Encyclopedia of Environmental Science and Engineering, Volume I and II

4 ACID RAIN

Sample Handling

Changes in the chemicals in the sample over time are

decreased through (1) the addition of preservatives to pre-

vent biological change, (2) refrigeration, (3) aliquoting, and

(4) filtering. Filtering is more effective than refrigeration for

stabilizing samples for some species such as calcium and

magnesium. For species such as organic acids, only chemi-

cal preservatives are certain to prevent change.

Analytical Methods

Several analytical methods are available to adequately measure

the major ions found in precipitation, but special precautions

are necessary because the concentrations are low and thus the

samples are easily contaminated. Measurement of the chemical

parameter pH, although deceptively easy with modern equip-

ment, requires special care in order to arrive at accurate results

because of the low ionic strength of rain and snow samples.

Frequent checks with low ionic strength reference solutions are

required to avoid the frequent problem of malfunctioning pH

electrodes. The ions SO 42 , NH 4 , Ca^2 , etc., are measured

in modern laboratories by ion chromatography, automated

colorimetry, flame atomic absorption, and other methods.

Quality Assurance/Quality Control

The chemical analysts actually performing measurements

should follow documented procedures, which include mea-

surements of “check” or “known” solutions to confirm imme-

diately and continuously that the work is “in control” and

thus is producing quality results. At an administrative level

above the analysts, procedures are developed to “assure” that

the results are of the quality level established for the pro-

gram. These quality assurance procedures should include the

submission of blind reference samples to the analysts on a

random basis. Quality assurance reports should routinely be

prepared to describe procedures and results so that the data

user can be assured (convinced) that the data are of the quality

level specified by the program. In the past, insufficient atten-

tion has been given to quality assurance and quality control.

As a minimum, from 10 to 20% of the cost of a monitoring

program should be devoted to quality assurance/quality con-

trol. This is especially true for measurements on precipitation

samples that have very low concentrations of the acid-rain-

related species and thus are easily contaminated.

CALCULATING PRECIPITATION pH

This section describes the procedures for calculating the

pH of a precipitation sample when the concentrations of the

major inorganic ions are known (Stensland and Semonin,

1982). Granat (1972), Cogbill and Likens (1974), and Reuss

(1975) demonstrated that the precipitation pH can be calcu-

lated if the major ion concentrations are known. The pro-

cedure described below is analogous to that used by these

previous workers but is formulated somewhat differently.

Three good reasons to have a method to calculate the pH

are that:

1) The pH can be calculated for older data sets when

pH was not measured but the major inorganic ions

were measured (e.g., the Junge (1963) data set),

2) The trends or patterns of pH can be interpreted in

terms of trends or patterns in the measured inor-

ganic ions such as sulfate or calcium, and

3) The calculated pH can be compared with the mea-

sured pH to provide an analytical quality control

check.

Gases (e.g., SO 2 and CO 2 ) and aerosols (e.g., NaCl and

(NH 4 ) 2 SO 4 ) scavenged by precipitation can remain as electri-

cally neutral entities in the water solution or can participate

in a variety of chemical transformations, including simple

dissociation, to form ions (charged entities). The basic prem-

ise that the solution must remain electrically neutral allows

one to develop an expression to calculate pH. Stated another

way, when chemical compounds become ions in a water

solution, the quantity of positive ions is equal to the quantity

of negative ions. This general concept is extremely useful in

discussing acid precipitation data.

As a simple example, consider a solution of only water

and sulfuric acid (H 2 SO 4 ). The solution contains H^ ^ , OH^ ^ ,

and ions. At equilibrium

(H^ ^ )(OH^ ^ )  10 ^14 (m/L)^2

if the ion concentrations are expressed in moles/liter

(m/L). Assuming pH  4, then from the defining relation

pH  log(H^ ^ ) it follows that

(H^ ^ )  10 ^4 m/L

Therefore (OH^ ^ )  10 ^10 m/L and thus (OH^ ^ ) is so small

that it can be ignored for further calculations. Since the dis-

sociation of the sulfuric acid in the water gives one sulfate

ion for each pair of hydrogen ions, it follows that

(SO 42  )  1/2(H)  0.5  10 ^4 m/L

It is useful to convert from moles/liter (which counts par-

ticles) to equivalents/liter (eq/L), as this allows one to count

electrical charge and thus do an “ion balance.” The conver-

sion is accomplished by multiplying the concentration in

m/L by the valance (or charge) associated with each ion. The

example solution contains

(0.5  10 ^4 m/L)  (2)  10 ^4 eq/L  100 m eq/L

of sulfate and

(1  10 ^4 m/L)  (1)  10 ^4 eq/L  100 m eq/L

of hydrogen ion. Thus the total amount of positive charge

(due to H^ ^ in this example) is equal to the total amount of

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