Chapter 6
Water Chemistry and Analysis
Introduction:
All natural waters contain varying amounts of suspended and dissolved matter as well as
dissolved gases. The type and amount of impurities in fresh water vary with the source (lake,
river, well) and with the area of location. Impurities in water become an important consideration
when water is to be used for steam generation. With the trend toward higher-pressure boilers,
pretreatment has become the key to successful operation of industrial power plants. Feedwater
must be pretreated to remove impurities to control deposition, carryover, and corrosion in the
boiler system. Poor quality water gives poor quality steam. The first step in any treatment is
filtration of suspended solids. On the basis of proven satisfactory performance, cost, and other
considerations, cartridge filters are a practical solution to most problems of water clean-up.
Water impurities include dissolved and suspended solids. Calcium bicarbonate is a soluble salt.
A solution of calcium bicarbonate is clear, because the calcium and bicarbonate are present as
atomic sized ions which are not large enough to reflect light. Some soluble minerals impart a
color to the solution. Soluble iron salts produce pale yellow or green solutions; some copper
salts form intensely blue solutions. Although colored, these solutions are clear. Suspended
solids are substances that are not completely soluble in water and are present as particles.
These particles usually impart a visible turbidity to the water. Dissolved and suspended solids
are present in most surface waters. Seawater is very high in soluble sodium chloride;
suspended sand and silt make it slightly cloudy. An extensive list of soluble and suspended
impurities found in water is given in Table 1-1.
Constituent
Turbidity
Hardness
Alkalinity
Chemical Formula
Difficulties Caused
Means of Treatment
imparts unsightly appearance to
non-expressed in analysis water; deposits in water lines,
coagulation, settling, and
as units
process equipment, etc.;
filtration
interferes with most process uses
chief source of scale in heat
calcium and magnesium
softening; demineralization;
exchange equipment, boilers,
salts, expressed as
internal boiler water treatment;
pipe lines, etc.; forms curds with
CaCO3
surface active agents
soap, interferes with dyeing, etc.
lime and lime-soda softening;
foam and carryover of solids with
bicarbonate(HCO3-),
acid treatment; hydrogen
steam; embrittlement of boiler
carbonate (CO32-), and
zeolite softening;
steel; bicarbonate and carbonate
hydroxide(OH-),
demineralization
produce CO2 in steam, a source
expressed as CaCO3
dealkalization by anion
of corrosion in condensate lines
exchange
Free Mineral
Acid
H2SO4 , HCI. etc.,
expressed as CaCO3
corrosion
neutralization with alkalies
Carbon
Dioxide
CO2
corrosion in water lines,
particularly steam and
condensate lines
aeration, deaeration,
neutralization with alkalies
PH
hydrogen ion
concentration defined as: pH varies according to acidic or
alkaline solids in water; most
1
natural waters have a pH of 6.0pH = log
8.0
[H+]
Sulfate
SO42-
Chloride
Cl -
Nitrate
NO3-
Fluoride
F-
Sodium
Na+
Silica
SiO2
Iron
Fe2+ (ferrous)
Fe3+ (ferric)
Manganese
Mn2+
Aluminum
AI3+
Oxygen
O2
pH can be increased by
alkalies and decreased by
acids
adds to solids content of water,
demineralization, reverse
but in itself is not usually
osmosis, electrodialysis,
significant, combines with calcium
evaporation
to form calcium sulfate scale
adds to solids content and
demineralization, reverse
increases corrosive character of
osmosis, electrodialysis,
water
evaporation
adds to solids content, but is not
usually significant industrially:
demineralization, reverse
high concentrations cause
osmosis, electrodialysis,
methemoglobinemia in infants;
evaporation
useful for control of boiler metal
embrittlement
cause of mottled enamel in teeth; adsorption with magnesium
also used for control of dental hydroxide, calcium phosphate,
decay: not usually significant
or bone black; alum
industrially
coagulation
adds to solids content of water:
