To perform calculation of characterization, following five data are required:

• Temperature (as °C)
  
• pH
  
• TAC (or Total Alcalinity, as french degree [°f], as mg/L HCO3, or as mg/L CaCO3)
  
• Calcium (as french degre [°F], as mg/L Ca/L, as milliequivalent/L or as mg CaCO3/L)
  
• Dry Residue (ou DR as mg/L - 180°C, or conductivity as µS [microsiemens] or resistivity [as Ohm.cm])
  

Temperature, pH, conductivity (or resistivity) must be measured "in situ" to avoid deterioration due to transport of samples (risk degassing exchange with the ambient air). This is true for freshwater especially and/or very aggressive water.

Concentrations of chlorides and sulfates are out si you want to calculate Larson-Index.
Note: concentrations of sulfate and chloride are given in mg/L.



Temperature.
Temperature (°C) into the calculations of pK1, pK2 and pK, so in those of pHs, IS and free CO2.
(see on this subject Equilibrium file) and finally affects parameters of calcocarbonic balance.

pH.
pH represents the hydrogen potential ie the concentration of hydrogen ions expressed by the negative logarithm of H⁺ (-H⁺)

Generally water is:

• "acid" with pH less of 7
• "basic" with pH higher of 7
• "neutral" with pH equal to 7

But it should be noted that pH of the calcocarbonic equilibrium does not match pH of electronic neutrality (pH 7). It may have a pH of equilibrium which is to be an acidic or basic pH of electronic point of view.
Note: Hallopeau graph represents the "figurative point" of water in question.

TAC.
TAC ("Titre Alcalimètrique Complet" in French) also called Total Alkalinity or simply Alkalinity (Alk) is water content of bicarbonates (hydrogencarbonates. HCO^3-). carbonates (CO3^2-), free alkali (OH^-), and positive ions associated with Ca²⁺, Mg²⁺, Na⁺ and K⁺

Note : "Titre Alcalimétrique simple" as TA (not used herein) measuring content of free water in alkali carbonates and alkali (Na⁺ associated with sodium and potassium K⁺). In natural waters TA can only occur si the pH is greater than or equal to 8.3 (TA = 0 si pH <= 8.3).
The distribution of constituent ions for alkalinity can be calculated from respective values of TA and TAC.

"Titre Temporaire" or dureté temporaire in French : "Temporary title" or Temporary Hardness.
Also called carbonate hardness is often carbonates and bicarbonates anions, mainly calcium and magnesium (anions disappear after boiling water). We can sometimes find the term "alkaline hardness."

Total Hardness (dureté totale orTH = "Titre Hydrotimètrique" in French).
corresponds to total amount of calcium and magnesium in water.

• si Alkalinity (Alk) is less than TH (Alk <TH), carbonate hardness is equal to TAC.
• si Alkalinity = TH. there can be other salts of Ca²⁺ and Mg²⁺ other than bicarbonates and in this case carbonate hardness is Alkalinity.
• si Alkalinity > TH : in latter case, water contains bicarbonates alkali metal. potassium or sodium (Na⁺or K⁺) and carbonate hardness is equal to total hardness.

"Titre Permanent" ou dureté permanente : "Permanent Title" or Permanent hardness (also called non-carbonated or non-alkaline hardness) :
concentration of calcium and magnesium, other than carbonates or bicarbonates. Thus mainly due to presence of anions sulfates. chlorides and nitrates anions.
It is also dsiference between the total hardness and the temporary hardness.

By definition, total hardness is equal to sum of permanent and temporary hardness. Total hardness is also equal to sum of carbonate and non-carbonate hardness.

Therefore, we have: permanent hardness = TH - Alk. so Alk ± TH (which is general case of natural waters almost).

By convention, these securities, which do not relate to a specsiic ion, are always expressed in french degree.
The following french table can deduct ions defining alkalinity of water from TA and TAC values (general):

Calcium.
Calcium or calcic hardnesss is global concentration in calcium salts. regardless of the associated anion. In french : Titre Calcique [TCa] of water.

Also. Magnesium hardness (or Title Magnesia. TMG). only with magnesium salts.

