Ahmed Hassen

Hallo,

why do we use 70% ethanol to disinfect?

I always assumed it was because 70% was the lowest dosage that was still working (the more you dilute, the cheaper it is). But is this even true?

I have heard someone saying that 70% is the % that works the best against bacteria etc..
But why does 70% work the best? And why not 80%?


I find it hard to believe that 100% would work as well or even better then 70% to kill of germs..

Today was a very bad day at the university... beside that some students answered in the quiz question that if you get blue color in Gram staining that means your bacteria is Gram negative I noticed most of the students including myself had trouble with the dilution process. So I thought maybe I'd make a quick explanation for what is dilution and how it is done and all the ratios and other confusing stuff. I tried my best to simplify it for people who find trouble in doing their homeworks, labwork, or other calculation problems regarding dilution questions and solutions.

So let's start out with definitions:

Dilution: is the mixing of a small accurately measured sample with a large volume of sterile water or normal saline called (diluents or dilution blank)

Laws:

Dilution = V of Sample / Total V of (sample + diluent)

Dilution Factor = Total V of (sample + diluent) / V of sample
** or we can simply say the reciprocal of Dilution

Starting off with this simple example to understand how these laws are applied.





It has been known that if we use a larger volume we obtain a more accurate dilution.
So for better results, we use 1:1000 dilution. And that is by adding 1ml of sample to 999 ml of diluent. But practically we cannot use 999 ml of diluent. So we do what is called a serial dilution.

Serial Dilution: is a dilution made of a series of smaller dilution, and the total dilution is the product of each dilution in the series.

To understand this more, let's see this example.







Here's an example combining all of the above.





I hope this brief explanation proves helpful to you...let me know if i've made any mistakes...

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thank u for your explanation..

what makes me always confused is the last volume used for inoculation (0.1ml in your example)..

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Yeah that's why i almost jumped with 'HAVE' there :blush: sorry about that
i hope my mini-tutorial and examples helped you through it strawberry...

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I'm very thankful to you to provide the easier explaination...it's true that many undergrad students have problem in dilution..and the worst thing is if the lecturers/tutors assume that the students are familiar with the concept. it is true that they have learned it in high school, but most of the time they will forget..

Regarding the example 2 given, you dilute a sample (10 ml the final volume) 10x10x10=10 to the power of 3. and u inoculate 0.1 ml in your medium.so supposed the final volume of your medium is 1ml right?

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