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Bromothymol blue is a yellow color when mixed with an acid and a blue color when mixed with a base or a neutral substance. It is often used to help maintain the pH of fish tanks and swimming pools.


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  4. Acid-Base Indicator Definition and Examples!
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A universal Indicator is a solution containing a mixture of indicators. It provides a gradual change in color over a wider pH range than the individual indicators would.


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  • The approximate pH of a solution can be identified by adding a few drops of universal indicator to it. Justin Schamotta began writing in Schamotta has a Bachelor of Science in psychology from Plymouth University and a postgraduate diploma in journalism from Cardiff University.

    Litmus is a weak acid. It has a seriously complicated molecule which we will simplify to HLit. The "H" is the proton which can be given away to something else. The "Lit" is the rest of the weak acid molecule. There will be an equilibrium established when this acid dissolves in water.

    Taking the simplified version of this equilibrium:. Now use Le Chatelier's Principle to work out what would happen if you added hydroxide ions or some more hydrogen ions to this equilibrium. At some point during the movement of the position of equilibrium, the concentrations of the two colours will become equal. The colour you see will be a mixture of the two. The reason for the inverted commas around "neutral" is that there is no reason why the two concentrations should become equal at pH 7.

    As you will see below, that isn't true for other indicators. Methyl orange is one of the indicators commonly used in titrations. In an alkaline solution, methyl orange is yellow and the structure is:. Now, you might think that when you add an acid, the hydrogen ion would be picked up by the negatively charged oxygen. That's the obvious place for it to go. Not so! In fact, the hydrogen ion attaches to one of the nitrogens in the nitrogen-nitrogen double bond to give a structure which might be drawn like this:.

    The truth is that there is delocalisation over the entire structure, and no simple picture will show it properly. Don't worry about this exact structure - it is just to show a real case where the colour of a compound is drastically changed by the presence or absence of a hydrogen ion. You have the same sort of equilibrium between the two forms of methyl orange as in the litmus case - but the colours are different. You should be able to work out for yourself why the colour changes when you add an acid or an alkali. The explanation is identical to the litmus case - all that differs are the colours.

    Acid - Base Indicators

    Sort it out! In the methyl orange case, the half-way stage where the mixture of red and yellow produces an orange colour happens at pH 3. This will be explored further down this page. In this case, the weak acid is colourless and its ion is bright pink. Adding extra hydrogen ions shifts the position of equilibrium to the left, and turns the indicator colourless.


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    • Adding hydroxide ions removes the hydrogen ions from the equilibrium which tips to the right to replace them - turning the indicator pink. The half-way stage happens at pH 9. Since a mixture of pink and colourless is simply a paler pink, this is difficult to detect with any accuracy! This is quite difficult stuff, and if you are coming at this from scratch you will have to explore at least one other page before you can make sense of what is on that page. The equilibrium shifts right, HIn decreases, and In - increases.

      As the pH increase between 8. The third beaker has only the NaOH but no phenolphthalein, so it remained colorless. The first beaker contain acetic acid and is skipped over at first.

      Bottom half of the graphic: When the pitcher is then poured back into beakers 2, 3, 4 it is a pink solution. In the first beaker, a strange thing happens in that the pink solution coming out of the pitcher now changes to colorless. This happens because the first beaker contains some vinegar or acetic acid which neutralizes the NaOH, and changes the solution from basic to acidic. Under acidic conditions, the phenolphthalein indicator is colorless. Use equilibrium principles to explain the color change for phenolphthalein at the end of the demonstration.

      Acid-base indicator

      As OH - ions are added, they are consumed by the excess of acid already in the beaker as expressed in the above equation. The hydroxide ions keep decreasing and the hydrogen ions increase, pH decreases. See lower equation: The indicator equilibrium shifts left, In - ions decrease. Below pH 8. Click for larger image.

      Color changes in molecules can be caused by changes in electron confinement. More confinement makes the light absorbed more blue, and less makes it more red.

      Handbook of Acid-Base Indicators - CRC Press Book

      How are electrons confined in phenolphthalein? There are three benzene rings in the molecule. Every atom involved in a double bond has a p orbital which can overlap side-to-side with similar atoms next to it. The overlap creates a 'pi bond' which allows the electrons in the p orbital to be found on either bonded atom.