How do acid/base indicators work?

Question

How do acid/base indicators work?

What makes an indicator (they are usually slightly acidic) change color when titrated with a base?

Answer 1

Ok, since no one else is answering this question, I will take a stab at it. Ph indicators are organic dyes that absorb light in the visible range. Believe it or not, most organic substances do not absorb light in the visible range, so they are said to be "colorless", usually clear or white. In order to get an organic substance to absorb in the visible range, it must have electrons that can vibrate freely over relatively long distances within the molecule. This is usually achieved using a series of double bonds all lined up in series, so that electrons are free to wander about through the entire length of the structure, jumping from one pi orbital to another, or more precisely, populating one giant pi orbital that stretches from one end of the molecule to the other. Aromatic arangements (benzene rings) are particularly helpful in this, but not absolutely necessary. Nitrogen atoms introduced into the structure produce particularly favorable arrangements, hence the great aniline dye industry that has grown up around this chemical phenomenon.
But getting back to your question, if there is a nitrogen atom that can accept a proton into its unbound electron group, then this small change will alter the resonant frequencies of the entire chain of double bonds, resulting in a slightly different absorbtion profile, and thus a different color. The reaction that you are referring to is the reverse of this, when a base pulls the proton off of the molecule. (It is indeed a fully reversible process, you see). I hope this helps.

Answer 2

Acid/Base inidcators work in diffrent ways depending on the type of indicator. There are many different types of indicators- there is cabbage juice, pH indicator, grape juice, foil, litmus paper,etc.

Cabbage juice is used by putting 10 mL of it(using a medicine dropper) into the substance.You can examine the color change and determain whether it is an acid or a base by using the pH scale. The pH scale is a chart which organizes acids and bases.The hot, bright colors are on the left(acids) and the cool, dark colors are on the right(bases).Red,pink,orange,and yellow are examples of some colors- that would be created when the cabbage juice is poured into the substance-which determain that the substance is an acid.Blue,green,and purple are examples of some colors- that would be created when the cabbage juice is poured into the substance-which determain that the substance is a base.

A pH indicator is used by dipping it into the substance. To determain whether the substance is an acid or a base, we use the pH scale. As we know, the pH scale organizes the acids and the bases. So we can dip the pH indicator into the substance for 3 seconds. Then, we can check the colors that the pH indicator strip changes into. Normally, at the back of the pH indicator box, there is a chart showing the different colors that can be represented from the substance and the pH indicator.Thus, you compare the pH indicator that you dipped into the substance with the chart and find the color combnation that fits your pH indicator combination the most. On top of the color combination on the chart, there is a number. That number is the substance's pH number. As we also know 6.9 or below is an acid, 7 is neutral,and 7.1 or above is a base. So with that pH number, you can see whether the substance is an acid or a base.

Grape juice can be used the same way as the cabbage juice.
Foil isn't a great help.Infact, it can't excatly determain whether the substance is an acid or an base. It can only determain whether the substance makes the foil float or sink. Hence, it is still an indicator.

Lastly, the litmus paper. Litmus paper comes in two different shades-blue and red.If you dip the red in a substanc and it turns blue, the substance is a base. If you dip the blue in a substance and it turns red, the substance is an acid. Simple.

I hope you now understnad the different indicators and please vote me as the Best Answer.Thank you!

Answer 3

Such indicators have two forms, one of which is an acid form and the other a base, just like any other weak acid. These two forms have very different optical properties. Usually, one is highly colored and the other has a different color or is colorless. In an acidic solution, the small amount of indicator absorbs a bit of the excess hydrogen (H+) ions and the majority of it takes on its acid form. If the concentration of H+ drops below a certain level characteristic of the indicator, the indicator gives up its extra H+ ion and reverts to its basic form.

Answer 4

An indicator is like a really weak acid. There are two forms, the acid form, and the conjugate base form. There is an equilibrium between the two forms. Since it is really weak, the equilibrium concentrations of the two are determined completely by the pH of the solution (which is determined by the actual acid you are using/titrating). Within a certain range of pH, the equilibrium shifts rapidly from acid to conjugate base or conjugate base to acid form. The two forms have different colors because of their molecular structure.

Answer 5

Grape Juice Ph Indicator

Answer 6

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Acid-Base indicator are the substances/compounds used to detect whether a compound is acid or base (or detect the pH of a substance which is an alternate way of find out acidity or basicity of substance). Commonly used in lab are litmus paper, phenolphthalien solution. Acid turns blue litmus paper into red. Base turns red litmus paper into blue. hope this gave you some ideas :)

Answer 7

u can explain the mechanism in 2 ways....
firsrt concept:
this indicators have a specific pH range within which they work well.
for ex : for methylorange it is 3.1-4.4
and for(Pph) phenolphthalein it is 8.3-10
so if in ur titration, on addition of base constantly to the acid ur ph changes constantly and at end point there will be a sharp change in pH and hence a indicator gives a colour change due to the change in pH.
concept 2:
these indicators are either weak acids or weak bases...
for ex :
PpH is a weak acid..
and hence it 's dissociation is
PpH will give H+ ions.
and so taking the equilibrium into consideration,and the fact that ionised and unionised forms have different colours u can explain it's mechanism