A facultative anaerobic organism is an organism, usually a bacterium that makes ATP by aerobic respiration if oxygen is present but is also capable of switching to fermentation. In contrast, obligate anaerobes die in the presence of oxygen.

Respitation: Aerobic CH2O + O2 = CO2+ H2O

Phosphate is commonly used as a fertilizer. Phosphate may be added to the lake through sewage, fertilizer and other sources. The excess nutrients stimulate the algal growth in the lake. The algae grow and die that becomes the food source for the aerobes. In such condition, the aerobe population in the lake multiplies exponentially consuming oxygen at a faster rate than it could diffuse through water.  This leads to anaerobic condition in the lake. In such situation, facultative anaerobes thrive killing entire fish population of the lake.

Respiration: Anaerobic 2CH2O + SO4-2 = 2HCO3 + H2S

Eutrophication can be human-caused or natural. Untreated sewage effluent and agricultural run-off carrying fertilizers are examples of human-caused eutrophication. However, it also occurs naturally in situations where nutrients accumulate (e.g. depositional environments), or where they flow into systems on an ephemeral basis.

Hard_water_and_drop

July 11, 2011 (Coal Geology) Hard water is water referes to the alkaline earth ion concentrations. The primary alkaline earth ions are Ca2+ and Mg2+. Calcium usually dominates over magnesium concentration.

Some important Points:

• Hard water is generally not harmful to one’s health
• pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling towers, and other equipment that handles water. In boilers, the deposits impair the flow of heat into water, reducing the heating efficiency and allowing the metal boiler components to overheat. In a pressurized system, this overheating can lead to failure of the boiler.
• Chelation of organics within soap by the alkaline ions is the main reason of concern for hard water. When soap is chelated, it fails to properly react with the water.
• Hard water also forms deposits that clog plumbing. These deposits, called “scale”, are composed mainly of calcium carbonate (CaCO₃), magnesium hydroxide (Mg(OH)₂), and calcium sulfate (CaSO₄).[
• Calcium and magnesium carbonates tend to be deposited as off-white solids on the surfaces of pipes and the surfaces of heat exchangers. This precipitation (formation of an insoluble solid) is principally caused by thermal decomposition of bi-carbonate ions but also happens to some extent even in the absence of such ions. The resulting build-up of scale restricts the flow of water in pipes.

Measurement Unit: "mg/L as of CaCO3"

How to convern both Ca+2 and Mg+2 concentration to "mg/L as of CaCO3" unit?

Formula for Hardness = {[Ca+2]x(Mol. Wt. of CaCO3/Mol Wt. of Ca+2)}  + { [Mg+2]x(Mol. Wt. of CaCO3/Mol Wt. of Mg+2)
= [Ca+2]x(100.08/40.08)    +    [Mg+2]x(100.08/24.31]

Use the attached excel spreadsheet to  Calculate hardness of water.

Hardness of Water Calculation Spreadsheet -CoalGeology.Com

[ReviewAZON asin="0521891485" display="inlinepost"]Hardness Scale (mg/L of CaCO3):
0-60 – Soft Water
61-120 Moderately Hard Water
121-180 Hard Water
>180    Very Hard Water

Hard water is commonly associated with limestone/dolomite rocks such as in Florida.

Sources:
Hardness in water is defined as concentration of multivalent cations. Multivalent cations are cations (positively charged metal complexes) with a charge greater than 1+. They mainly have the charge of 2+. These cations include Ca2+ and Mg2+. These ions enter a water supply by leaching from minerals within an aquifer. Common calcium-containing minerals are calcite and gypsum. A common magnesium mineral is dolomite (which also contains calcium). Rainwater and distilled water are soft, because they also contain few ions.

The following equilibrium reaction describes the dissolving/formation of calcium carbonate scales:

CaCO3 + CO2 + H2O ? Ca2+ + 2HCO3-

Resources:

http://en.wikipedia.org/wiki/Hard_water

• Chemical Formula:     MnS
• Composition:     Molecular Weight = 87.00 gm
• General physical properties and color photograph of Alabandite can be found at http://www.mindat.org/min-89.html
• Google Image also has tons of great Alabandite photographs.