demineralization, reverse
when combined with OH-, causes
osmosis, electrodialysis,
corrosion in boilers under certain
evaporation
conditions
hot and warm process
removal by magnesium salts;
scale in boilers and cooling water adsorption by highly basic
systems; insoluble turbine blade
anion exchange resins, in
deposits due to silica vaporization
conjunction with
demineralization, reverse
osmosis, evaporation
discolors water on precipitation;
aeration; coagulation and
source of deposits in water lines, filtration; lime softening; cation
boilers. etc.; interferes with
exchange; contact filtration;
dyeing, tanning, papermaking, surface active agents for iron
etc.
retention
same as iron
same as iron
usually present as a result of floc
carryover from clarifier; can cause
improved clarifier and filter
deposits in cooling systems and
operation
contribute to complex boiler
scales
corrosion of water lines, heat
deaeration; sodium sulfite;
exchange equipment, boilers,
corrosion inhibitors
return lines, etc.
Hydrogen
Sulfide
H2S
Ammonia
NH3
Dissolved
Solids
none
Suspended
Solids
none
Total Solids
none
cause of "rotten egg" odor;
aeration; chlorination; highly
corrosion
basic anion exchange
corrosion of copper and zinc
cation exchange with
alloys by formation of complex hydrogen zeolite; chlorination;
soluble ion
deaeration
refers to total amount of dissolved
lime softening and cation
matter, determined by
exchange by hydrogen zeolite;
evaporation; high concentrations
demineralization, reverse
are objectionable because of
osmosis, electrodialysis,
process interference and as a
evaporation
cause of foaming in boilers
refers to the measure of
undissolved matter, determined subsidence; filtration, usually
gravimetrically; deposits in heat preceded by coagulation and
exchange equipment, boilers,
settling
water lines, etc.
refers to the sum of dissolved and
see "Dissolved Solids" and
suspended solids, determined
"Suspended Solids"
gravimetrically
CHEMICAL REACTIONS
Numerous chemical tests must be conducted to ensure effective control of a water treatment program.
Most of these tests are addressed in detail in Chapters 39-71. Because of their significance in many
systems, three tests, pH, alkalinity, and silica, are discussed here as well.
pH Control
Good pH control is essential for effective control of deposition and corrosion in many water systems.
Therefore, it is important to have a good understanding of the meaning of pH and the factors that affect
it.
Pure H2O exists as an equilibrium between the acid species, H+ (more correctly expressed as a
protonated water molecule, the hydronium ion, H30+) and the hydroxyl radical, OH -. In neutral water
the acid concentration equals the hydroxyl concentration and at room temperature they both are
present at 10-7 gram equivalents (or moles) per liter.
The "p" function is used in chemistry to handle very small numbers. It is the negative logarithm of the
number being expressed. Water that has 10-7 gram equivalents per liter of hydrogen ions is said to have
a pH of 7. Thus, a neutral solution exhibits a pH of 7. Table 1-3 lists the concentration of H+ over 14
orders of magnitude. As it varies, the concentration of OH - must also vary, but in the opposite direction,
such that the product of the two remains constant.
Table 1-3. pH relationships.
H+ Concentration,
+
pHa H Concentration
Exponential Notation, Normality
OH - Concentration,
Normality
OH - Concentration,
pOH Exponential Notation,
gram moles/L
gram moles/L
0
100
1
0.00000000000001
10-14
14
1
10-1
0.1
0.0000000000001
10--13
13
2
10-2
0.01
0.000000000001
10--12
12
3
10-3
0.001
0.00000000001
10-11
11
4
10-4
0.0001
0.0000000001
10-10
10
5
10-5
0.00001
0.000000001
10-9
9
6
10-6
0.000001
0.00000001
10-8
8
7
10-7
0.0000001
0.0000001
10-7
7
8
10-8
0.00000001
0.000001
10-6
6
9
10-9
0.000000001
0.00001
10-5
5
10 10-10
0.0000000001
0.0001
10-4
4
11 10-11
0.00000000001
0.001
10-3
3
12 10-12
0.000000000001
0.01
10-2
2
13 10-13
0.0000000000001
0.1
10-1
1
14 10-14
0.00000000000001
1
100
0
a
pH+pOH=14.
Confusion regarding pH arises from two sources:

