The Handy Chemistry Answer Book (2014)

THE MODERN CHEMISTRY LAB

OTHER MEASUREMENTS

How do those new scanners at airports work?

There are two main types of these scanners: one uses radio waves to generate a three-dimensional image of what’s underneath a person’s clothes, while the other uses low-intensity X-rays to generate a two-dimensional image. Both of these techniques rely on measuring the radiation that is scattered back off of one’s body to generate an image. The purpose of these scanners is to look for basically the same things a security officer would look for in a pat-down: weapons, explosive devices, or anything else someone is trying to hide.

Image

Airport scanners work by using radiation to see under clothing, whether it is X-ray or radio-wave radiation. The levels of radiation are kept low enough to be safe.

How is X-ray diffraction used by chemists?

Chemists use X-ray crystallography to determine the exact structure of chemical compounds. This involves taking a solid crystal of a pure compound and diffracting X-rays off of it, which produces a complex diffraction pattern. Using computer software, the diffraction pattern can be processed to yield a structure that describes the structure of an individual molecule of the compound making up the crystal. This is a powerful technique, but it can only be used on compounds that can be crystallized. It’s also worth pointing out that the solid phase structure of a molecule is not always the same as that in a solution, so caution should be used when relating crystal structures to chemical reactivity in the solution phase.

How do you measure conductivity of a solution?

Conductivity describes the ability of a solution to conduct an electric current. There are a few methods for measuring the conductivity of a solution, and the most straightforward to understand is probably the amperometric method. This method simply applies a voltage between two electrodes and measures the current. While this method is simple to describe, it can have complications in practice and it isn’t always the most accurate method. Another way of measuring conductivity is with a potentiometric method, which makes use of two pairs of rings. There are two outer rings that apply an alternating voltage, and this results in a loop of current being generated in the solution. The other pair of rings sits inside the first and measures the change in voltage between the pairs of rings; this change in voltage is directly related to the magnitude of the current loop induced by the outer rings, which is in turn directly related to the conductivity of the solution. Other methods exist to measure conductivity, but these are probably the easiest to both describe and understand.

Why would a chemist want to measure conductivity?

Most practical applications of solution-conductivity measurements involve determining the quality of water samples. Conductivity measurements can provide a measure of the total amount of dissolved solids in a water sample. This information can be put to use in different ways, depending on the context, but in general it provides a measurement of the purity of the water. Chemists sometimes use a method called ion chromatography, which is a type of liquid chromatography (LC). Ion chromatography often uses a detector that measures conductivity to detect when different analytes pass through the detector.

Image

A simple way to test the pH of a solution is with pH paper; the color of the paper is then compared to samples that show how acidic or alkaline a solution is.

How do you measure the pH of a solution?

One of the first ways science students usually learn to test the pH of a solution is by using pH paper. This is a pretty simple test that only requires you to place a drop of the solution onto the paper and to look at its color. The color change accompanying changes in pH is due to a chemical indicator whose absorption spectrum changes with changes in pH. Another way to measure pH is with a pH-sensitive electrode. This can provide a more accurate measure of pH, as it digitally outputs the pH as a number value and does not rely on a person visually inspecting a color change to interpret the result.

What is a centrifuge?

A centrifuge is a machine that spins its contents very quickly and is used to separate the components of a mixture. The rapid rotation applies a force that causes the more-dense components of a mixture to collect on the bottom of the centrifuge tube and the less-dense components to rise toward the top.

What is a centrifuge used for in a chemistry lab?

In chemistry labs, centrifuges are typically used for separating suspensions. A suspension is a heterogeneous mixture containing small solid particles suspended in a liquid. The number of applications in biochemistry and biology laboratories probably far outweighs those in pure synthetic chemistry; centrifuges are often used to separate the contents of homogenized cellular material to isolate the proteins or cellular organelles. Centrifuges have also found applications in controlling the rates of reactions by simple partitioning of reactants; a centrifuge can be used to separate enzymes from their substrates in solution, which can serve to stop or significantly slow a reaction that is already taking place. In other cases, centrifuges have been used to attempt to accelerate reactions by forcing reactants together at the bottom of the centrifuge tube.

Image

Centrifuges like this one are used to separate components in solution by spinning them rapidly, causing denser components to collect at the bottom of tubes.

How are centrifuges used for nuclear power?

Centrifuges are used to enrich the uranium that is used in nuclear power plants. The two main isotopes of uranium are U–235 and U–238, with U–235 being the isotope used to generate nuclear power via fission processes. Unfortunately, over 99% of naturally occurring uranium is U–238, so a lot of effort has to go into enriching the fraction of U–235 present in a sample. Centrifuges are often used to isotopically enrich a sample of uranium in the U–235 isotope. As described in an earlier question, this is accomplished by spinning a centrifuge tube, and in this case the heavier U–238 isotope is weighed down more, allowing a greater fraction of U–235 to be collected (in the gas phase) from the top of the centrifuge. This process can also be facilitated by heating the bottom of the centrifuge tube, which also helps the U–235 to move toward the top of the tube where it is collected. The process is typically repeated many times before the desired fraction of U–235 is reached. Highly enriched uranium often contains >85% Uranium-235 though, so clearly people have gotten pretty good at carrying out the isotopic enrichment process.