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Negative pH is possible, however, whether an acidic solution actually has a negative pH is not something that can easily be determined in a lab, therefore you cannot accurately measure/detect negative pH values with a pH probe.
A pH probe is used to detect the potential hydrogen (pH), which has a usual range from 0-14. Measuring pH tells us how much hydrogen is in a substance, usually a liquid, like water. It can also tell us how active a hydrogen ion is. A solution with a high amount of hydrogen-ion activity is an acid. In contrast, a solution with a high amount of hydroxide ion activity is a base.
Using pH probes when measuring pH is important for a wide range of industries, which is why there are different pH sensors for different applications.
While the pH scale typically runs from 0 to 14, it is definitely possible to calculate a negative pH. A negative pH occurs when the molarity of hydrogen ions in a strong acid is greater than 1 N (Normality). You can calculate a negative pH when an acid solution yields a concentration of hydrogen ions with molarity greater than 1.
For example, the pH of 12M HCl (hydrochloric acid) is calculated like this:
pH = -log[H+]
pH = -log[12]
pH = -1.08
By all means, calculating a negative pH is not the same as a solution being measured with a pH probe, and actually having a negative pH.
Using a pH probe to detect negative pH is not very accurate, since there are no standards for very low pH values. The majority of the inaccuracy comes from the large potential developed at the liquid junction of the reference electrode inside the pH probe.
Even though many textbooks may state that a negative pH is possible using a pH probe, no examples are given. This is probably as negative pH values cannot be easily measured or determined in a lab, and the poor availability of buffer standards suitable for a pH <1.
Another point we should mention is the dissociation of ions.
Despite commonly calculating hydrochloric acid this way, the above pH equation for HCl is not accurate, as it presumes a complete dissociation of ions has taken place inside the strong acid solution.
But, we must take into account that the hydrogen ion activity is generally higher in a concentrated strong acid compared to a more dilute solution. This is due to the solution containing a low concentration of water per unit of acid.
Because stronger acids cannot fully dissociate in water at higher concentrations when measuring the pH of HCl with a pH probe, some hydrogen ions would remain bound to the chlorine atoms, and therefore the true pH value would be higher than the calculated pH.
To understand negative pH, we must work out whether the incomplete dissociation of ions or an increased hydrogen ion activity has a greater effect. If the increased hydrogen ion activity has a greater effect, the acid is likely to have a negative pH value.
As previously mentioned, experimentally detecting negative pH values is a little complicated.
You cannot measure a negative pH value with a pH probe, nor is there a special pH litmus paper that turns a specific color when a negative pH is detected.
So, if litmus paper doesn’t work, then why can’t we just dip a pH probe into a solution like HCl?
Well, if you were to place a commercial glass pH electrode (probe) into HCl and measure the negative pH, it would be subject to significant errors, usually displaying an “acid error” to the reader. This error causes the pH probe to measure a higher pH than the real pH value of HCl. A glass pH probe that gives this high reading cannot be corrected to obtain the true pH value of solutions such as HCl.
When a negative pH is detected in a real-world situation, special correction factors are applied to the pH probe measurements. The two methods frequently used to scale these measurements are known as “The Pitzer approach and MacInnes assumption”.
The Pitzer approach to a solution’s ion concentration is widely accepted to estimate single-ion activity coefficients, and to understand the MacInnes assumption, we can look at HCl. The MacInnes assumption states that the individual coefficients for an aqueous solution such as H+ and Cl- are equal.
Negative pH can be found from natural water acidic streams to mine drainage.
The two most dominant sources of extremely low pH in natural waters are magmatic gasses (found in vents and crater lakes) and hot springs .
Some examples of the lowest pH values currently reported in environmental samples are:
Negative pH is possible, however, whether or not an acidic solution actually has a negative pH is not something that can easily be determined in a lab, therefore you cannot accurately measure extremely low pH values with a commercial glass pH electrode.
It is also difficult to use a pH to detect if the pH of a solution is lowered by the increase of hydrogen ion activity or via incomplete dissociation. To measure extremely low pH values, special electrodes would have to be used that incorporate special correction factors, which is why negative pH is currently calculated and not detected.
If you have any questions regarding pH, or what pH probes we have to offer, do not hesitate to contact our world-class team at Atlas Scientific.
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