Nn indicator for calcium carbonate assay test

Please describe use of nn indicator for assay of calcium carbonate in Japanese Pharmacopeia .
nn indicator formula? , Appearance? and any other name if available

NN Indicator is similar to “Eriochrome Black T” indicator. The procedure of preparation as per Japanese pharmacopoeia is as follows,

Mix 0.5 g of 2-hydroxy-1-(2-hydroxy-4-sulfo-1-naphthylazo)-3-naphthoic
acid with 50 g of anhydrous sodium sulfate, and triturate until the mixture becomes
126 homogeneous.
Eriochrome Black T molecular name: 2-Hydroxy-1-(1-hydroxy-2-naphthylazo)-6-nitronaphthalene-4-sulfonic acid sodium salt

This is also known as Patton & Reeder’s reagent having formula 3-Hydroxy-4-(2-hydroxy-4-sulfo-1-naphthylazo)-2-naphthalenecarboxylic acid.