Absorbance vs. Transmittance: Empowering Yourself with Spectrometry Knowledge
Spectrometry is an analytical technique widely used in chemistry, biology, physics, and environmental sciences.
It enables scientists to measure the amount of light absorbed or transmitted by a substance at different wavelengths. Two of the most important concepts in spectrometry are absorbance and transmittance, which are related but distinct.
In this article, we will explore these concepts in-depth, including their definitions, calculations, and relationships. We hope that by the end of this article, you will have a greater understanding of absorbance and transmittance, and the nuances of using them in a spectrometric analysis.
Definition and Difference
Let’s begin with the definitions of absorbance and transmittance. Absorbance (A) is the measure of the amount of light that a substance absorbs.
More specifically, it is the logarithm to the base 10 of the ratio of the incident light (I0) to the transmitted light (I), multiplied by negative one:
A = – log10 (I/I0)
On the other hand, transmittance (T) is the measure of the amount of light that passes through a substance. It is calculated as the ratio of the transmitted light to the incident light, multiplied by 100 to express it as a percentage:
T = (I/I0) x 100
It’s important to understand the difference between these two concepts.
Absorbance measures the reduction of light intensity due to the interaction between the substance and the light, while transmittance measures the fraction of the initial light that passes through the substance. To put it simply, absorbance is the opposite of transmittance, as A and T have an inverse relationship.
If the absorbance increases, the transmittance decreases, and vice versa.
Calculation and Relationship
Now that we’ve established the definitions of absorbance and transmittance, let’s delve a little deeper into the calculation and relationship between these two concepts. We use the Beer-Lambert law, also known as the Beer-Lambert-Bouguer law, to relate the absorbance, transmittance, quantity of the substance, and other variables involved in spectrometry.
The Beer-Lambert law states that the absorbance of a solution is directly proportional to the concentration (c) of the absorbing species and the path length (l) of the spectrophotometer cell, and the molar absorptivity () of the substance at the given wavelength:
A = cl
This equation tells us that the greater the concentration of a substance, the more light it will absorb, and the greater the path length through which the light travels, the more light it will absorb. Meanwhile, is a constant that depends on the nature of the substance and the wavelength of the light used.
We can also derive the relationship between absorbance and transmittance from the Beer-Lambert law. Using the formula for transmittance described earlier, we can express the absorbance in terms of transmittance:
A = – log10 (T/100)
If the transmittance is measured in percent, as is typically the case, then we divide it by 100 to convert it to a decimal.
Once we have the absorbance, we can use the Beer-Lambert law to calculate the concentration of the substance:
c = A/(l)
Using these relationships, we can make quantitative measurements of substances absorbing light at specific wavelengths.
Definition
Now that we have covered the basics of absorbance and transmittance, let’s move on to a more focused discussion of transmittance. As mentioned earlier, transmittance is a measure of the amount of light that passes through a substance.
It is defined as the ratio of the intensity of transmitted light to the intensity of incident light, expressed as a percentage. Transmittance is affected by various factors, including the thickness and composition of the material through which the light passes, as well as the wavelength of the light.
Measurement
The measurement of transmittance is a crucial part of spectrophotometric analysis. Scientists use it to determine the concentration of a substance in a solution by comparing the amount of light transmitted through the solution with that transmitted through a solvent.
Generally, the spectrophotometer is set to read the amount of transmitted light and convert it to a percentage of the initial light detected. This percentage is known as the percentage transmittance.
To measure transmittance, the light source is shined onto the sample cell containing the substance and its solvent. The ratio of the intensity of transmitted light to the intensity of the incident light is then measured by the detector.
This ratio is converted to a percentage and entered into a formula to determine the concentration of the substance. This measurement is typically done at specific wavelengths, which are chosen based on the nature of the substance being analyzed.
Conclusion
Absorbance and transmittance are essential concepts in spectrometry. While they are closely related, they have their own definitions, calculations, and relationships.
By understanding these concepts, scientists can make quantitative measurements of substances that absorb or transmit light at particular wavelengths. We hope that this article has provided you with a better understanding of absorbance and transmittance in spectrometry and has empowered you with knowledge to further explore the fascinating world of analytical chemistry.
Absorbance
Definition
Absorbance is the extent to which a substance interacts with light passing through it. It is determined by measuring the amount of incident light (I0) that is absorbed by a substance and comparing it to the amount of transmitted light (I) that passes through the material.
Absorbance is a measure of the concentration of molecules in a solution that absorb a particular wavelength of light. The atoms and molecules in a compound absorb specific wavelengths of light proportional to their concentration, and this absorption produces a decrease in the intensity of the incident light.
The proportion of light absorbed is related to the concentration of the molecules in the substance, the path length of the incident light, and the molar absorptivity or extinction coefficient () of the substance. Calculation and
Measurement
The relationship between absorbance and concentration of a substance is exponential, and therefore, it is possible to calculate the concentration of an unknown solution by measuring its absorbance.