the inverse nature of the function

the pH meter scale
It is important to remember that as the acid concentration increases, the pH value decreases (see Table
1-4).
Table 1-4. Comparative pH levels of common solutions.
a
12
OH - alkalinity 500 ppm as CaCO3
11
OH - alkalinity 50 ppm as CaCO3
Columbus. OH, drinking water, a
10
OH - alkalinity 5 ppm as CaCO3
9
strong base anion exchanger effluents
8
phenolphthalein end point
7
neutral point at 25 °C
6
Weymouth, NIA, drinking water, a
5
methyl orange end point
4
FMA 4 ppm as CaCO3
3
FMA 40 ppm as CaCO3
strong acid cation exchanger effluent
2
FMA 400 ppm as CaCO3
Extremes of drinking water pH
The pH meter can be a source of confusion, because the pH scale on the meter is linear, extending from
0 to 14 in even increments. Because pH is a logarithmic function, a change of I pH unit corresponds to a
10 fold change in acid concentration. A decrease of 2 pH units represents a 100 fold change in acid
concentration.
Alkalinity
Alkalinity tests are used to control lime-soda softening processes and boiler blowdown and to predict
the potential for calcium scaling in cooling water systems. For most water systems, it is important to
recognize the sources of alkalinity and maintain proper alkalinity control.
Carbon dioxide dissolves in water as a gas. The dissolved carbon dioxide reacts with solvent water
molecules and forms carbonic acid according to the following reaction:
CO2 + H2O = H2CO3
Only a trace amount of carbonic acid is formed, but it is acidic enough to lower pH from the neutral
point of 7. Carbonic acid is a weak acid, so it does not lower pH below 4.3. However, this level is low
enough to cause significant corrosion of system metals.
If the initial loading of CO2 is held constant and the pH is raised, a gradual transformation into the
bicarbonate ion HCO3- occurs.
The transformation is complete at pH 8.3. Further elevation of the pH forces a second transformation
into carbonate, CO32-. The three species carbonic acid, bicarbonate, and carbonate can be converted
from one to another by means of changing the pH of the water.
Variations in pH can be reduced through "buffering" the addition of acid (or caustic). When acid (or
caustic) is added to a water containing carbonate/bicarbonate species, the pH of the system does not
change as quickly as it does in pure water. Much of the added acid (or caustic) is consumed as the
carbonate/bicarbonate (or bicarbonate/carbonic acid) ratio is shifted.
Alkalinity is the ability of a natural water to neutralize acid (i.e., to reduce the pH depression expected
from a strong acid by the buffering mechanism mentioned above). Confusion arises in that alkaline pH
conditions exist at a pH above 7, whereas alkalinity in a natural water exists at a pH above 4.4.
Alkalinity is measured by a double titration; acid is added to a sample to the Phenolphthalein end point
(pH 8.3) and the Methyl Orange end point (pH 4.4). Titration to the Phenolphthalein end point (the Palkalinity) measures OH - and 1/2 CO32-; titration to the Methyl Orange end point (the M-alkalinity)
measures OH -, CO32- and HCO3 .
Silica
When not properly controlled, silica forms highly insulating, difficult to remove deposits in cooling
systems, boilers, and turbines. An understanding of some of the possible variations in silica testing is
valuable.
Most salts, although present as complicated crystalline structures in the solid phase, assume fairly
simple ionic forms in solution. Silica exhibits complicated structures even in solution.
Silica exists in a wide range of structures, from a simple silicate to a complicated polymeric material. The
polymeric structure can persist when the material is dissolved in surface waters.
The size of the silica polymer can be substantial, ranging up to the colloidal state. Colloidal silica is rarely
present in groundwaters. It is most commonly present in surface waters during periods of high runoff.
The polymeric form of silica does not produce color in the standard molybdate based colorimetric test
for silica. This form of silica is termed "nonreactive". The polymeric form of silica is not thermally stable
and when heated in a boiler reverts to the basic silicate monomer, which is reactive with molybdate.
As a result, molybdate testing of a boiler feedwater may reveal little or no silica, while boiler blowdown
measurements show a level of silica that is above control limits. High boiler water silica and low
feedwater values are often a first sign that colloidal silica is present in the makeup.
One method of identifying colloidal silica problems is the use of atomic emission or absorption to
measure feedwater silica. This method, unlike the molybdate chemistry, measures total silica
irrespective of the degree of polymerization.
Boiler Water Testing:
In the draft "Code of practice for the design, safe operation, maintenance and servicing of boilers", a
requirement is made for regular water-quality monitoring of both limited-attendance boilers and unattended
boilers.
OSH require monthly tests for these boiler types and stipulate that the testing be conducted by an IANZ
accredited laboratory.
If you supervise a limited-attendance or unattended boiler then ELS can assist you with your water testing
requirements.
A boiler requires testing of three different water types as shown below:
Feedwater
Boiler feedwater is sourced from many different places. Some supplies come from industry owned bores and
treatment plants, while others come directly from a council supply, however all feedwater should be analysed in
order to correctly determine dose rates of treatment chemicals.
Water quality can change as it passes through a delivery or reticulation system, so it is important to check for
various parameters at point of use - ie where it enters the boiler or pre-treatment system.
Boiler feedwater is usually a combination of returned condensate plus pre-treated makeup water from a
softener, reverse osmosis, or other purification system. Typical tests used for boiler feedwater include:

