Litmus Paper - Everything you need to know

WHAT IS LITMUS PAPER?

One of the first indicators used to determine the pH value of an aqueous solution by treating the filter paper with any of the usual pH indicators was litmus.

The word "litmus" is derived from a Norse word that means "colour" or "dye." Arnaldus de Villa Nova, a Spanish alchemist and physician, invented litmus approximately 1300 CE.

Litmus is a water-soluble combination of various dyes derived from lichens which is usually absorbed onto the filter paper to make one of the earliest kinds of pH indicator which in turn are used to evaluate the acidity of materials in the chemistry lab.

From the 16th century until the present, lichens have been used as a source of litmus. Between 10 and 15 indicator chemicals can be found in lichens and before becoming litmus, the lichens are thoroughly treated.

Litmus' chemical components are most likely identical to those found in the comparable mixture known as orcein, although they are in different proportions. The very essential element of Litmus, in contrast to orcein, has an average molecular mass of 3300.

Litmus paper is made by dissolving the colour in water and treating filter paper.

All pH paper is sometimes referred to as "litmus paper." However, this is incorrect. Many indicators are used as dyes in pH paper, and some of them can change colour.

USES OF LITMUS PAPER IN CHEMISTRY LAB:

Litmus paper is cost-effective and portable chemistry lab apparatus to test acidity and alkalinity that requires only a little amount of solution. It is essential to determine the pH of the substances.

Testing the pH of the material is very crucial because most life forms can only exist in a narrow pH range and hence pH is an important biological indicator. The acid-base balance in the human body, for example, is one of the delicate balance. Even a small shift in the pH value of the blood in body in either direction can cause mortality. Not only human or animals but plants are also affected by minor pH shifts in the soil. This is why overly acidic soil is balanced using calcium carbonate fertilizer, a base.

 Litmus paper comprise of various categories: Red litmus paper, Blue litmus paper, and Neutral litmus paper which is purple in nature as it consists of dyes used in both red and blue litmus paper.

 Generally blue litmus paper converts into red litmus paper when the blue litmus paper comes across the acidic material. Similarly when the red litmus paper turns comes in contact with the basic or alkaline material it turns into blue litmus paper. Neutral litmus paper will change its colour to either blue or red on the basis of solution which is being tested if its acidic or alkaline.

 In simple words, litmus paper converts into red colour in acidic solutions and changes to blue colour in simple solutions.

 A little drop of sample is placed on the coloured paper for the litmus test.

 A mild diprotic acid is found in red litmus. The hydrogen ions react with the additional base when it is exposed to a basic molecule. Wet red litmus paper turns blue in alkaline solution because the conjugate base generated from litmus acid has a blue tint.

 Though the litmus test is a quick response test and is used in the chemistry lab apparatus to determine whether the material is acidic or alkaline but it cannot provide the level of acidity or alkalinity of a solution.

 Usually litmus paper is most commonly used to determine the pH of liquids, but one can use it to determine the pH of gases but only under the condition if the paper is dampened with distilled water before being exposed to the gas.

 

LITMUS PAPER
pH VALUE
COLOUR CHANGE
Purple Litmus Paper

NEUTRAL

pH value below 4.5

pH value above pH 8.3

NO CHANGE

RED

BLUE

Blue Litmus Paper

NEUTRAL

pH value below 4.5

pH value above pH 8.3

NO CHANGE/ Slight Purple

RED

BLUE

Red Litmus Paper

NEUTRAL

pH value below 4.5

pH value above pH 8.3

NO CHANGE

RED

BLUE

 

Blue litmus paper when not changed to red indicates the solution is basic.

Red litmus paper when not changed to blue indicates the solution is acidic.

 

LITMUS PAPER TEST AT HOME:

One can also do litmus paper testing at home,

  1. Pour some lemon juice or vinegar on red litmus paper and it will not change colour.
  2. Pour some milk of magnesia on the blue litmus paper and it will not change the colour.
  3. Now Switch the colour of the litmus papers then repeat the process and closely observe the change on the litmus paper.

 

LIMITATIONS OF USING THE LITMUS PAPER IN CHEMISTRY LAB:

The litmus paper test seems to be the quick and easy test to perform, but it has a few limitations and drawbacks. For starters, it isn't a reliable pH indicator because it doesn't produce a numerical pH value. Instead, it gives an approximate indication of whether a sample is acidic or basic. Second, aside from an acid-base reaction, the paper can change colour for other reasons.

As per the research Blue litmus paper turns white when exposed to chlorine gas. The dye is bleached by hypochlorite ions and not acidity/basicity, which causes the colour change.

 

Also Read: Types of Tubes Used in Science Laboratories

 

  • Feb 19, 2022
  • Category: News
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