What color do neutral solutions turn the universal indicator
pH scale, neutralisation, ionic theory of acids & alkalis
This page introduces and explains the pH scale measuring the relative acidity and alkalinity of aqueous solutions, that is solutions of substances dissolved in water. The use of indicators is described and several well known indicators are tabulated showing their different colours in solutions of different pH. The ionic theory of acids, bases and neutralisation is simply described and why explains why solutions are either acid, neutral or alkaline.
2. The pH scale, indicators acids, alkalis (bases), neutralisation & ionic theory
2a. Introduction to the pH scale What is the pH scale?
The colours observed in solutions when universal indicator is added
The pH scale
is a measure of the relative acidity or alkalinity of a solution (see diagram).
So, knowing the pH of a solution, you know how acid or alkaline it is by reference to the pH scale (diagram above) or whether the solution is neutral. The smaller the pH number, the more acid it is, the greater the pH number, the more alkaline it is, and if the pH is close to 7, you have a more or less neutral solution that has neither acidic or alkaline chemical properties. Lots of examples of solution pH values are tabulated with everyday examples of acid/alkaline chemistry are described in section 1 . pH can be approximately measured using indicator solution by putting a few drops of universal indicator into a solution and comparing the colour formed with a standard chart (picture above). You can also used paper impregnated with an indicator solution (pH paper ), the paper is dipped in the solution and again the colour matched with a pH chart. This is quite handy for testing soil mixed and shaken with water. You can get special soil testing kits which use indicator solution and the colour of the indicator in the water is matched with a chart after the soil has settled out. pH can be very accurately measured with a special instrument called a pH meter using a glass electrode probe which is calibrated with standard buffer solutions of accurately known pH (see photographs and note at the end of the page). WHAT IS AN INDICATOR? An indicator is a dye substance or mixture of coloured substances that when added to the solution gives a different colour depending on the pH of the solution. Universal indicator solution or paper, is prepared from mixing several indicators to give a variety of colours to
match a wide range of pH values from very acid to very alkaline. The mixture of dyes responds to changes in pH, so depending on what the pH is, i.e. how acid, how alkaline or neutral the solution is, the indicator tells which it is. Not only that, an indicator like universal indicator' can tell you how strongly acid or strongly alkaline the solution is by giving you the pH to about the nearest indicator. A calibrated instrument called a pH meter can give the pH to two decimal places. pH meters are calibrated using buffer solutions which have an accurately known pH. It is a very handy indicator for showing whether the solution is very weakly/strongly acidic (pH <7) or alkaline (pH > 7) or neutral (pH = 7) and gives the pH to the nearest pH unit.
The diagram above gives the sort of range of colours you get from using universal indicator. which is a complex mixture of different dye molecules that respond to changes in pH. Theoretically there is no limit to the pH scale, but most solutions are between pH 0 and pH 14. For example, looking at the 'extremes', 1M hydrochloric acid (HCl) has a pH of 0 and 10M HCl has a pH of –1 and these would be described as strongly acidic solutions. 1M sodium hydroxide (NaOH) has a pH of 14, but 10M potassium hydroxide (KOH) has a pH of 15 and these would be described as strongly alkaline solutions. The closer the pH is to 7, the less strong is the acid or alkali. However the solubility limits of substances in water ensures that its almost impossible to get below –1 or above 15 and most laboratory measurements will be in the range pH 1 to pH 14 . Note 1: M is the old shorthand for solubility in mol/litre or mol dm –3 . Note 2: The pH scale is known as a logarithmic scale of base 10. At GCSE/IGCSE level, to put it more simply, a change of one pH unit means a 10x change in the acidity or alkalinity of the solution e.g. from pH 5 to pH 2 means an increase in acidity of 1000x
or to change from pH 13 to pH 11 means to become 100x less alkaline.).