector should be less than, the same, or more than the light that enters the sample? Explain your answer.

3.Brass is a metal alloy composed of: copper, zinc, tin, and lead. This is due to copper being too soft in its pure state. Brass can be dissolved in concentrated nitric acid, which then gives off a bright blue color due to the copper (zinc, lead, and tin ions are colorless). If you were to do a spectrophotometric analysis of the amount of copper found in a brass sample, approximately which wavelength would you select to conduct your analysis and why?

4.Scientists measure the amount of light passing through a sample in terms of percent transmittance (%T). Percent transmittance is the fraction of original light that passes through a sample. Recall that P0 is the amount of light entering the sample and P is the amount of light reaching the detector.

                                                           %T=(P/P0)X100

Using a spectrometer, a sample was analyzed to have a percent transmittance of 85%. What percentage of light was actually absorbed by the sample?

5.The equation below shows how percent transmittance (%T) can easily be converted into a quantity known as absorbance (A). Though most spectrometers give readings in terms of both %T and A, measurements should be made in %T and mathematically converted to A because %T can be determined more accurately.

                                                                                  A=-log(%T/100)

Calculate the absorbance of a sample that has a percent transmittance of 75%.

Explanation / Answer

1) White light is a composite of several wavelengths. When a sample is irradiated with wite light then it will absorb at different characteristic wavelengths and result in a broad absorption spectrum in which the individual peaks would be very poorly resolved. This difficulty in spectral interpretation can be overcome by selecting light of a single wavelength,i.e., using a monochromatic radiation.

2) Since the sample would absorb light at wavelengths characteristic of the molecules in the sample, the light reaching the detector would be less than the incident light.

3) As the brass solution gives off a bright blue color, the absorption would be in the complimentary orange region. Hence, it would be appropriate to choose a wavelength between 580 - 620 nm.

4) If the %T = 85, then %absorbance = 100 - %T = 100 - 85 = 15%

5) A = -log (%T/100) where %T = 75

A = -log (75/100) = 0.125

Related Questions

Navigate

• Browse (All)
• Browse 1
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.