[BaO]

for example : orange juice, ...?

[Will]

Concentration of what in orange juice? For example, the concentration of H⁺ in my orange juice is about 0.0004 moles dm^-3, measured using a pH meter.

[BaO]

yeah , that is what i meant
sorry , my question wasnt clear,
 
how did you measure it? is it a machine or something ?

[Will]

yeah , that is what i meant
sorry , my question wasnt clear,

That's OK

how did you measure it? is it a machine or something ?

Yes; a pH meter is a small device which normally uses a glass electrode with a very accurate voltmeter to work out the concentration of H⁺ ions in solution.
More info here:
http://www.ph-meter.info/pH-meter
http://en.wikipedia.org/wiki/pH_meter

You have to use other devices/methods to work out the concentration of other stuff in orange juice though.