Why 1.2 % KCL used for stray light calibration in UV Vis Spectrophotometer

"Stray light" is light from wavelengths other than the desired wavelength.

A solution of 1.2% (I'm not exactly sure why it's always 1.2%) provides ionic strength to the solution (hence, it changes the refractive index so as to be similar to comparable solutions), but it doesn't absorb much in the UV or Visible region. Therefore, light hitting the detector will be almost all stray light.

KCl cuts off at 200 nM.

Thanks

I think the limit for the stray light by using KCl is not 200 nm rather it is 198 nm now.

The Stray Light Reference Materials are useful in determining the amount of stray light in your instrument because each material stops transmitting light below a specific wavelength. Hence, below a specified "cur-off" wavelength, any indication of light transmission must be stray light.

See: http://www.starnacells.com/d_ref/d_sl/Stray.html