If a substance is "soluble in alkaline pHs", does that mean its a base?

[thebillsfan]

Or an acid? Or just polar in general?

 

[austinap]

Or an acid? Or just polar in general?

It only means that it's soluble at alkaline pH. Any other information you can garner from this requires that you know the solvent. If it's a polar solvent, this suggests that your molecule is charged at alkaline pH, which implies it is an acid. If it's in a non-polar solvent, this suggests that the molecule is almost certainly not charged at alkaline pH, and is more than likely basic.

 

[thebillsfan]

It only means that it's soluble at alkaline pH. Any other information you can garner from this requires that you know the solvent. If it's a polar solvent, this suggests that your molecule is charged at alkaline pH, which implies it is an acid. If it's in a non-polar solvent, this suggests that the molecule is almost certainly not charged at alkaline pH, and is more than likely basic.

brilliant.

 

[Tokspor]

It only means that it's soluble at alkaline pH. Any other information you can garner from this requires that you know the solvent. If it's a polar solvent, this suggests that your molecule is charged at alkaline pH, which implies it is an acid. If it's in a non-polar solvent, this suggests that the molecule is almost certainly not charged at alkaline pH, and is more than likely basic.

If it's in a polar solvent at alkaline pH, it does suggest that it's charged, but it could either be positively or negatively charged couldn't it? Couldn't the solute be a base if it's negatively charged like NH2-?

 

[austinap]

If it's in a polar solvent at alkaline pH, it does suggest that it's charged, but it could either be positively or negatively charged couldn't it? Couldn't the solute be a base if it's positively charged like NH4+?

Sure, but is a base (such as ammonia) more likely to be charged at high or low pH? How about an acid?

 

[austinap]

If it's in a polar solvent at alkaline pH, it does suggest that it's charged, but it could either be positively or negatively charged couldn't it? Couldn't the solute be a base if it's negatively charged like NH2-?

Nice bait and switch there. Any idea what the pKa of NH3 is?

 

[Tokspor]

Nice bait and switch there. Any idea what the pKa of NH3 is?

Ok I looked it up and the pKa of NH4+ is 9.25 while NH3 is 9.75. So shouldn't this mean that NH2-, which is basic and negatively charged, can exist at alkaline pH, in this case something greater than 9.75?

 

[loveoforganic]

The pKa of NH3 is around 40 if I remember right.

 

[Tokspor]

The pKa of NH3 is around 40 if I remember right.

I think the pKa for a compound is when half of the compound is in its usual protonated form and the other half is in its deprotonated form. NH3 is the protonated form in this case and NH2- is the deprotonated form. NH3 --> NH2- occurs at around 9.75.

I also think you're right that the pKa for NH4+ is somewhere around 4. In fact, the pKb of NH3 is 4.75. I said that it was 9.25, but I must have misread. Can anyone verify 4.75?