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Analytical Chemistry: Techniques & Methods

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will learn about analytical chemistry. We will describe some common methods and techniques that are used in analytical chemistry, including wet chemistry and instruments used.

Analytical Chemistry

What, exactly, is in the water that you are drinking? How much nitrogen is in the soil on your lawn? These and many other questions can be answered using analytical chemistry techniques and methods. Analytical chemistry is the science of determining what a compound is, separating it out, and measuring how much of it there is.

The two main methods used are wet chemistry and instrument methods. Wet chemistry, which is the classical approach to analytical chemistry, has been used for centuries. The instrument method is a fairly recent development. Both methods are still used today, but the instrument method is gaining popularity.

Wet Chemistry

Wet chemistry uses known chemicals to react with unknown chemicals (or known amounts of chemicals with an unknown amount of chemicals) and uses known reaction results to determine what or how much of a chemical is present. These reactions may also isolate the desired compound.

One common wet chemistry technique is titrations. A titration takes a known chemical reaction that typically results in a change of color, because color change is easy to identify. By carefully measuring how much of a titrant is added before the color change occurs we can calculate how much of the other compound is present.

The flame method was discovered by Robert Bunsen and Gustav Kirchoff in 1860, leading to the discovery of many of the trace elements (elements that are not commonly found on the earth) such as cesium and rubidium. Each element burns at a very specific color. By burning the elements and watching the colors we can determine what elements are in a compound.

Many different types of chemical methods are also used. These tests are often used to identify the presence or absence of a compound. For example, the Kastle-Meyer test can determine if there is any blood present by swabbing an unknown area with hydrogen peroxide and observing if it rapidly turns pink.

Other tests such as chromatography can isolate specific compounds. This method isolates compounds based on their size, charge, or other quality. Chromatography can isolate by size in the same way that you strain spaghetti from water using a colander, only the 'holes' are much smaller. It can also isolate compounds by charge or by other attracting forces. A chromatography column is prepared with substances that are meant to retain some products and let others fall through. The compound being analyzed is then poured into the column. After everything falls through the first time, a solvent is used to extract what was left behind in the column.

Instrument Methods

The instrument methods are often based upon wet chemistry methods but have been automated or made more precise. These methods use specific instruments to measure, isolate, and identify unknown compounds. They are able to do this quicker and more easily than by using traditional wet chemistry methods. However, these instruments are often very expensive, which is why wet chemistry is often still used.

There are several different types of spectroscopy instruments that are used to identify and measure compounds. Often these instruments use the same techniques as wet chemistry. For example, color spectroscopy can be used in a similar method as with titrations. However, the instrument is able to detect slight color changes that the human eye cannot detect, so it is able to more accurately and easily determine how much of a compound is present, or if even a small amount of the compound is present.

Spectroscopy works by sending waves of light (or x-rays, or whichever waves are used) through the unknown sample. The sample bends, reflects, and refracts the waves. Different compounds bend, reflect, and refract the waves at different angles. The spectrometer measures how the waves were bent; this measurement can then be compared to known standards.

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