How to make a partially diluted solution of a chemical


The Sunday Nation


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 20-ml 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 three-quarters 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 semi-diluted 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 semi-diluted 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 spray-can That will give me the recommended final concentration.

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