Why do we need to do neutralization in clause 54.1 of ASTM D460?

1 Answer
Jul 26, 2017

See below.


The determination of chloride by titration with silver nitrate using potassium chromate as an indicator is called the Mohr method.

It is based on the formation of a red precipitate of #"Ag"_2"CrO"_4# at the endpoint after all the chloride has been precipitated as #"AgCl"#.

#"2Ag"^"+""(aq)" + "CrO"_4^"2-""(aq)" → underbrace("Ag"_2"CrO"_4"(s)")_color(red)("red precipitate")#

If the solution is acidic, the chromate is converted to dichromate.

#"2CrO"_4^"2-" + 2"H"^"+" → "Cr"_2"O"_7^"2-" + "H"_2"O"#

The endpoint then either comes too late or cannot be detected at all.