Saturday, 28 January 2017

Water analysis details

Water analysis details

Introduction

This page covers the water analysis required for ion exchange applications. It is considerably simpler than the analysis you would use to assess the quality of drinking water.
Inorganic componentsThe general feed water characteristics are described in another page, with recommended limits for certain contaminants and parameters. Here, we focus on the inorganic components dissolved in the water.
The picture here (click it for bigger size) shows only the components usually found in surface or deep well water and important for the ion exchange processes.
Some of the components are traditionally grouped:
Ca++ + Mg++= TH
HCO3 + CO3= + OH= m-Alk
Cl + SO4= + NO3= EMA
  • Calcium and magnesium are the Total Hardness (TH).
  • Bicarbonate, carbonate and hydroxide are the Total Alkalinity (m-Alk). Usually, natural water does not contain carbonate or hydroxide.
  • Chloride, sulphate and nitrate are the Equivalent Mineral Acidity (EMA), also called Salts of Strong Acids (SSA) or, after cation exchange, Free Mineral Acidity (FMA).
  • When hardness is greater than alkalinity (in meq/L) the bicarbonate hardness is called "temporary hardness" (= TH – Alk) and the remaining hardness is called "permanent hardness". The value of temporary hardness is then equal to that of alkalinity in meq/L.
  • When hardness is smaller than alkalinity (in meq/L) there is no permanent hardness and temporary hardness is equal to total hardness.
  • All natural waters are ionically balanced, i.e. the sum of cations in meq/L is equal to the sum of anions.
Other ions, usually present as traces but sometimes not completely negligible, can be combined with the above:
  • barium (Ba++) and strontium (Sr++) are alkaline earth metals (see important note below) and belong thus to the hardness;
  • for calculation with an ion exchange software, you would also add divalent iron (Fe++), nickel (Ni++) and copper (Cu++) to the hardness group, by convenience;
  • ammonium (NH4+) and potassium (K+) are handled like sodium;
  • lithium (Li+) also reacts like sodium (Na+);
  • phosphate (PO43–) belongs to the EMA;
  • fluoride (F), bromide (Br) and iodide (I) are halogenides and behave like chloride.
Beware that standard resins may have poor affinity for some of these ions, such as Li and F. Also, other possible components, such as aluminium, arsenic and many other metals may be complexed and behave as anions, and sometimes their removal is difficult.
Barium and strontium specific behaviour:
  • The solubility of barium sulphate is only 2 mg/L, thousand times lower than that of calcium sulphate.
  • Ba and Sr are not well removed on WAC resins. These resins have a lower affinity for Ba and Sr than for Ca and Mg. See the table of selectivity values.
  • Ba (and Ra) are very well removed on SAC resins. So well that regeneration may be difficult. Using H2SO4 to regenerate a SAC resin loaded — even partially — with barium may be close to impossible.
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Units of concentration and capacity

Because we need to know the number of ions to be exchanged — their mass is not helpful here — the concentration of all these ions must be converted into chemical "equivalent" units, of which the international unit is eq·kg–1, which we traditionally re-name as equivalents per litre eq/L, and in case of low concentrations, meq/L. Other units of concentrations are still used regionally:
 
Units of concentration
(per volume of water)
Name Abbreviation
meq/L
ppm as calcium carbonate 1 ppm as CaCO3 = 0.02
French degree 1 °f = 0.2
German hardness degree 1 °dH = 0.357
Grain as CaCO3 per US gallon 1 gr as CaCO3/gal = 0.342
 
Units of capacity
(per volume of resin)
Name Abbreviation
eq/L
Gram as CaCO3 per litre 1 g as CaCO3/L = 0.02
French degree 1 °f = 0.0002
Gram as CaO per litre g CaO/L = 0.0357
kgr as CaCO3 per cubic foot kgr CaCO3/ft3 = 0.0458
The complete tables of conversion can be seen in a separate window.
The unit of mole should be avoided altogether in ion exchange, as it does not take valence into account and brings only confusion. For reference: 1 eq = 1 mole / valence.
For those curious, a mole contains 6.02×1023 atoms, ions or molecules. This big number is called Avogadro constant.
Note: in Germany and some other Central and Eastern European countries, mval/L and val/L are used instead of meq/L and eq/L.

Examples

The table shows the most common ions in water and their equivalent mass.
Name Ion g/mole g/eq
Calcium Ca++ 40 20
Magnesium Mg++ 24 12
Sodium Na+ 23 23
Potassium K+ 39 39
Ammonium NH4+ 18 18
Chloride Cl 35.5 35.5
Sulphate SO4= 96 48
Nitrate NO3 62 62
Bicarbonate HCO3 61 61
Carbonate CO3= 60 30
In water, the concentrations are expressed in meq/L. For instance, if you have a calcium concentration of 90 mg/L, the equivalent concentration is 90/20=4.5 meq/L.
Silica (SiO2), not ionised in normal water, has a molar mass of 60. For ion exchange (with a strongly basic resin in OH form), it is considered monovalent, so the equivalent mass is also 60.
Carbon dioxide (CO2) is very slightly ionised in normal water, and is also considered monovalent, with a molar and equivalent mass of 44. The equilibrium between CO2 and HCO3 is shown at the bottom of this page.
Don't be confused: 1 equivalent CaCO3 (50 g), for instance, contains 1 eq Ca (20 g) and 1 eq CO3 (30 g). You don't add these (one eq cation and one eq anion): it is still only 1 eq CaCO3, not 2!

