Acids, Alkalis and Neutralisation
The pH scale determines how many hydrogen ions are present in an aqueous solution. It's logarithmic, progressing from 0 being the most acidic to 14 being the most alkaline, 7 being neutral - as in, water. The closer something is to neutral, the more likely you can handle it without experiencing severe burns.
To get to 7 in the first place, a solution needs to be neutralised. In other words, this reaction has to occur:
H+ + OH- → H2O
As in, those specific ions need to react in such a way. Say, for example, you're reacting hydrochloric acid (HCl) and sodium hydroxide (NaOH) because you're bored. You should get sodium chloride (NaCl) and water. As water is a neutral substance, a neutralisation reaction has occurred.
This also means we can broaden the definition of what is acidic and what is alkaline. Much like the equation above, it's clear that HCl or NaOH will contain either hydrogen ions or hydroxide ions. And from looking at the chemicals in question, it's clear that:
- substances predominantly containing H+ ions are acidic, and;
- substances predominantly containing OH- ions are alkaline.
You can then broaden this definition by differentiating between the acids/alkalis with higher pHs and those with lower pHs. Weak acids only partially ionise (release H+ ions) in solution, and thus have a higher pH - since pH determines how many H+ ions are present, the fewer there are, the less acidic it is, and thus it's closer to being an alkali compared to strong acids which fully ionise.
A weak acid could be citric acid; a strong acid could be hydrochloric acid. This definition also applies to alkalis, where a weak alkali could be calcium hydroxide; a strong alkali could be bleach.
That's part 1 - now it's time for part 2. Intermission...
Titrations
How could you neutralise two solutions? There aren't particularly large ranges in pH values - just a few drops of solution could cause you to go from an acidic solution to an alkaline one. Yes, you could go very close to pH 7, but if you're producing chemicals which ought to be neutral, that wouldn't be enough.
That is where titrations come in - a strategy in which you take two solutions and attempt to react them so they're exactly neutral. Since different solutions have different concentrations and the volumes may be different to each other, titrations are usually performed as such:
you know the concentrations of both solutions, but you know the volume of only one of them (call it solution2). (Occasionally, you may know both volumes, but only one concentration - the method remains rather the same.) After adding the exact volume of solution2, you can use a burette (which is a long measuring tube for storing volumes of solutions) to measure out what volume of solution1 you'd need to neutralise solution2. Obviously you'd apply a few drops of pH indicator first, with all the various types having different colour schemes and accuracy levels depending on which solutions you're using.
And there are so many indicators, it's ridiculous. I've only ever heard of three - universal indicator (which goes from red to violet), phenolphthalein (which goes from pink to colourless), and the one I used most recently, methyl orange (which goes from red to yellow, and the one used in the titration photo). Some of them only operate at certain pH levels, however, which explains why there are loads out there, often with names which are of the format "chemical name, colour".
You'd do one trial, not being particularly cautious about what value you get, rather wanting to see in what range of volumes does the solution go from acidic to alkaline or to neutral. After this, each subsequent trial is intended to allow you to narrow down what the exact volume needed is, and should you achieve results 10cm3 from each other, they're concordant, and you can calculate a mean titre. That therefore should be the volume at which the two solutions will neutralise each other - yet indicators are notoriously unreliable and can transition within wide ranges, so your assumptions might not actually be completely correct. That and chemistry is a subject which suits the avid perfectionist.
I did some titrations a few days back, and they were frustrating to say the least. So much of chemistry is dependent on accuracy - if you're slightly off, that could have major repercussions on your outcomes, so being a couple cubic centimetres out is no excuse. And after nearly five titres, I still couldn't get concordant results, so I couldn't accurately determine what volume I needed. I did get a nice picture out of it though, so all's well that ends well.
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