preface to fifteenth edition

11.116 SECTION 11
Mercurous nitrate, HgNO 3 : mix 1 part of HgNO 3 , 20 parts of H 2 O, and 1 part of HNO 3 .
Metacresol purple, m-cresol-sulfonphthalein (indicator): dissolve0.1 g in 13.6 mL 0.02N NaOH
and dilute with water to 250 mL; acid pH range red 0.5–2.5 yellow, alkaline pH range yellow
7.4–9.0 purple.
Metanil yellow, diphenylaminoazo-m-benzene sulfonic acid (indicator): dissolve 0.25 g in
100 mL alcohol; pH range red 1.2–2.3 yellow.
Methyl green, hexamethylpararosaniline hydroxymethylate (component of mixed indicator): dissolve
0.1 g in 100 mL alcohol; when used with equal parts of hexamethoxytriphenyl carbinol
gives color change fromviolet to green at a titration exponent (pI) of 4.0.
Methyl orange, orange III, tropeolin D, sodium p-dimethylaminoazobenzenesulfonate (indicator):
dissolve 0.1 g in 100 mL water; pH range red 3.0–4.4 orange-yellow. If during a titration
where methyl yellow is being used a precipitate forms which tends to remove the indicator from
the aqueous phase, methyl orange will be found to be a more suitable indicator. This occurs, for
example, in titrations of soaps with acids. The fatty acids, liberated by the titration, extract the
methyl yellow so that the endpoint cannot be perceived. Likewise methyl orange is more suitable
for titrations in the presence of immiscible organic solvents such as carbon tetrachloride or ether
used in the extraction of alkaloids for analysis. Iodeosin (q.v.) has also been proposed as an
indicator for such cases. Cf. Mylius and Foerster, Ber. 24:1482 (1891); Z. Anal. Chem. 31:240
(1892).
Methyl red, p-dimethylaminoazobenzene-o-carboxylic acid (indicator): dissolve 0.1 g in
18.6 mL of 0.02N NaOH and dilute with water to 250 mL; or, 0.1 g in 60% alcohol; pH range
red 4.4–6.2 yellow.
Methyl violet (indicator): dissolve 0.25 g in 100 mL water, pH range blue 1.5–3.2 violet.
Methyl yellow, p-dimethylaminoazobenzene, benzeneazodimethylaniline (indicator): dissolve
0.1 g in 200 mL alcohol; pH range red 2.9–4.0 yellow. The color change fromyellow to orange
can be perceived somewhat more sharply than the change of methyl orange from orange to rose,
so that methyl yellow seems to deserve preference in many cases. See also under methyl orange.
Methylene blue, N,N,N,N-tetramethylthionine (component of mixed indicator): dissolve
0.1 g in 100 mL alcohol; when used with equal part of methyl yellow gives color change from
blue-violet to green at a titration exponent (pI) of 3.25; when used with equal part of 0.2% methyl
red in alcohol gives color change fromred-violet to green at a titration exponent (pI) of 5.4;
when used with an equal part of neutral red gives color change fromviolet-blue to green at a
titration exponent (pI) of 7.0.
Millon’s reagent (gives a red precipitate with certain proteins and with various phenols): dissolve
1 part of mercury in 1 part of HNO 3 (sp. gr. 1.40) with gentle heating, then add 2 parts of water;
a few crystals of KNO 3 help to maintain the strength of the reagent.
Mohr’s salt: see ferrous ammonium sulfate.
-Naphthol solution: dissolve 144 g of -naphthol in enough alcohol to make a liter of solution.
-Naphtholbenzein (indicator): dissolve 0.1 g in 100 mL 70% alcohol; pH range colorless 9.0–
11.0 blue.
-Naphtholphthalein (indicator): dissolve 0.1 g in 50 mL alcohol and dilute with water to
100 mL; pH range pale yellow-red 7.3–8.7 green.
Nessler’s reagent (for free ammonia): dissolve 50 g of KI in the least possible amount of cold
water; add a saturated solution of HgCl 2 until a very slight excess is indicated; add 400 mL of a
50% solution of KOH; allow to settle, make up to a liter with water, and decant.