The Absorbance (A) of a substance is calculated using the Beer-Lambert law A = lc, where is the molar absorptivity or extinction coefficient, l is the light path (distance the light travels through the solution), and c is the concentration of the solution. To determine the absorbance of a solution, a spectrophotometer is commonly used.
A spectrophotometer measures the amount of light absorbed by a substance by comparing the intensity of the incident light with the intensity of the transmitted light. The spectrophotometer calculates the absorbance based on the measured light intensity and analyzes the concentration of the substance in the solution.
Difference between Absorbance and Transmittance
Definition and Explanation
Absorbance and transmittance are two fundamental characteristics of a substance when it interacts with light. While absorbance is the amount of light absorbed by a substance, transmittance is the amount of light that passes through a substance.
Absorbance and transmittance are not complementary measurements, and a material may have both a high absorbance and high transmittance at different wavelengths.
Value Changes
The changes in absorbance and transmittance values differ based on the changes in path length, concentration, and molar absorptivity. The absorbance of a solution is related to the concentration, path length, and molar absorptivity of the substance in the solution.
As the concentration of a solution increases, the absorbance of light generally increases in an exponential decrease. The path length works with the concentration to affect the absorbance of light, as longer path lengths will create a greater decrease in light intensity.
The absorptivity coefficient, termed epsilon, and is related to the wavelength of the light being absorbed. Thus, each molecule reacts differently for different wavelengths, and molar absorptivity or extinction coefficient values are dependent on the substance or species.
In contrast, the transmittance of light passing through a substance is directly proportional to the concentration of the substance. As the concentration of the substance in a solution increases, the transmittance of light passing through the solution decreases in a linear increase.
In other words, the transmittance will decrease as the concentration of the substance increases, and it increases as the concentration of the solution decreases.
Range
The range of values for absorbance and transmittance is crucial in analyzing their differences. Absorbance values are limited from 0 to infinity since the amount of light absorbed by a substance is related to the concentration of the substance in solution.
In contrast, transmittance values are limited to the range of 0 to 1 or 0% to 100%. To avoid confusion, scientists often use absorbance instead of transmittance, and they report absorbed light as a negative number.
Furthermore, because absorbance is an exponential function, even tiny absorbance changes indicate significant concentration changes. Transmittance offers an easier comparison since it does not relate to any exponential relationship.
Conclusion
In conclusion, absorbance and transmittance are two critical concepts in spectrometry analysis. Absorbance is defined as the reduction of light intensity due to the interaction between light and a substance, while transmittance is the fraction of initial light that passes through a substance.
Absorbance and transmittance are not complementary measurements, and they have different value ranges and value changing formulas. These measurements are fundamental in analyzing the characteristics of substances, specifically the wavelength absorption patterns of the various molecules.
By understanding these concepts, scientists can make accurate quantitative measurements in a wide range of scientific disciplines.
Image Courtesy
Description
In this article, we have discussed the important concepts of absorbance and transmittance and their significance in spectrometry analysis. To supplement this information, an image of a spectrophotometer can provide a visual aid for understanding the physical layout of the instrument.
The image below is an example of a spectrophotometer that shows the main components of the instrument and how they work together to perform a spectrometric analysis. The image displays the main parts of a spectrophotometer, including the light source, monochromator, sample holder, and detector.
The light source emits light at a specific wavelength, which is then filtered and separated into individual wavelengths by the monochromator. The sample holder contains the solution being analyzed and is placed in the path of the light beam.
The detector measures the amount of light that passes through the sample and calculates the absorbance or transmittance, depending on the selected mode.
Source
The image used in this article is courtesy of Skorpion87 on Wikimedia Commons. Wikemedia Commons is a free media repository containing images and other media files that are freely licensed.
These files can be used for educational purposes and provide valuable support for learning about various topics, including spectrometry. Skorpion87’s image of a spectrophotometer is a good example of how images can enhance learning and make complex concepts more accessible to a wider audience.
It is important to give credit to the original source of an image or media file when using it in an article. This not only acknowledges the creator’s work but also ensures that the material is used ethically and legally.
Use of images without appropriate credit can violate copyright laws and can lead to legal issues.
Conclusion
In conclusion, using an image to supplement written content can enhance understanding of the topic. The image shows the physical layout of the instrument and how its components work together.
It also serves as a great visual for spectrophotometer beginners and students alike. It is important to give credit to the original source of an image or media file when using them in articles or other publication to ensure that the material is used ethically and legally.
In this article, we explored the concepts of absorbance and transmittance in spectrometry analysis. Absorbance measures the amount of light that a substance absorbs, while transmittance measures the amount of light that passes through a substance.
We discussed their definitions, calculations, and relationships, highlighting the exponential relationship of absorbance and the linear relationship of transmittance with concentration. We also emphasized the importance of using spectrophotometers to measure absorbance and transmittance accurately.
Additionally, we touched on the value changes and range of these measurements and the significance of properly crediting image sources. The key takeaway is that understanding absorbance and transmittance enables scientists to quantitatively analyze substances and make informed decisions in various scientific disciplines.
This knowledge empowers researchers to uncover the unique characteristics of molecules and deepen our understanding of the world around us.