A compound that is added to indicate the pH of the solution is called a pH indicator and it’s a halochromic chemical compound that’s added to a solution in small amount to determine if the compound is acidic or basic. It detects the hydronium (H3O+) ions or the H+ ion in an Arrhenius model. The color of the indicator normally changes depending on the solution’s pH value. At a temperature of 25 degrees C which is considered as standard, a pH of 7 is the neutral pH of a solution. If it goes below 7 then it is considered acidic, over 7 and it’s basic. Most of the organic compounds that are naturally found are considered as weak protolytes such as amines and carboxylic acids, the pH indicator therefore finds lots of applications in analytical chemistry as well as biology. pH indicators form 3 types of compounds for indication based on the chemical analysis. For metal cation’s quantitative analysis, the complexometric indicator is more preferred while the redox indicator, which is the third compound, is used for titration involving redox reactions.
Application
pH indicators are usually used in analytic chemistry as well as in biology experiments in titrating solutions to determine up to what extent the chemicals had reacted to. Because in titration, the determination of color is subjective, it can lead to imprecise readings. For precise measurement of pH, a pH meter is more adequate.
Commercial preparations
Indicator papers are used for pH measurement. Universal indicators or the litmus test as well as the Hydrion papers are indicators that exhibit color changes with a great range of pH values.
Universal indicator
A universal indicator for pH is made up of a blend of compounds that exhibit color changes over a value of pH from 1 to 14 to indicate the solution’s acidity or basicity levels. Though there are several commercially available universal indicators, most of them are just variants of the patented formula from 1923 by Yamada. This indicator is usually composed of propan-1-ol, methanol, phenolphthalein Na+, water, methyl red, bromothymol blue monosodium salt as well as thymol blue monosodium salt.
Naturally occurring pH indicators
Many plants and parts of plants contain the naturally-colored chemical from the anthocyanin group of compounds. They’re red when placed in an acidic solution and they turn blue in a basic solution. Cabbage leaves contain anthocyanins and also on lemon’s skin. Extracting this is a popular chemistry demonstration of a crude acid and base indicator.
The anthocyanins are extracted from colored plants and/or plant parts that includes the leaves such as with the red cabbage, flowers such as with rose petals, poppy or geranium and with berries from blackcurrant or blueberries as well as with the stems such as from rhubarb.
Litmus Paper
The litmus is a water soluble mixture with different dyes that had been extracted from lichens most especially the Roccella tinctoria. It’s often dropped onto the filter paper for it to absorb and this is the type of pH paper that had been used for many decades to test for acidity. The blue litmus paper would turn red if it’s dipped in an acidic solution while the red litmus paper would turn blue in a basic solution and the color changes occur at a pH range of 4.5 to 8.3 at normal temperature of 25 degrees Celsius. The neutral type of litmus paper is colored purple. It can also be prepared in aqueous solution and it turns red under an acidic condition then blue under basic condition.
The CAS number for the litmus mixture is 1393-92-6 and this mixture has about 10 to 15 different types of dyes. Most of the components of the litmus is almost the same as the orcein mixture but just in a different proportion level. As opposed to orcein, the average MW for litmus weighs about 3300. The properties for acid-base indicators get it from the 7-hydroxyphenoxazone chromophore. Some of the litmus has names such as erythrolitmin, spaniolitmin, azolitmin, leucazolitmin and leucoorcein. Azolitmin has almost the same effect as the litmus.
How to Use the pH Paper
Step 1
First learn the scale for pH. pH is the measure of the substance’ acidity. Normal water has a pH of 7 and anything less than this is acidic and a pH that is higher than 7 is basic.
Step 2
A general purpose type of pH paper is able to study a pH level from 1 to 14. Dip this strip in your unknown liquid for around 2 seconds then take it and wait for the change in color. The pH paper must have a color charge to tell you what the color change means. Compare the dipped strip to the chart to come up with an approximation of the pH reading.
Step 3
Repeat step 2 by using a more precise strip for pH determination. As an example, if the result showed a pH of 3, use a pH paper that will measure pH level from pH 2-4. Compare the dipped strip to the chart to get a more accurate reading for the pH.
Step 4
By using the acidity of a known substance would help you in gauging accurately the pH of your unknown substance. As an example, if your unknown solution has a pH near to 7, you can compare it by dipping a strip of the paper in distilled water then compare the color of the 2.
Step 5
For a more accurate reading, use a pH meter since color can be subjective so it can be difficult to exactly get the right pH of a substance if you only base it on a pH paper.
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