Today, I am just going to do some “educational” tasks using Geochemist’s Workbench. Let’s begin with a solubility reaction. Alabandite is highly unstable in oxidized environment and breaks down to Mn+2 and SO4– rapidly. The oxidation of Alabandite can be presented by the simple reaction:

• Alabandite  + 2 O2(aq)  = Mn++  + SO4–

• Log K’s:

0 °C:  152.1275        150 °C:   89.7573
25 °C:  137.9632        200 °C:   76.7436
60 °C:  121.2437        250 °C:   65.5350
100 °C:  105.6272        300 °C:   55.4212

• Polynomial fit:  log K = 152.1 – .6062 × T + .001732 × T^2 – 3.537e-6 × T^3 + 3.056e-9 × T^4

• Equilibrium equation for Alabandite: log K = log a[Mn++] + log a[SO4--] – 2 × log a[O2(aq)] ————————–(1)

Like many other minerals, the log K for Alabandite gets smaller with higher temperature.  So, Alabandite is more soluble at higher temperature. Geochemist’s Workbench only allow calculations upto 300 degree centigrade. Using the polynomial fit, we can plot the log K of Alabandite vs temperature. Figure 1 shows the log K curve.

Alabandite Log K

Stability Diagrams:

Now I am going to present some stability diagrams. Lets start with a simple Tempeature-log f(O2) diagram. I am using a .001 as activity for Alabandite. Now crystallized MnSO4 can also be formed by the oxydation of Alabandite.

Alabandite  + 2 O2(aq)  = MnSO4(c)

• Log K’s:

0 °C:  148.5194        150 °C:   91.3790
25 °C:  135.2914        200 °C:   79.9353
60 °C:  119.8758        250 °C:   70.4165
100 °C:  105.6417        300 °C:   62.4797

Polynomial fit:
log K = 148.5 – .5684 × T + .001736 × T^2 – 3.806e-6 × T^3 + 3.829e-9 × T^4

Equilibrium equation:
log K = – 2 × log a[O2(aq)]

Notice about the difference between equilibrium reactions.

Temp-log f O2 for Alabandite

So, the diagram is showing that at higher temperature the reaction needs less oxygen to proceed.

Some more stability field diagrams:

Albandite - Solubility-SO4-Eh Diagram 25 C

This diagram tells you that at a very reducing condition (low Eh), Mn will be stable as solid  phase alabandite. In  highly oxidized environment Pyrolusite will be the most stable phase.

Alabandite  + 2.5 O2(aq)  + H2O  = Pyrolusite  + SO4–  + 2 H+

Eh-pH with Alabandite activity 0.001 in pretense of free so4 25 C

Eh-pH with Alabandite activity 1 in pretense of free so4 25 C

Eh-pH with Alabandite activity 1 in presence of free so4 at 300 C

Just enjoy the different Eh-pH diagrams that shows different solid and soluble phases of Mn. Notice how the MnSO4 becomes crystalline at higher temperature.

Now to model sea water composition, we are going to make some assumptions:

• CO2 fugacity controls the pH of the sea water which can be written in terms of the reaction: H+ + HCO3- = CO2 + H2O. Log fugacity of CO2 in the atmosphere = -3.5
• We also assume that the sea water dissolved oxygen (O2(aq)) is in equilibrium with atmospheric oxygen (O2(g)).  or, f(O2)=.2

So, in our model we do not specify any pH and let the model predict the pH of the sea water. It would be nice to see how close we get to the actual pH of the sea water.

Analysis of sea water composition is widely available in numerous geochemistry text books and over the web. I used the sea water composition as presented in the website: http://www.seafriends.org.nz/oceano/seawater.htm

Chemical analysis of Sea Water:

Before doing anything, I used the GSS module of the Geochemist’s module to check for the Charge Imbalance Error. It came out to be 0.023%. This gave me confidence that we have all of the major ions that we need to properly construct the geochemical model. Table 1 shows the result that I got.

