# Instrumental analysis finding the concenration

• thea831
In summary, the method of standard addition can be simplified by keeping the volume of the test solution constant, but in some cases, volume changes after spiking with known concentrations of the analyte must be taken into account. In this particular case, the concentration of caffeine in a sample of coffee is being determined using two portions of the sample. The first portion is mixed with water, and the second portion is spiked with a known concentration of caffeine. The absorbance of each portion is measured, and the concentration of caffeine can be calculated using Beer–Lambert law.
thea831

## Homework Statement

The analysis for method of standard addition can be simplified significantly if the volume of the test solution is held constant. However, in some cases, it is difficult to avoid significant volume change after the sample is spiked with known concentrations of the analyte. The change in volume needs to be considered. A sample of coffee is analyzed to determine its caffeine concentration. Two portions of the sample are prepared for analysis. The first portion contains 50.0 ML of brewed coffee, which is added 10.0 ml of water. The second portion contains 50.0 ml of brewed coffee that has been spiked with 10.ml of an aqueous solution that contains 1.0 x10‐2 M caffeine. The first portion of the sample is found to give a measured absorbance of 243 units. And the second portion gives an absorbance of 387 units. What is the concentration of caffeine in the brewed coffee?

## Homework Equations

We need to use standard addition. a background and intensity for each. I am not sure if beer's law is applicable

## The Attempt at a Solution

So I understand how standard addition works. But I cannot think of a way to find the concentration without the background intensity and the intensity for each portion.

As I told you in other thread - you don't need intensity, if absorbance is given. And yes, Beer–Lambert law is applicable.

Last edited by a moderator:
Ok, I will try beer's law

## 1. What is instrumental analysis and how is it used to find the concentration of a substance?

Instrumental analysis is a branch of analytical chemistry that uses instruments to measure and quantify the composition of a sample. It is used to find the concentration of a substance by using various techniques such as spectrophotometry, chromatography, and electrochemical methods.

## 2. What is the principle behind using spectrophotometry for concentration determination?

Spectrophotometry is based on the principle that different substances absorb light at different wavelengths. By measuring the amount of light absorbed by a sample, the concentration of a substance can be determined using Beer-Lambert's Law.

## 3. Can instrumental analysis be used to determine the concentration of both organic and inorganic substances?

Yes, instrumental analysis techniques can be used to determine the concentration of both organic and inorganic substances. However, the specific technique used may vary depending on the type of substance being analyzed.

## 4. What are some common sources of error in instrumental analysis for concentration determination?

Some common sources of error in instrumental analysis include instrumental drift, chemical interference, and human error. It is important to carefully calibrate and maintain instruments and to use proper techniques to minimize these errors.

## 5. How does sample preparation affect the accuracy of concentration determination using instrumental analysis?

Sample preparation is crucial in instrumental analysis as it can greatly impact the accuracy of concentration determination. Proper sample preparation techniques, such as ensuring homogeneity and avoiding contamination, can help to minimize errors and ensure accurate results.

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