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

Jul 26, 2017

See below.

#### Explanation:

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 ${\text{Ag"_2"CrO}}_{4}$ at the endpoint after all the chloride has been precipitated as $\text{AgCl}$.

"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.

$\text{2CrO"_4^"2-" + 2"H"^"+" → "Cr"_2"O"_7^"2-" + "H"_2"O}$

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