Now it is time to construct the geochemical model using the SpecE8 model. Many people like to hand code all commands into the Geochemist’s workbench. I find it easier just to use their user friendly interface. However, sometime you need to know some simple tricks to set up the basis panel. “SWAP” could be little confusing in the beginning. To construct the model we would have to swap fugacity of CO2 with H+ ion concentration. This will keep the f(CO2) constant (so at atmospheric equilibrium). We also need to SWAP between atmospheric O2 and dissolved O2.  Figure 1 shows how to construct the basis for sea water speciation model.

Once the basis is ready, we can run the model. Below I am going to focus on some of the results and at the end I am going to attach the complete output file as TXT. you can download an play with the txt if you like:

Part 1 Result: This part gives you the some of the important parameters. First of all, the model has predicted a  pH of 8.34 for the sea water. In reality the pH varies between 7.8 to 8.5 depending on the place of sea water collection. Secondly, the CBE is 0.04%. This again tells you that the chemical analysis of the sea water was great. It gives you the carbonate alkalinity 122.22 mg/kg sol’n as CaCO3 and also tells you that you have a Na-Cl water.

• Temperature =  25.0 C    Pressure =  1.013 bars
• pH =  8.344              log fO2 =   -0.699
• Eh =   0.7253 volts      pe =  12.2612
• Ionic strength      =    0.635602
• Charge imbalance    =    0.040118 eq/kg (7.153% error)
• Activity of water   =    0.982060
• Solvent mass        =    1.000000 kg
• Solution mass       =    1.037308 kg
• Solution density    =    1.030    g/cm3
• Chlorinity          =    0.566009 molal
• Dissolved solids    =       35966 mg/kg sol’n
• Hardness            =     6371.71 mg/kg sol’n as CaCO3
• carbonate         =      122.22 mg/kg sol’n as CaCO3
• non-carbonate     =     6249.49 mg/kg sol’n as CaCO3
• Rock mass           =    0.000000 kg
• Carbonate alkalinity=      122.22 mg/kg sol’n as CaCO3
• Water type          =    Na-Cl

Result part 2: this part details the mollalities and log activities of the different species present in sea water. We can easily tell that the sea water is dominated by free Cl- and Na+ ion followed by Mg++, K+, SO4– and other complex species.

