With varying pH, temperature and salinity: CaCO3 scaling in swimming pools Calcium carbonate

in contrast open equilibrium scenario above, many swimming pools managed addition of sodium bicarbonate (nahco3) 2 mm buffer, control of ph through use of hcl, nahso4, na2co3, naoh or chlorine formulations acidic or basic. in situation, dissolved inorganic carbon (dic) far equilibrium atmospheric co2. progress towards equilibrium through outgassing of co2 slowed (i) slow reaction h2co3 ⇌ co2(aq) + h2o; (ii) limited aeration in deep water column , (iii) periodic replenishment of bicarbonate maintain buffer capacity (often estimated through measurement of ‘total alkalinity’).

in situation, dissociation constants faster reactions h2co3 ⇌ h + hco3 ⇌ 2 h + co3 allow prediction of concentrations of each dic species in solution, added concentration of hco3 (which comprises more 90% of total dic ph 7 ph 8 @ 25 ˚c in fresh water. addition of hco3 increase co3 concentration @ ph. rearranging equations given above, can see [ca] = ksp / [co3], , [co3] = ka2 × [hco3] / [h]. therefore, when hco3 concentration known, maximum concentration of ca ions before scaling through caco3 precipitation can predicted formula:

camax = (ksp / ka2) × ([h] / [hco3])

the solubility product caco3 (ksp) , dissociation constants dic species (including ka2) substantially affected temperature , salinity, overall effect camax increases fresh salt water, , decreases rising temperature, ph, or added bicarbonate level, illustrated in accompanying graphs.

the trends illustrative pool management, whether scaling occurs depends on other factors including interactions mg, b(oh)4 , other ions in pool, supersaturation effects. scaling commonly observed in electrolytic chlorine generators, there high ph near cathode surface , scale deposition further increases temperature. 1 reason pool operators prefer borate on bicarbonate primary ph buffer, , avoid use of pool chemicals containing calcium.