Fundamentals of Sample Preparation for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 164

the digestion process. As a strong acid, it dissolves inorganic oxides into solution
(Eq. 7.1). As an oxidizing agent/acid combo, HNO 3 can oxidize zero valence
inorganic metals and non-metals into ionic form (Eqs. 7.2 and 7.3):

CaOþ2H 3 O þ ! Ca 2þ þ3H 2 O ð7 : 1Þ

Fe 0 þ3H 3 O þ þ3 HNO 3 ðconc :Þ!Fe 3þ þ3NO 2 ðbrownÞþ6H 2 O

ð7 : 2Þ

3Cu 0 þ6H 3 O þ þ2 HNO 3 ðdiluteÞ!2NOðclearÞþ3Cu 2þ þ10 H 2 O ð7 : 3Þ

Nitric acid undergoes both one electron (Eq. 7.2) and three electron (Eq. 7.3)
changes. The one electron change is observed with concentrated HNO 3 , whereas the
three electron change is observed when dilute HNO 3 is in reaction. The presence of
brown fumes during digestion is indicative of reactions going by one electron.

H 3 O þ þHNO 3 þe

* NO 2 "ðbrownÞþ2H 2 OðconcentratedÞ 7 : 4Þ

3H 3 O þ þHNO 3 þ3e

* NO"ðclearÞþ5H 2 OðdiluteÞ ð7 : 5Þ

The second advantage of using HNO 3 is that it does not form any insoluble
compounds with metals and nonmetals. The same cannot be said for H 2 SO 4 , HCl,
HF, H 3 PO 4 , or HClO 4 . An additional advantage in using HNO 3 is that nitrate ðNO 3 Þ
is the acceptable matrix for both flame and electrothermal atomic absorption and the
preferred matrix for ICP-MS. When digesting highly aromatic samples, or samples
containing -OH functionality, a pretreatment with concentrated sulfuric acid as a
dehydrating agent is recommended as shown in the following equation:

RCH 2 CHðOHÞR 0 þH 2 SO 4 ! RCH

CðOHÞR 0 þH 2 O þ H 2 SO 3 ð7 : 6Þ

Practical tips

Evaporate HNO 3 as much as possible, but not to dryness in the digestion
container. Volatile metals such as Hg, Pb, Cd, Ca, As, Sb, Cr, and Cu are
subject to loss if solution becomes dry. A complete digestion usually can be
seen from the light color of the digestate or the digestate does not change in
appearance with continued refluxing.
Avoid metal contamination from any potential sources, including air borne
dusts, acids, containers, and pipettes. Plastic pipet tips are often contaminated
with Cu, Fe, Zn, and Cd. Avoid using colored plastics that can contain
metals. Use certified metal-free plastic containers and pipet tips when
possible. Avoid using glass if analyzing Al and Si. Use metal-free water
and ultra-pure acids (mineral acids can be sources of many elements
including Al, As, Mn, Cr, Ni, and Zn).
Take extreme cautions for samples with high organic contents. Pretreat
HNO 3 before adding HClO 4 to oxidize most of the organic matter. Do not
use HClO 4 alone and never evaporate it to dryness. Use metal-free hood and
perchloric acid hood (stainless steel or PVC liner) if HClO 4 is being used.
Never heat HClO 4 in fume hood unless it has been designed for use with