Aqueous species       molality    mg/kg sol’n    act. coef.     log act.
—————————————————————————
Cl-                     0.5491    1.877e+004      0.6290       -0.4617
Na+                     0.4780    1.059e+004      0.6726       -0.4928
Mg++                   0.04301         1008.      0.3171       -1.8652
SiO2(aq)               0.02210         1280.      1.1689       -1.5877
K+                     0.01029         387.8      0.6290       -2.1890
MgCl+                 0.009901         570.4      0.6726       -2.1766
Ca++                  0.006497         251.0      0.2479       -2.7930
CaCl+                 0.004146         301.9      0.6726       -2.5547
H6(H2SiO4)4–         0.004042         1490.      0.1709       -3.1606
NaH3SiO4              0.003871         440.7      1.0000       -2.4121
NaCl                  0.002793         157.4      1.0000       -2.5539
HCO3-                 0.001506         88.57      0.6913       -2.9826
H3SiO4-               0.001270         116.4      0.6726       -3.0686
Mg(H3SiO4)2           0.001194         247.0      1.0000       -2.9228
Br-                  0.0008568         66.00      0.6290       -3.2685
MgH3SiO4+            0.0005698         65.60      0.6726       -3.4165
NaHCO3               0.0004503         36.47      1.0000       -3.3465
MgHCO3+              0.0002160         17.77      0.6726       -3.8377
O2(aq)               0.0002159         6.660      1.1689       -3.5980
MgH2SiO4             0.0001743         19.89      1.0000       -3.7588
Sr++                 0.0001529         12.92      0.2102       -4.4928
MgCO3                0.0001173         9.533      1.0000       -3.9308
KCl                 5.778e-005         4.152      1.0000       -4.2382
CO3–               5.454e-005         3.155      0.1907       -4.9829
CaHCO3+             3.911e-005         3.812      0.7147       -4.5536
CaH3SiO4+           3.823e-005         4.982      0.6726       -4.5899
F-                  2.903e-005        0.5317      0.6519       -4.7229
CaCO3               2.754e-005         2.658      1.0000       -4.5600
Mg2CO3++            2.653e-005         2.778      0.2102       -5.2536
MgF+                2.419e-005         1.010      0.6726       -4.7886
Ca(H3SiO4)2         1.668e-005         3.703      1.0000       -4.7778
NaCO3-              1.588e-005         1.271      0.6726       -4.9714
CO2(aq)             1.116e-005        0.4734      1.0000       -4.9524
MgOH+               7.113e-006        0.2833      0.6726       -5.3202
OH-                 3.426e-006       0.05617      0.6519       -5.6510
H4(H2SiO4)4—-     2.379e-006        0.8725      0.0007       -8.7707
CaH2SiO4            1.736e-006        0.2246      1.0000       -5.7603
NaF                 8.028e-007       0.03250      1.0000       -6.0954
SrHCO3+             7.507e-007        0.1076      0.6726       -6.2968
CaF+                5.709e-007       0.03251      0.6726       -6.4157
NaOH                4.507e-007       0.01738      1.0000       -6.3462
SrCO3               2.146e-007       0.03054      1.0000       -6.6684
CaOH+               1.063e-007      0.005848      0.6726       -7.1459
H2SiO4–            8.788e-008      0.007972      0.1709       -7.8234
Mg2OH+++            2.483e-008      0.001571      0.0700       -8.7598
H+                  5.640e-009    5.480e-006      0.8037       -8.3436
SrF+                5.085e-009     0.0005227      0.6726       -8.4659
KOH                 4.588e-009     0.0002482      1.0000       -8.3384
SrOH+               5.478e-010    5.526e-005      0.6726       -9.4336
HF                  1.267e-010    2.444e-006      1.0000       -9.8972
Mg4(OH)4++++        7.598e-013    1.210e-007      0.0225      -13.7677
HF2-                1.127e-014    4.239e-010      0.6726      -14.1202
HCl                 1.244e-015    4.371e-011      1.0000      -14.9053
H2F2                4.303e-020    1.660e-015      1.0000      -19.3663
ClO4-               6.569e-024    6.298e-019      0.6519      -23.3683
SiF6–              4.689e-033    6.422e-028      0.1709      -33.0962
H2(aq)              4.078e-045    7.924e-042      1.1689      -44.3218
CH4(aq)             4.853e-149    7.506e-145      1.1689     -148.2462
CH3COO-             1.989e-154    1.132e-149      0.6913     -153.8616
MgCH3COO+           5.197e-155    4.176e-150      0.6726     -154.4565
NaCH3COO            2.904e-155    2.297e-150      1.0000     -154.5370
SrCH3COO+           9.039e-158    1.278e-152      0.6726     -157.2161
HCH3COO             3.556e-158    2.059e-153      1.0000     -157.4490
CaCH3COO+           2.168e-158    2.072e-153      0.6726     -157.8362
Ca(O-phth)              0.0000        0.0000      1.0000     -300.0000
Na(O-phth)-             0.0000        0.0000      0.6726     -300.0000
H2(O-phth)              0.0000        0.0000      1.0000     -300.0000
H(O-phth)-              0.0000        0.0000      0.6726     -300.0000
(O-phth)–              0.0000        0.0000      0.1709     -300.0000