Total Hardness (TH) corresponds to total amount of calcium and magnesium in water. Hard water is one leaves encrusting carbonate deposits (scale generator) when heated. This feature is also reflected on dsificulty of soap lather.
Waters are classified as follows generally:

• TH < 10 °F (100 mg/L as CaCO3) : very soft waters
• 10 °F <= TH < 20 °F : soft waters
• 20 °F <= TH < 30 °F : hard waters
• 30 °F < TH : very hard waters

Reminder to milliequivalents and French degrees (°F):
In analyzes, concentration of components is almost always expressed in milligrams per liter (mg/L) or micrograms per liter (mg/L) for trace elements (1 mg/L=10^-3 mg/L=10^-6 g/L).
It is also sometimes, especially in US documents, from an expression of the concentration in ppm (parts per million). Strictly speaking, ppm refers to concentration of weight to weight. But there is not a big mistake by equating as mg/L, or as g/m³ (in case of extremely diluted solutions as natural water). Expression mg/L is not always convenient to monitor results of an analysis. It should in this case be replaced by milliequivalent per liter (meq/L).

By definition gram equivalent (g eq) is quotient of atomic weight for simple substance considered. On the number of electric charge (formerly the valence). For example, atomic weight of calcium is about 40 and this body being divalent (Ca²⁺). The g equivalent is therefore 40/2 or 20.
A solution with 1 g/L as calcium contains 1/20 = 0.05 equivalent/L, or 50 meq/L. So the item is considered monovalent, e.g. sodium Na⁺ mass 23, therefore the equivalent worth g as 23/1 = 23 g/L.

This notation has several advantages:
It allows summation of all elements analyzed. Which leads simply to assess mineralization. ionic = balance and allows immediate calculation of salt concentrations.

In water chemistry. we often need to know not details of various ions. But rather the sum of some of them (Ca²⁺, Mg²⁺, carbonates, bicarbonates, etc.).
It is for example titles: a measure expressed in mg/L would obviously meaningless. While meq/L allows immediate evaluation.
However, an old habit was retained by French water caterers of assessing these securities in French degrees (°F).

We need to know : 1 equivalent = 5000 ° F. So, 1 meq/L = 5 °F (or 1°F=0.2 meq/L).

French degree is a concentration unit may be used as meq/L, to express dose of any soluble salt in water. Widely used there few decades, this notation is much less applicable than "securities" such as TAC. TH. etc.

However, its use is still widespread in field of water treatment by ion exchange.

The units are as follows:

• A French degree (°F or f) is defined as 10 mg/L CaCO3, equivalent to 10 ppm. The lowercase f is often used to prevent confusion with degrees Fahrenheit.
• A degree of General Hardness (dGH or 'German degree (°dH, deutsche Härte))' is defined as 10 mg CaO/L, equivalent to 17.85 ppm (mg CaCO3/L).
• A Clark degree (°Clark) or English degrees (°e or e) is defined as one grain (64.8 mg) of CaCO3 per Imperial gallon (4.54609 litres) of water, equivalent to 14.254 ppm / mg/L.
• A US degree (gr/gal) is defined as grain CaCO3/gal[US, 3.78541 litres], equivalent to 17.118 ppm / mg/L.
• Parts per million (ppm) is usually defined as 1 mg/L CaCO3. It is equivalent to mg/L without chemical compound specsiied, and to American degree.

 Dry Residue (DR).
Dry Residue - measured after evaporation of filtered water and steamed (drying) to 180 °C - assesses dissolved solids. It allows to approach mineralization value. It can be deduced from conductivity value (or its inverse, as resistivity):

• DR (mg / l) as a function of conductivity (as microsiemens/cm):
  • so conductivity <50 then DR = 1.365079 x conductivity
  • so conductivity > 50 then <= 166 then DR = .947658 x conductivity
  • so conductivity > 166 then <= 333 then DR = .769574 x conductivity
  • so conductivity > 333 then <= 833 then DR =.71592 x conductivity
  • so conductivity > 833 then <= 10000 then DR = .758544 x conductivity
  • so conductivity > 10000 then DR = .850432 x conductivity
    
• DR (mg/l) as a function of resistivity (as Ohm/cm) :
  • so resistivity> 20000 then DR = 1365079 / resistivity
  • so resistivity> 6024 then <= 20000 then DR = 947658 / resistivity
  • so resistivity> 3003 then <= 6024 then DR = 769574 / resistivity
  • so resistivity> 1200 then <= 3003 then DR = 715920 / resistivity
  • so resistivity> 100 then <= 1200 then DR = 758544 / resistivity
  • so resistivity <= 100 then DR = 850432 / resistivity

Notes :
Conversion unit : resistivity (Ohm.cm) / conductivity (microsiemens / cm)

• resistivity = (1 000 000 / conductivity)
• conductivity = (1 000 000 / resistivity)

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