Chloride or salinity

Conductivity

Dissolved Oxygen

Hardness

Iron and Manganese

pH

Silica

Sulphide

Suspended Solids

Total Dissolved Solids

Turbidity
Not all water supplies will require all the tests shown here, and if the supply is constant the tests will not need to
be repeated very often.
Boiler Water
The boiler water itself must be dosed in order for the boiler to run efficiently and safely. A chemical imbalance
can lead to corrosion and damage to the system and this damage can ultimately lead to boiler failure and
injury.
Boiler water analyses are basically aimed at keeping the parameters within established limits. Tests include

Chloride

Hydroxide P2 Alkalinity

Nitrate

pH

Phenolphthalein P1 Alkalinity

Phosphate

Silica

Sulphite

Total Alkalinity

Total Dissolved Solids
Condensate
Good condensate is the best quality, least expensive water most systems can generate. You do not want to
lose it, or contaminate it unnecessarily.
Steam condensate analysis should include

Ammonia

Conductivity

Copper

Iron

pH
Steam Purity Measurement:
Accurate measurement of steam purity is essential to identifying the cause of potential or existing steam
purity problems in modern boiler plants. One reason for this is that superheated steam turbines have an
extremely low tolerance for solids contamination in the steam. Fortunately, techniques are available to
determine steam contamination in the parts per billion range to satisfy the demands of most systems. The
test results make it possible to determine the effect of changing boiler operation on steam purity.
IMPURITIES
Impurities present in steam can be solid, liquid, or gaseous. Solids are usually dissolved in water droplets
or are present as dust. Because water treatment practices are such that most soluble chemical
constituents of boiler feedwater are converted to sodium salts, most solids present in steam are sodium
salts, with minor amounts of calcium, magnesium, iron, and copper also present.
Gaseous constituents commonly found in low-pressure steam (less than 2000 psig) are ammonia, carbon
dioxide, nitrogen, amines, and silica. Of these, only silica contributes to the difficulties commonly
associated with impure steam; the other constituents are of concern only where they interfere with the
measurement of steam purity.
Water Testing Instruments:

Embrittlement Detector:
Caustic embrittlement is a corrosion process, Caustic embrittlement, a specific
form of stress corrosion, results in the intercrystalline cracking of steel.
Intercrystalline cracking results only when all of the following are present: specific
conditions of stress, a mechanism for concentration such as leakage, and free
NaOH in the boiler water. Therefore, boiler tubes usually fail from caustic
embrittlement at points where tubes are rolled into sheets, drums, or headers.
The possibility of embrittlement may not be ignored even when the boiler is of an
all-welded design. Cracked welds or tube-end leakage can provide the
mechanism by which drum metal may be adversely affected. When free caustic
is present, embrittlement is possible.
An embrittlement detector, as shown in figure is a device used to test boiler water
for embrittlement tendencies. The detector is installed in the continuous
blowdown line of a boiler. The boiler water is kept at boiler temperature as it
circulates through the detector.
The detector has a metal test specimen (the same metal as the boiler is
constructed of) that is subjected to boiler water. The specimen is bolted in place
and-clamped to put it under stress. Adjustment of the adjusting screw allows
water to leak out very slowly under the specimen. The escaping water and steam
leaves a concentrated solution in contact with the stressed surface of the metal
specimen. The specimen cracks if the water has embrittlement tendencies. The
specimen is checked at monthly intervals for signs of cracking.
If boiler water possesses embrittling characteristics, steps must be taken to
protect the boiler from embrittlement-related failure.

Total Dissolved Solids (TDS):

Conductivity Meters:
Conductivity meters are Jab instruments that have a probe that is inserted into
the sample. The probe of a conductivity meter is connected to an electronic
instrument. The meter has an output needle or digital output to read the
conductivity of the sample. The meter may be calibrated to read directly in TDS.
The meter may have a probe with two electrodes for high purity waters and a
four-electrode probe for more concentrated solutions.
The four-electrode model in Fig. has two drive electrodes and two sensing
electrodes. The current is applied to the two d1ive electrodes and the sensing
electrodes measure the current flow through the solution.
A similar setup is used for an on-line conductivity meter. In this type a small flow
of water is passed through the cell. The meter output is connected to the control