Part 3:-Saturation index : mineral saturation index An index showing whether a water will tend to dissolve or precipitate a particular mineral. Its value is negative when the mineral may be dissolved, positive when it may be precipitated, and zero when the water and mineral are at chemical equilibrium. The result shows that although the system has acquired  internal equilibrium within the fluid, there are still 23 metastable mineral phases present. However, it is important to remember that the minerals could be more soluble that the derived values based on the LLNL database.

log Q/K                          log Q/K
—————————————————————-
Antigorite        86.4430s/sat Bischofite        -7.3648
Tremolite         27.4628s/sat KNaCO3^6H2O       -7.6905
Anthophyllite     23.2014s/sat Antarcticite      -7.8763
Sepiolite         18.6718s/sat SrCl2^2H2O        -7.9548
Talc              16.5211s/sat Na2SiO3           -8.1699
Chrysotile         9.7027s/sat CaCl2^4H2O        -8.6398
Diopside           4.5557s/sat   Portlandite       -8.6988
Dolomite-ord       3.5432s/sat Ca(OH)2(c)        -8.6988
Dolomite           3.5432s/sat SrCl2^H2O         -9.4232
Quartz             2.4116s/sat Ca5Si6O17^3H2O    -9.7641
Huntite            2.3004s/sat Carnallite        -9.9444
Tridymite          2.2458s/sat MgOHCl           -10.0385
Chalcedony         2.1404s/sat MgCl2^4H2O       -10.2774
Dolomite-dis       1.9988s/sat Ca2SiO4^7/6H2O   -10.7288
Strontianite       1.9515s/sat Ca2SiO4(gamma)   -11.3250
Cristobalite       1.8611s/sat CaCl2^2H2O       -11.8283
Enstatite          1.7577s/sat CaCl2^H2O        -11.9552
Amrph^silica       1.1259s/sat K2CO3^3/2H2O     -12.5017
Magnesite          1.0593s/sat SrCl2(c)         -12.5775
Calcite            0.8550s/sat Larnite          -12.7860
CaSi2O5^2H2O       0.7341s/sat Rankinite        -13.4744
Aragonite          0.6901s/sat MgBr2^6H2O       -13.6887
Forsterite        -0.1057        SrBr2^6H2O       -13.9048
Monohydrocalcite  -0.1467        Ca4Si3O10^3/2H2O -14.0754
Ca2Si3O8^5/2H2O   -0.3435        Sr(OH)2(c)       -14.3787
Fluorite          -1.2800        Merwinite        -14.9477
Wollastonite      -1.3251        Hydrophilite     -15.5350
Brucite           -1.6324        MgCl2^2H2O       -15.6990
Artinite          -1.6564        Ca2Cl2(OH)2^H2O  -16.1839
Nesquehonite      -1.6630        Sr2SiO4(c)       -18.4683
Pseudowollastoni  -1.7160        Ca6Si6O18^H2O    -18.7464
Halite            -2.5473        Lime             -18.8076
Monticellite      -2.6387        MgCl2^H2O        -19.0437
Hydromagnesite    -2.8020        KMgCl3^2H2O      -19.5978
MgF2(c)           -3.1325        SrBr2^H2O        -19.8539
SrSiO3(c)         -3.4642        Ca3Si2O7^3H2O    -21.7603
Sylvite           -3.6101        SrBr2(c)         -23.3666
Ca5Si6O17^21/2H2  -4.2514        Chloromagnesite  -24.7928
Gaylussite        -4.3245        Tachyhydrite     -26.8322
Pirssonite        -4.4636        KMgCl3           -26.8883
Mg2Cl(OH)3^4H2O   -5.3251        SrO(c)           -28.9331
SrF2(c)           -5.4002        Ca4Cl2(OH)6^13H2 -30.5893
Kalicinite        -5.5036        Ca3SiO5          -33.9276
Na2Si2O5          -5.6438        Na4SiO4          -36.7405
Akermanite        -5.7759        MgBr2            -37.0691
Ca5Si6O17^11/2H2  -6.0696        K8H4(CO3)6^3H2O  -46.8361
SrCl2^6H2O        -6.1348        Na6Si2O7         -57.8214
KBr               -6.5701        Graphite         -75.5362
NaBr              -6.6932        O-phth acid(c)  -596.6112

Part 4: Gas Fugacities: This part calculates the different gases in sea water.

