The difference between NaOH% and Free NaOH%

[hazel]

Could some one tell me what is the difference between NaOH % and free NaOH %?
From analysis report for the caustic soda solution, only NaOH % data was detected and didn’t any free NaOH % contain in the solution. The laboratory person said the caustic soda was losing ability to remove hydrogen sulfide (H2S) which should be removed from our waste gas to flare.
What is the free NaOH% in the fresh 15% NaOH solution? Why would the free NaOH be losing during the caustic scrubber operation?

 

[bimr]

You lab person is just telling you that sodium hydroxide is consumed in the process to remove hydrogen sulfide. If you do not add fresh caustic to the scrubber, then the removal hydrogen sulfide will stop.

The free NaOH is just the unreacted NaOH.

http://www.lantecp.com/technical/H2S_DesignExample.pdf