room. The signal can be used to control such variables as boiler blowdown or
chemical feeds.
pH meters:
The pH is an indication for the acidity of a substance. It is determined by the number of free hydrogen
ions (H+) in a substance. Acidity is one of the most important properties of water. Water is a solvent
for nearly all ions. The pH serves as an indicator that compares some of the most water-soluble ions.
The outcome of a pH-measurement is determined by a consideration between the number of H+ ions
and the number of hydroxide (OH-) ions. When the number of H+ ions equals the number of OH- ions,
the water is neutral. It will than have a pH of about 7.
The pH of water can vary between 0 and 14. When the pH of a substance is above 7, it is a basic
substance. When the pH of a substance is below 7, it is an acid substance. The further the pH lies
above or below 7, the more basic or acid a solution is.The pH is a logarithmic factor; when a solution
becomes ten times more acidic, the pH will fall by one unit. When a solution becomes a hundred times
more acidic the pH will fall by two units.
The word pH is short for "pondus Hydrogenium". This literally means the weight of
hydrogen. De pH is an indication for the number of hydrogen ions. It consisted when we
discovered that water consists of hydrogen ions (H+) and hydroxide ions (OH-).
The pH does not have a unit; it is merely expressed as a number. When a solution is neutral,
the number of hydrogen ions equals the number of hydroxide ions. When the number of
hydroxide ions is higher, the solution is basic. When the number of hydrogen ions is higher,
the solution is acid.
Methods to determine the pH
There are several different methods to measure the pH. One of these is using a peace of pH indicator
paper. When the paper is pushed into a solution it will change colour. Each different colour indicates a
different pH-value. This method is not very accurate and it is not suitable to determine more exact pH
values. That is why there are now test-strings available, which are able to determine smaller pHvalues, such as 3.5 or 8.5.
The most accurate method to determine the pH is measuring a colour change in a chemical lab
experiment. With this method one can determine pH values, such as 5.07 and 2.03.
All of these methods are not suitable to determine a pH development in time.
The pH-electrode:
Fig of pH Meter Operation
A pH electrode is a tube that is small enough to put it in sample jars. It is tied to a pH-meter by
means of a cable. A special type of fluid is located within the electrode; this is usually "3M Kalium
Chlorine ". Some electrodes contain a gel that has the same properties as the 3M-fluid. In the fluid
there are silver and platinum wires. The system is quite fragile, because it contains a small
membrane. The H+ and OH- ions will enter the electrode through this membrane. The ions will create
a slightly positive charge and a slightly negative charge in each end of the electrode. The potential of
the charges determines the number of H+ and OH- ions and when this is determined the pH will
appear digitally on the pH-meter. The potential is co-dependent on the temperature of the solution.
That is why the temperature is also presented on the pH-meter.
pH
product
14
Sodium hydroxide
13
lye
12.4
lyme
11
ammonia
10.5
manganese
8.3
backing powder
7.4
human blood
7.0
pure water
6.6
milk
4.5
tomatoes
4.0
win
3.0
apples
2.0
lemon juice
0
hydrochloric acid
Representative Steam Sampling:
In order to ensure accurate analysis, samples must be truly representative of the steam being generated.
When the sampling procedures are not followed properly, the steam purity evaluation is of little or no
value.
Sampling nozzles recommended by the ASTM and ASME have been in use for many years. The nozzles
have ports spaced in such a way that they sample equal cross sectional areas of the steam line.
Instructions for these nozzles can be found in ASTM Standard D 1066, "Standard Method of Sampling
Steam" and ASME PTC 19.11. Field steam studies have shown that sampling nozzles of designs other
than these often fail to provide a reliable steam sample.
Isokinetic flow is established when steam velocity entering the sampling nozzle is equal to the velocity of
the steam in the header. This condition helps to ensure representative sampling for more reliable test
results. The isokinetic sampling rate for many nozzles that do not conform to ASME or ASTM
specifications cannot be determined.
Accurate sampling of superheated steam presents problems not encountered in saturated steam
sampling. The solubility of sodium salts in steam decreases as steam temperature decreases. If a
superheated steam sample is gradually cooled as it flows through the sample line, solids deposit on
sample line surfaces. To eliminate this problem, the steam can be desuperheated at the sampling point.
What Is Isokinetic Sampling?
Isokinetic sampling ensures that all phases (solid oxides and precipitates, liquid droplets, and vapor) of
the sampled fluid enter the sampling nozzle with the same velocity vector (velocity and direction of flow)
and the flow velocity into the nozzle is the same as the sampled stream velocity. The main reason
isokinetic sampling is necessary is that the sampled stream is almost always a two-phase fluid (gas-liquid,
gas-solid, liquid-solid) and the second phase has a very different chemical composition than the steam or
water. In addition, the second phase (droplets or particles) has a different density and inertia than the primary
phase (gas or liquid) and therefore wouldn’t be proportionally represented in a sample that’s not withdrawn
isokinetically.
Locations where isokinetic sampling nozzles should be used include: feedwater, boiler downcomer,
saturated steam, and main steam (or reheat steam).
Isokinetic sampling means sampling in which the linear velocity of the Steam entering the
sampling nozzle is equal to that of the undisturbed steam stream at the sample point.