A balanced analysis ?

Water is electrically neutral, even when it contains large quantities of ions. This means that the number of anionic charges is exactly the same as that of cationic charges. Otherwise you would have an electric shock when putting your hand in water. Therefore, once you have carefully converted all the elements of your water analysis in meq/L units, the sum of anions should be the same as the sum of cations. The only exceptions to that rule are:
  • A small difference due to imprecision in the analytical procedures is acceptable as long as the difference between total cations and total anions is less than 3 %.
  • At high pH (> 8.2), e.g. in the presence of ammonia or after lime decarbonation, there will be hydroxide or carbonate ions. Hydroxide ions are usually not reported separately. Carbonate ions are not always reported. In such a case, you would have more cations than anions.
  • At low pH (say < 6.8), the water may contain either free mineral acidity (very rare for natural water) or free carbon dioxide, both producing H ions which are usually not reported separately.

An example of water analysis

Here is an analysis as required to calculate an ion exchange plant (softening, demineralisation, de-alkalisation, nitrate removal). This is a real water (1), from the Oise river, in France, dated 28 September 2005.
Cations mg/L meq/L Anions mg/L meq/L
Ca++ 93 4.65 Cl 67 1.89
Mg++ 12 1.00 SO4= 33 0.69
Na+ 26 1.13 NO3 6 0.10
K+ 4 0.10 HCO3 259 4.23
Total cations 6.90 Total anions 6.91

SiO2 2.4 0.04
pH value 7.04 Free CO2 45 1.02
Conductivity ┬ÁS/cm 627 Anion load 7.97
Organic matter (2) 2.6  
Temperature °C 16
(1) Important note: To calculate an ion exchange plant, a real water analysis should always be used, not an average. If necessary, two or three analyses should be considered when the salinity shows seasonal variations.
(2) Organic matter (COD) is important because it can foul anion exchange resins. It is usually expressed in mg/L as KMnO4.
This particular analysis is typical of Western Europe, with relatively high hardness and alkalinity, and little silica. Silica and free carbon dioxide are removed by the strong base anion resin in a demineralisation system. However, carbon dioxide can be reduced with a degasifier after cation exchange to reduce the anion load.

m- and p-Alkalinity

Alkalinity includes following anions:
  • Hydrogencarbonate HCO3, often called bicarbonate
  • Carbonate CO3=
  • Hydroxide OH
Alkalinity in water is measured by titration with an acid. Two different indicators are used:
  • Phenolphthalein changing colour at pH 8.3 measures p-alkalinity
  • Methylorange changing colour at pH 4.5 measures m-alkalinity
The total alkalinity is m-Alk, and can include OH, CO3, and HCO3 ions. p-Alk measures only the OH and half of the CO3 ions. When the pH value of the water is smaller than 8.3, p-Alk is equal to zero, and the water can contain only bicarbonate. At a higher pH, carbonate can exist. At even higher pH values, hydroxide ions can exist, but then there will be no bicarbonate ions left, as those would combine with OH to produce carbonate ions and water:
HCO3 + OH ---> CO3= + H2O
You will have thus with increasing pH either only bicarbonate, or bicarbonate + carbonate, or only carbonate, or carbonate + hydroxide, or only hydroxide. This gives the following table, from which the components of alkalinity can be calculated:
 
Ion
p = 0 p < m/2 p = m/2 m/2 < p < m p = m
OH = 0 0 0 2 p - m p
CO3 = 0 2 p m = 2 p 2 (m - p) 0
HCO3 = m m - 2 p 0 0 0
The values in the table are expressed in equivalent units, i.e. in meq/L, ppm CaCO3, French or German degrees, not in mol/L or mg/L!
Let us see examples with values in meq/L, with waters of increasing pH
 
Example 1 m-Alk = 5 p-Alk = 0
OH = 0 CO3 = 0 HCO3 = 5

Example 2 m-Alk = 5 p-Alk = 1.5
OH = 0 CO3 = 3 HCO3 = 2

Example 3 m-Alk = 5 p-Alk = 3
OH = 1 CO3 = 4 HCO3 = 0


If p-Alkalinity is > 0, which means the pH value is more than 8.3, you don't have free CO2, because it would combine with CO3 to produce HCO3.
CO2 + CO3= + H2O ---> 2 HCO3

Free CO2 and pH

pH vs Alk to CO2 ratioA low pH value means that there are H+ ions in solution. In the presence of bicarbonate, the following equilibrium exists:
H+ + HCO3 <---> H2CO3 <---> CO2 + H2O
The two pictures illustrate this equilibrium. Use the second graph to verify that the water analysis given by your customer makes sense, and to estimate the concentration of free carbon dioxide if it is not given. You also see there that at a pH of more than 7.2, this concentration is practically negligible.
When treating RO permeate, however, this relationship is very important, as CO2 is the largest part of the anion load on the resin. In this case, you can use the third graph, which is a close-up of the other one for low concentrations.

CO2 vs pH and Alk  CO2 vs pH and Alk

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