# Re: ppm (parts per million)

```Hello Quek,

On Mon, 18 Jan 1999, you wrote:

Could some kind soul help me with the above term.  I was offered a couple of
equivalent for dosage. They are 1mg/lit and 10mg/lit.  How do you arrived at
the equivalent. Thank you.

Dave Lorenzen responded to you and is right in stating for concentrations in
water 1mg/L= 1ppm.  If you are curious to know more about the derivation

A concentration of one part per million (ppm) corresponds to one part of the
substance per one million parts of the gas, liquid or solid mixture in which
the substance occurs.  In the case of liquids and solids the reference is
generally to ppm by weight.   (In the case of a gas mixture, the reference
is generally ppm by volume).

The density of water is 1g/mL. There are 1,000 milligrams (mg) in one gram
and 1,000 milliliters (mL) = 1 liter.  Therefore, 0.001g(of your substance)
in one mL of water (1mg/mL)  = one part/thousand, and so it further follows
that 1mg/1000mL (1mg/L) = one part per million (1 ppm).   1mg/L = 1ppm and
of course 10mg/L would be 10 ppm.

Craig David

```