How to make a partially diluted solution of a chemical
By MUNGAI KIHANYA
The Sunday Nation
Nairobi,
02 April 2023
Recently, I needed a pesticide for my houseplants, so, I bought some
from an agrochemicals shop. I chose the smallest package (just 100ml)
available but I soon discovered that these chemicals are targeted to
farmers with many crops in the field: the instructions for use said;
“Mix 5ml in 20L of water”. That sounds fair enough if you are a farmer –
the small container will produce 400 litres of the mixture!
But all I needed was about 2L of the mixture. To get the correct
concentration would require that I measure out 0.5ml of the chemical.
The package came with a 20ml measuring cap that has 5ml subdivisions.
How can one use this to measure 0.5ml? It is impossible!
I decided to make 5L of the mixture. That meant measuring 1.25ml of the
chemical. This was still not possible with the measuring cap provided so
I proceeded as follows: I measured out 5ml of chemical then topped the
measuring cap with water. I stirred the mixture gently with a stick and
then poured away 15ml leaving only 5ml in the measuring cap.
I figured that, since I had started with 5ml of chemical, and then
removed threequarters of it (15ml out of 20ml), one quarter of 5ml was
left in the measuring cap. This is what I need to mix with 5L of water.
After two weeks of spraying, the 5L mixture was exhausted and I needed
some more. This time I approached the problem differently. What if I
made 5L of semidiluted solution from which I can take out 20ml and mix
with 1L of water to get the desired dilution?
At the rate of 5ml chemical in 20L of water, I am getting a
concentration of 0.25ml per litre. Thus, my semidiluted solution needs
to have 0.25ml of chemical in every 20ml measuring cap.
So, the first question is, how many 20ml measuring caps are there in 5L?
The answer is 5 divided by 0.02, that is, 250 caps. Now, each of these
should contain 0.25ml of the chemical, therefore, I need 250 x 0.25 =
62.5ml of chemical in 5L of water.
I repeated the calculation with 4L of water and found that I need 50ml
of the chemical. This is easily measurable with my measuring cap. I
added the 50ml to 4L of water and from now henceforth, I will be pouring
20ml of this solution 1L of water in my spraycan That will give me the
recommended final concentration.
