pH of Hard water and Soft Water

[shrikant]

Dear All,
In an dissolution test procedure usually water is not considered as a medium(or in that case not favoured) as itspH varies from place to place.
But in theory ,Water is known to be of pH=7.0 (thats Neutral).
Again ,Water is classified as 'HARD' and 'SOFT' in a broad way.
So my question is:-
Can we hv a official pH range declared separately for Hard water and Soft Water.
I hope i am clear with the query.I mean just as we say that upto 7.0 its acidic pH & there afterwards its a basic domain upto 14,In the same way can we have well defined and distingiushed domain for Hard and Soft water, just to make its use to be more specific .Water being amphoteric in nature ,it should be possible.Pl. comment on this.

[ravenwork]

As I understand it, hard water is defined as water containing alkaline earth ions, divalent metal cations, typically calcium and magnesium. I am not aware of a direct link between water acidic/alkalinity and hardness/softness. In my experience, SOME hard waters can be alkaline due to the anions associated with the divalent cations, but this need not be the case.

[DR]

By water, any compendial method means deionised water. DI water is neither hard (loaded with lime forming salts) nor soft (using Na to replace Ca and/or Mg in these same salts) as it is essentially free of metal and salts.

[jitender]

Agree with DR. What is the objective of any such classification. Hard and Soft Water term is originally linked with ground water in which monovalent sodium (soft water) or/and divalent calcium or/and magnesium (hard water) salts are dissolved.


Probably the Compendial water is purified water which can be produced by distillation/deionization or reverse osmosis.


I never found the pH of water to be 7.0.

First its difficult to measure the pH of purified or distilled water because of low conductivity of water.

Second water rapidly pick up carbon dioxide from atmosphere and form carbonic acid which results to acidic pH.


Water Purified by reverse osmosis is first made alkaline to remove carbon dioxide to form bicarbonates in first step and later in second step made slighly acidic to change dissolved ammonia to ammonium ions. Formed Ions are removed by reverse osmosis at each stage. Water produced by RO is invariably acidic in many RO apparatus due to final stage of slight acidification.

In context of ions in Phamaceutical water, the important parameter is its conductivity or resistivity. The source of ions may be gases, salts or organic chemicals. Conducitivity of water must be in some specified range.

[syx]

By water, any compendial method means deionised water.

Water for dissolution media is deaerated water, though it is not mention by name in USP. The definition of deaerated water is purified water that has been treated to reduce the content of dissolved air by “suitable means”. The suggested methods for deaerating dissolution media (including water) including warming to 41 degree, vacuum filtering through a 0.45 um rated membrane, and vigorously stirring the filtrate while maintaining the vacuum. Other validated approaches may be used. Please see USP General Chapter about Dissolution <711>.

... I ever think that hard water is ice cube.