Gases                fugacity      log fug.
———————————————–
O2(g)                   0.2000      -0.699
Steam                  0.03075      -1.512
CO2(g)               0.0003162      -3.500
H2(g)               6.169e-042     -41.210
CH4(g)              3.750e-146    -145.426

Part 5: Initial basis and elemental composition

Original basis total moles   moles     mg/kg      moles     mg/kg      L/kg
——————————————————————————-
Br-             0.000857   0.000857      66.0
Ca++              0.0108     0.0108      416.
Cl-                0.566      0.566 1.93e+004
F-             5.46e-005  5.46e-005      1.00
H+               -0.0169    -0.0169     -16.4
H2O                 55.6       55.6 9.65e+005
HCO3-            0.00247    0.00247      145.
K+                0.0103     0.0103      390.
Mg++              0.0553     0.0553 1.29e+003
Na+                0.485      0.485 1.08e+004
O2(aq)          0.000216   0.000216      6.66
SiO2(aq)          0.0466     0.0466 2.70e+003
Sr++            0.000154   0.000154      13.0

Elemental composition               In fluid                  Sorbed
total moles     moles       mg/kg        moles       mg/kg
——————————————————————————-
Bromine          0.0008568    0.0008568       66.00
Calcium            0.01077      0.01077       416.0
Carbon            0.002465     0.002465       28.54
Chlorine            0.5660       0.5660  1.934e+004
Fluorine        5.460e-005   5.460e-005       1.000
Hydrogen             111.1        111.1  1.080e+005
Magnesium          0.05527      0.05527       1295.
Oxygen               55.66        55.66  8.585e+005
Potassium          0.01035      0.01035       390.0
Silicon            0.04663      0.04663       1263.
Sodium              0.4851       0.4851  1.075e+004
Strontium        0.0001539    0.0001539       13.00

Graphical presentation:

The output of the geochemical modeling can be presented in various different ways. Below are some examples.

Figure 2: Bar Chart of SpecE8 Model

Figure 3: Durov Diagram of Sea Water

Figure 4: Piper Diagram of SpecE8 model of Sea Water

Figure 5: Radial Plot of SpecE8 Sea Water speciation

Figure 6: Stiff Diagram of SpecE8 model of sea water

Well, we are going to do nothing fancy. We will be converting our field Eh (mV) measurements to pe. I just used simple formula and make the spread sheet in excel that I use for my own quick reference. If you have lost your calculator or could not remember the value of “F” or “R” then my spreadsheet might be handy for you. Feel free to use it and distribute with your friends.

Download Spreadsheet Eh to pe
Simply change the Eh value in the first column. That will do. Rest has been set up for you!

Quick Background on Eh (reference)

Eh, or redox potential, is the electrochemical potential of a solution relative to the standard hydrogen electrode. The standard hydrogen electrode is a fictive solution with a hydrogen ion (H+)activity of 1 molal at equilibrium with 1 Atmosphere of H2 at 25 degrees C. In practice, the potential is usually measured by a platinum electrode relative to a reference electrode is measured and converted to the appropriate value.

The Eh and pH of a solution are related. For a half-cell equation (conventionally written as reduction, or with electrons on the right side):

aA + bB + n e- + h H+ = cC + dD

The half-cell standard potential Eo is given by:

Eo (volts) = -Delta G/nF

where Delta G is the Gibbs free energy change, n is the number of electrons involved, and F is Faradays Constant. The Nernst Equation relates pH and Eh:

