The story of a little molecular crab
what do salad dressing and shampoo have in common?
No, I'm not saying that they all contain water, or that they are all white and turbid liquids. ). In fact, they are likely to contain the same chemical, and this molecule is also the protagonist of today's story.
the following picture lists the ingredients of several salad dressings and daily necessities I photographed in the supermarket:
have you seen it? Although it is not in a prominent position, it is hidden in many everyday products, which is EDTA (ethylenediamine tetraacetic acid, which is often added with its sodium salt).
this is a common chelating agent-a little crab always comes to mind when it comes to the word. Many metal ions exist in solution in the form of complexes, such as copper hydrate. This common complex is not very stable, the molecules that coordinate with metal ions can still be exchanged, and metal ions can still have various reactions.
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but the case of EDTA is different. It has six positions on one molecule for complexation, so it can form 1:1 complexes with many metal ions, and the stability of these complexes is quite good. It is really like a little crab, firmly holding on to the metal ions and will not let go easily.
A schematic diagram:
this property of firmly grasping metal ions makes it have a lot of application value. Sometimes, some metal ions will bring unwanted results, for example, free iron ions may catalyze unwanted chemical reactions, making the color, flavor or stability of food worse. At this time, adding a little chelating agent to hold on to the ions can help keep the product stable. EDTA can also catch lead ions, so it is also used to save patients with lead poisoning.
besides, EDTA can be used in complexometric titration. This is similar to acid-base titration, except that EDTA is used to bind metal ions and requires special indicators. I used it to determine the amount of calcium and magnesium ions in eggshells in my college chemistry class.
with the help of EDTA, we can also achieve a beautiful experimental effect:
this is a method of luminol luminescence experiment, in which copper ions near the copper wire catalyze the reaction and emit beautiful blue light. With the reaction, the copper ion will spread into the solution, so if you use the common practice, the blue light will soon fill the whole solution, but as long as you add EDTA to the solution to catch the copper ion, you can make the light only light around the copper wire ~ more read: moving picture appreciation: lighting the copper wire
this is today's molecular story, remember this little crab?