Stock solutions

[Pharmguru]

If potassium permanganate solution 1 in 8000 is prepared from a stock of 10 times this strength, why would the stock solution be 1 in 800. I thought 10 TIMES MORE would be 1 in 80,000????

[Nik]

Think of it this way - if you have 1 drug molecule in 8000 molecules of water, how would 1 drug molecule in 80,000 molecules of water be more concentrated ? You are under the (mistaken) impression that bigger numbers means more concentrated solutions.

[johnep]

Must be a homeopath.
johnep

[Pharmguru]

Think of it this way - if you have 1 drug molecule in 8000 molecules of water, how would 1 drug molecule in 80,000 molecules of water be more concentrated ? You are under the (mistaken) impression that bigger numbers means more concentrated solutions.

could you explain it slightly differently please, i am still a bit lost lol

[Nik]

OK let me use a different example - you have a drop of lemon juice in say 100 drops of water. You drink the solution and it tastes a bit lemony. What happens when you put more water in ? It gets diluted and you don't get the flavour of the lemon, or the ribena or orange squash... Same principle applies with the above example. Adding in more water dilutes the original solution, making it less concentrated.

[johnep]

Remember, the maths used in pharmacy is usually proportion and percentages. Master these and you will be fine. For homework, calculate the w/v of the 1:800. 1:8,000 and 1:80,000 solutions as a percentage. (1:100 = 1%)
johnep

[Web Ferret]

Less is more.

1: 50 (1ml in 50ml = 2%)
1: 500 (1ml in 500ml 0.2%)

[Pharmguru]

1:800 = 12.5% = 0.125g??
1:8000 = 1.25% = -0.0125g?
1g in 800ml so im trying to work out how many g in 100ml right?

[Pharmguru]

Thanks guys I understand now, is my calculation correct?

the only other thing I am now confused with is, when you dilute something the active mass doesnt change but volume does so if there is 10g in 800ml, there is obviously going to be less in 100ml I would have thought??!

[Pharmguru]

You are required to prepare 5 litres of an aqueous solution of chlorhexidine gluconate which when diluted 1 in 10 produces a 1 in 1000 solution, to be prepared from an aqueous stock solution of chlorhexidine gluconate 20 per cent. What is the correct formula for this preparation?


so i am trying to work this out

when diluted 1 in 10 produces 1 in 100 so are we adding 100ml! grrr