Eh = Eo + (0.059/n) x log {([A]^a [B]^b) / ([C]^c [D]^d)} – (0.059 h/n) pH

where square brackets indicate activities and exponents are shown in the conventional manner (using ^). This equation is the equation of a straight line for Eh as a function of pH with a slope of -0.059h/n volt (pH has no units.) This equation predicts lower Eh at higher pH values – This is observed for reduction of O2 to OH- and for reduction of H+ to H2. If H+ were on the opposite side of the equation from H+, the slope of the line would be reversed (higher Eh at higher pH). An example of that would be the formation of magnetite (Fe3O4) from HFeO2-(aq) (Garrels and Christ):

3 HFeO2- + H+ = Fe3O4 + 2 H2O + 2 e-

where Eh = -1.1819 – 0.0885 log[HFeO2-] + 0.0295 pH. Note that the slope of the line is -1/2 the -0.059 value above, since h/n = -1/2.

Half-cell equations can be combined if one is reversed to an oxidation in a manner that cancels out the electrons to obtain an equation without electrons in it.

Eh-pH (Pourbaix) diagrams are commonly used in mining and geology for assessment of the stability fields of minerals and dissolved species (See Eh (geology) for a very limited discussion.) Under conditions where a mineral (solid) phase is the most stable form of an element, these diagrams show that mineral. As with results from all thermodynamic (equilibrium) evaluations, these diagrams should be used with caution. Although teh formation of a miineral or its dissolution) may be predicted to occur under a set of conditions, the process may be negligible becaus its rate is so slow. Under those circumstances, kinetic evaluations are necessary. However, the equilibrium conbdiutions can be used to ebaluate the direction of spontaneous changes and the magnitude of the driving force behind them.

Temperature and pressure can affect Eh values. Temperature changes of a few degrees typically do not affect Eh – pH diagrams significantly. Changes from increases in pressures of the order of tens of atmospheres are generally small when gaseous reactants are involved. Chapter 9 of Garrels and Christ discusses the effects of temperature changes on geochemical equilibria.

Alkalinity is the total acid – neutralizing capacity of the water. Alkalinity is also one of the most common parameter you will observe in any kind of water quality analysis. But alkalinity is commonly reported as of CaCO3 mg/L. To use this value for any kind of calculations we must convert it to proper unit.

For example, say we are plotting a piper or stiff diagram. We will need activity of all major cations and anions to be used in plot making. In Piper or Stiff diagram, one of the input parameter is HCO3- (bicarbonate). We need to convert bicarbonate ion concentration (as of CaCO3) to bicarbonate ion activity.

Well, here is the trick.If you pH is less than about 8.4, just multiple the Alkalinity as of CaCO3 by 1.22 to convert to HCO3-. We assume that all alkalinity present in the solution is as HCO3-. Which is true in most cases as HCO3- is truly most dominant component of alkalinity in natural water.

So, for example, if we have 100 mg/L Alkalinity as of CaCO3, we have -
HCO3- mg/L = 100 x 1.22 = 122 mg/L HCO3 in the system.
To convert to activity, divide 122 by (61*1000).

You can also use the spreadsheet to calculate the activity of Bicarbonate in your system.
Activity, molality, normality calculations-Download Spreadsheet

If you want more sophistication in your calculation use the following equations to calculate bicarbonate in your system.

I am attaching the “Magic” Spreadsheet that will help you to calculate the HCO3 as mg/L just with few simple entries. Change the Alkalinity value and pH from your lab sheet to the spreadsheet and get the result.

Download : Alkalinity (as of CaCO3) to HCO3 mg/L conversion Spreadsheet

Note: As I told before, if you are below pH 8.4, just multiply your alkalinity by 1.22 to make it simple. Check the table using sophisticated HCO3 calculation in various pH range:

You can see that withing pH Range 5 to 9 the sophisticated HCO3 calculation differ in only 122-110.89=11.11 when we have 100 mg/l as of CaCO3 in the system. So, within that range, multiplication by 1.22 will be close to the “actual value” calculated by the sophisticated equation. But when you go to pH 10 multiplication by 1.22 will not be valid any more as you can see that we have only 60 mg/L HCO3- instead of 122 mg/L!

Why so?
At higher pH carbonic acid becomes more stable than the bicarbonate ion.