[dingleberry]

Okay. I have a 1% liquid methylene blue. I assume this means 10 mg of methylene blue per ml. Please correct me if I'm wrong.

Okay. Lets say I take 1 ml of this and mix it with 100 ml of water. This will give me ~100mcg of methylene blue per ml, yes?

So, starting with a 1% methylene blue solution, how many ml of water do I need to mix one ml with to get 60mcg of methylene blue per ml?

Thanks.

[DevaDevil]

Okay. I have a 1% liquid methylene blue. I assume this means 10 mg of methylene blue per ml. Please correct me if I'm wrong.

a % is always mass/mass, or volume/volume (in case of water your assumption is almost correct as water has a density of close to 1 g/ml)
so 1% solution means either 1ml of methylene blue per 100ml water, or 1 mg methylene blue per 100mg water, depending on if it is mass% or vol%

[Vidya]

your first two steps are perfectly right
now you can work for the last part in the same manner

[dingleberry]

I'm 39, and exhausted.  Please help.

[DevaDevil]

1 ml of your solution added to 100 ml of water will have a concentration of 10mg / 0.101 l is about 100 mg / l, or 100 microg. / l.

in formula:
you start with 10 mg = 10000 microg.
you need to add V_w volume of water to reach 60 microg per milliliter;
C_end = 60 microg/ml = 10000 / V_total
V_total = V_w + 1 ml (starting volume of your methylene blue)

so:
60 microg/ml = 10000 / V_total ==> V_t = 10000/60 = 167 ml
and
V_w = V_t - 1 = 166 ml.

[dingleberry]

God bless you, DevaDevil.

[JGK]

A % is always mass/mass, or volume/volume

Have to disagree there, % units can be w/w, v/v or w/v.

In Analytical chemisty the latter two ar more common than %w/w

[DevaDevil]

so what is the convention for %w/v (essentially a mass concentration)
[g/m³]?

I have not encountered it in my field I have to admit, and the dimensionlessness of a % would not be observed in w/v, so it must be well-defined.

[rabolisk]

Typically it's g/mL. Mathematically, this is flawed since % have to be dimensionless, and since it is possible for a solution to be >100% concentrated. Nevertheless, I have also seen w/v used rather often.