While choosing titration indicators certain things should be kept in mind. First of all, it should be remembered that the comparison point of a titration is at the place where you mix the 2 contents of precisely equation proportions. You should select an indicator that modifies color as close as possible to that comparison point. This may vary from titration to titration. When you use strong acid against strong base it will place on top the pH ranges for phenolphthalein and methyl orange. Then the indicator will not change the color at the comparison point. Nevertheless, the process will be so steep at the point where there will be practically no change in the quantity of acid added whichever indicator you select.
It will also make sense to titrate to the best possible color with each indicator. If you employ phenolphthalein, then you would titrate until it becomes colorless as it is as close as you can get to the comparison point. To the contrary, through methyl orange, you would titrate until there is the first trace of orange in the solution. When the solution becomes red, you will get further from the comparison point. When you use strong acid against weak base it is sure that phenolphthalein would be fully useless. Anyway, methyl orange begins changing from yellow to orange very close to the comparison point. So, you need to select an indicator that modifies color on the steep bit of the curve.
If you make weak acid against strong base, then the methyl orange will be hopeless. Anyway, the phenolphthalein modifies color precisely where you need it to. When using weak acid against weak base the acid and base would be equally weak. For instance, ammonia solution and ethnoic acid. In other cases, the comparison point will be at some other pH. You will see that neither indicator is any application. Phenolphthalein will have completed modification before the comparison point and methyl orange falls off altogether. It may be potential to discover an indicator that begins changing or completing modification at the comparison point. However, the pH of the comparison point will be distinctive from case to case that you cannot generalize.
Generally, you would never titrate a weak acid and a weak base with an indicator. Using dilute hydrochloric acid and sodium carbonate solution is an interesting case. If you employ methyl orange or phenolphthalein, then both will provide a valid titration result. However, the value with phenolphthalein will be precisely half the methyl orange one. It occurs so that the phenolphthalein has completed its color change at precisely the ph of the comparison point of the first half of the reaction wherein sodium hydrogencarbonate is generated. The methyl orange modifies color at precisely the pH of the comparison point of the second phase of the reaction. You should be careful and attentive while selecting titration indicators for different titration purposes. Online will help you choose the right indicator for your requirements.
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