Let's try to make Litmus test paper from wild lichens

A litmus test paper, used to test acidity and alkalinity, is a familiar sight in science classes of Junior high school.

In the past, a litmus test paper was made from the purple dye extracted from a lichen called litmus lichens that grew in the Mediterranean region. Representative species is Roccella tinctoria. As you already know, a lichen is a symbiotic organism of fungi and algae. But currently, litmus test papers commercially available are made from chemically synthesized pigments. 　

A friend of mine (a former science teacher at junior high school) recently held a Science Cafe to try to make a litmus test paper from wild lichens at the antiquarian bookshop & cafe he runs for 5 years. Unfortunately, I wasn't able to attend, but I went to his bookshop later to hear how the science cafe was and to take a look at the experimental materials used at that time. 

Since litmus lichens are not distributed in Japan, in this experiment, he used the lichen called Umenoki-goke in Japanese.  Japanese name means lichen on a ume tree.  If I had to say, ume is an apricot. We can easily find this lichen in Japan. Its scientific name is Parmotrema tinctorum.

This is the Umenoki-goke growing on an old tree trunk of ume in my orchard.

What looks light green powder is called soredia, powdery asexual reproductive organs of lichens.

This is a dried Umenoki-goke. But, in his experiment, he collected the lichen that was growing on the trunk of cherry trees at the riverside. 

The back side of the dried lichen. Dark brown color, but the pigment to be extracted from the lichen is not present here.

Material and Method　by his procedures

Add 100 milliliters of 3% ammonia solution and 5 milliliters of oxidol to 10 grams of dried lichens that are torn into small pieces. Oxidol is a commercial disinfectant (3% hydrogen peroxide solution).

He said that adding oxidol promoted the extraction of the red-purple pigment. By the way, this bottle is 5 days after immersion as of photography.

Stir once a day and leave for about a month. The contents eventually turned a deep red purple. It has a slight smell of ammonia. Lichens in this bottle have been immersed for about one month. 

Filter through gauze, etc., to obtain a deep red purple solution. 

Dilute the extracted solution with water 10-fold and soak the filter paper in this diluted solution. Dry the filter paper. See below photo.

Next, dip this dried filter paper in 1% sodium hydroxide solution and dry again. After drying, the blue litmus test paper has done.　As for the red litmus paper, soak this blue litmus paper in 1% acetic acid solution and dry it.

Do you mind if I add another point?　Why is the above filter paper necessary to be treated by weak alkalinity or weak acidity in advance?

The litmus test paper is blue or red due to the above treatment before usage, so blue turns red when dipped in acidity, and red turns blue when dipped in alkalinity. Therefore, there is no color change when dipped in the neutral solution.

After learning detailed explanation from him and I received the materials distributed on the day of the event. Further, I got rest of the litmus paper made at the Science Cafe. And I made some simple experiments with it at home.

I dipped a piece of red test paper in a diluted solution of kitchen bleach. Of course, this is alkaline solution. It turned blue, but strictly speaking it seems purple in appearance. 

This is a blue test paper soaked in vinegar (acidic). It turned reddish. The blue test paper, relatively speaking, looks more purple.

Lastly, we can check whether various aqueous solution is acidic or alkaline without making test paper. 

Dilute the deep red purple stock solution 100-fold and place 10 milliliters of the solution in a test tube. Use a dropper to add the aqueous solution, such as lemon juice, soap solution, etc. that you want to test. Then, compare the color with an untreated test tube.