characteristics solution constants

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specific gravities

gravity - concentration acid solutionsSecured image
Table 24. Equivalence between specific gravity and concentration in acid and base solutions
gravity - concentration saline solutions lime slurrySecured image
Table 25. Equivalence between specific gravity and concentration in saline solutions (and lime slurry)

Note : the characteristic of bleach, the commercial designation for sodium hypochlorite, is its active chlorine content, evaluated in chlorometric degrees, 1° chlorometric = 3.17 g · L–1 active chlorine. 1 L of bleach at 18° C contains 57 g of active Cℓ2.1 L of bleach at 48° C` contains 152 g of active Cℓ2.

conductivity – resistivity

Conductivity is measured by the conductance of a water column positioned between two metal electrodes having a 1 cm2 area and separated from each other by 1 cm. Conductivity increases with the dissolved salts content and will fluctuate depending on temperature (figure 7). Conductivity is expressed in siemens per metre (S · m–1). The sub-multiple, microsiemens per cm (μS · cm–1) is the unit mostly used in water treatment.

Pure water conductivity temperatureSecured image
Figure 7. Pure water conductivity based on temperature

Resistivity is the opposite of conductivity and is measured in ohms·cm (Ω·cm). The relation between these two measurements is as following :

formula: characteristic solution constants - Resistivity in ohms . cm

E.g. 10 μS · cm–1 conductivity corresponds to a 100,000 ohms · cm resistivity.

calibrating resistivity metres

Resistivity measurement cells have to be recalibrated at regular intervals using N/50 or N/100 potassium chloride solutions (table 26) :

formula: characteristic solution constants - Calibrating resistivity metres

relation with dissolved salts

For each type of dissolved salt, there is an empirical relation between conductivity and salt concentration; for instance, for a 1 mg · L–1 concentration variation in a dilute solution, conductivity at 25°C can vary from 1.25 μS · cm–1 for a pure solution of calcium bicarbonate to 2.5 μS · cm–1 for a pure solution of sodium chloride (broken down into very mobile ions, figure 8), through 1.5 to 1.6 μS · cm–1 for natural freshwater where all the most common salts are represented; this principle also applies to seawater (figure 9). The relation between conductivity and concentration is even higher in a pure solution of acid or base (figure 10).

Conductivity salinity NaCℓ solutionSecured image
Figure 8. Conductivity based on salinity for a NaCℓ solution

A measurement of conductivity variation (or resistivity) will therefore provide accurate information on changes in water salinity, even at very low concentrations (figure 11). Additionally, natural water can be classified according to their conductivity (e.g. low mineralisation of less than 200 μS · cm–1, average mineralization of between 200 and 600 200 μS · cm–1, high mineralisation above 600 200 μS · cm–1).

demineralised water conductivity electrolyte concentration Secured image
Figure 10. Demineralised water conductivity based on electrolyte concentration (25 °C)
impact sodium demineralised water resistivitySecured image
Figure 11. Impact of a sodium leak on the demineralised water resistivity

liquids viscosity

kinematic viscosity few liquidsSecured image
Table 27. Kinematic viscosity of a few liquids

Note: some flocculation additives have a high viscosity and the supplier must be asked for details (see ain reagents used in water treatment).

Absolute viscosity water temperatureSecured image
Figure 12. Absolute viscosity of water depending on temperature

unusual solutions


Solubility solid reagentsSecured image
Table 29. Solubility of a few solid reagents(1) (in grammes of substance applicable to the formula, per litre of water)

solubility of calcium sulphate (figures 13 and 14)

solubility product Ks calcium sulphate pure water temperatureSecured image
Figure 13. Solubility product Ks of calcium sulphate in pure water depending on temperature
solubility product salinitySecured image
Figure 14. Solubility product of CaSO4, 2H2O at 25 °C depending on salinity

solubility of calcium fluoride (figures 15 and 16)

Solubility pHSecured image
Figure 15. Solubility of CaF2 depending on pH
Solubility salinitySecured image
Figure 16. Solubility of CaF2 depending on salinity

solubility of silica

Note : in the absence of polymerisation inhibitors : the presence of a salt also has an effect on the solubility of silica, Ca2+ et Mg2+ will particularly reduce this solubility significantly.

Note : quartz (the solubility of the natural crystalline form of silica) and of amorphous silica in water will vary with pH and with temperature. Silica solubilises as silicic acid H4SiO4. In an aqueous solution quartz is less soluble than amorphous silica.

Solubility silica temperature pHSecured image
Figure 17. Solubility of silica quartz according to temperature and pH
Solubility amorphous silica pure water Secured image
Figure 18. Solubility of amorphous silica in pure water at 25 °C

approximate pH values for pure solutions

pH pure solutionsSecured image
Table 30. Approximate pH values for pure solutions

sodium hydroxide solution (table 31, figures 19 and 20)

temperature dissolving pelletsSecured image
Figure 19. Maximum temperature reached when dissolving pellets

Dissolution reactions are highly exothermic.

sulphuric acid solutions (table 32, figure 21)

reaction water sulphuric acidSecured image
Figure 21. Exothermic reaction when mixing water and sulphuric acid

Dissolution reactions are highly exothermic. Care is required when mixing: always pour the acid into the water and not vice-versa.

solutions of ammonia and morpholine (figures 22 and 23)

pH conductivity ammonia solutionSecured image
Figure 22. pH and conductivity of an ammonia solution at 25 °C
pH conductivity morpholine solution Secured image
Figure 23. pH and conductivity of a morpholine solution at 25 °C

solution of iron chloride (figures 24, 25, and 26)

Solubility temperaturesSecured image
Figure 24. Solubility of FeC 3 at different temperatures
Freezing point solutionsSecured image
Figure 25. Freezing point for FeCℓ3 solutions
rise dissolving ferric chloride waterSecured image
Figure 26. Temperature rise achieved when dissolving ferric chloride in water

quick lime

Figure 27 gives :

  • temperature rise;
  • respective concentrations in g·L–1 CaO or Ca(OH)2;
  • specific gravity,

based on the grammes of water per gramme of CaO dilution when quick lime slaking .

pK table

constant acid/base couple waterSecured image
Table 33. Common constant of acid/base couple in water at 25 °C

E.g. disintegration of boron (figure 28) :

pH TAC ratio, free CO2 (figure 29)

Relation  pH, M-alk. free CO2Secured image
Figure 29. Relation between pH, M-alk. and free CO2

COD - BOD5 – TThOD equivalences of some organic compounds in g per g of compound (table 34)

COD – BOD5 – ThOD organic compoundsSecured image
Table 34. COD – BOD5 – ThOD equivalences of some organic compounds in g per g of compound

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