5.
Evaluating chemical water analysis results
William Mremi
Evaluating chemical water analysis results
Hydrologists and others, who use water analyses must interpret individual
analyses or large numbers of analyses at the same time Although the details of
water chemistry often must play an important part in the water analysis
interpretation, a fundamental need is for means of correlating analyses with
each other and with hydrologic or other kinds of information that are relatively
simple as well as scientifically reasonable and correct.
For example, it may be necessary in the process of making an organized
evaluation in a summary report of the water resources of a region, to correlate
water quality with environmental influences and to develop plans for
management of water quality, control of pollution, setting of water quality
standards or selecting and treating public or industrial water supplies
❖ Therefore, first of all it is important to understand the data and evaluate the
goodness of the analysis using
Concentrations
Ionic balance
Total Dissolved Solids (TDS) and Electrical Conductivity (EC)
Typical ion correlations
➢ Mass fraction in percentage (%)
Is the amount of a dissolved substance in gram per 100 g solution (
g/100g)
➢ Percentage per volume (vol %)
Is the volume of a dissolved gaseous substance in 100 parts of
volume (ml,ccm,etc.) of the solution.
❖ Mass concentration (g/ mg/l, ppm, µg/l, ppb, g/kg, etc
Is the mass of a dissolved substance per litre or kg solution
❑ Molar concentration (molarity mol/l, mmol/l)
Defines the number of moles of a species per liter solution (mol/
❑ Molal concentration (molality mol/kg , mmol/kg)
Is the number of moles per kg solution
Equivalent concentration(normality eq/l, meq/l, eq/kg)
Is the number of equivalent charges of anion per litre or per kg
solution
Molar Concentration
Molar concentration or molarity (mol/l, mmol/l)
Defines the number of moles of a species per liter solution.
1 mole consists of NA = 6.022x1023 parts (molecules ions etc.),
NA is called the Avogadro constant used to calculate the
relative atomic mass u of a substance expressed in grams: u =
1g / NA
1 g is defined as the mass of 1/12 of the mass of a carbon atom
C (isotope C12). Thus the atom mass is a relative mass unit.
Molal Concentration
Molal concentration or molality (mol/kg or mmol/kg)
Is the number of moles per kg solution
❑ In dilute solutions this scale is almost the same as molar concentrations
because a 1 liter solution has a mass of approximately 1 kg
❑ For concentrated solutions, the two scales become increasingly
different!
❑ Substances react in molar proportions, that is why thermodynamic
calculations use molar concentration!
Equivalent concentration
Equivalent concentration (eq/l, meq/l, eq/kg)
Is the number of equivalent charges of an ion per litre or per
kg solution
❑ Equivalent mass is the mass of one equivalent, that is the
mass of a given substance which will combine with or
displace a fixed quantity of another substance.
❑ These values correspond to the atomic weight divided by
the usual valence.
Equivalent mass = atomic mass / Z
Concentration conversion
Ion Balance
Principle of Electroneutrality
In a solution in equilibria:
❑ Therefore, it is possible to calculate possible inconsistencies
from this principle, according to this formula
❑ These inconsistencies are considered analytical errors and/or
sampling errors end/or samples storage mistakes!
Some usually accepted limits are:
10% for water with TDS <200 mg/L
5% for water with TDS >200 mg/L
Ion Balance
Exercise: Ionic Balance
Total Dissolved Solids (TDS)
Principle of Mass Conservation
Another measure to evaluate the goodness of a water sample
uses the principle of mass conservation
The total dissolved solids ( are a measure for the content of
solids, organics and inorganics contained in water
It can also be approximated according to the following
formula
The relation between the measured and the calculated TDS
need to be with in the following range.
Total Dissolved Solids (TDS)
Exercise: Ionic Balance
Calculate the TDS and compare
with the measured value
Electrical Conductivity (EC)
The electrical conductivity EC of water is the reciprocal
value of the electrical resistance at 2020°C related to the
volume of a water cube with the length of its edge of 1 cm
The unit of the electrical conductivity is 1 S/cm (" 1 /Ohm
[electrical resistance] 1 mho)
Because most of the natural waters have low electrical
conductivity, the units mS/cm 10 3 S/cm) and µS/cm 10 6
S/cm) are normally used
The EC of water depends on
➢ The temperature
➢ the concentration of dissolved salts
➢ the degree of dissociation
➢ the electrical charges of ions
➢ the migration velocities of the ions being present
The electrical conductivity of pure water at 25°C amounts to 4.2*10-2μS/cm.
Electrical Conductivity (EC)
At low concentrations, the electrical conductivity is
proportional the concentration of dissolved salts
This proportions is particularly strong with Cl and
helps to detect anomalous samples
