Tuesday, July 03, 2012

A guest post by Chem Prof

Chem Prof wrote this; blame him for this shyte.

Over two months ago, a news article appeared in the Oregonian:

I wondered how look it would take for the complaints to appear. It took until June 30.

Chuck Weise wrote in Anthony Watts' blog, WattsUpWithThat a post called

I was surprised by the basic errors in chemistry made by the writer and those who commented on WUWT, so I wrote a few comments (they appear under the name Chem Prof). I was dismayed by the barrage that followed.

This is not about politics. It is about arithmetic and simple arithmetic functions.

Chuck Weise's WUWT post says
nothing about sources of error in measurements of pH,
nothing about sources or sinks of hydrogen ions in ocean brine,
nothing about complex equilibria,
nothing about the effects on hydrogen ion concentration due to other species,
nothing about non-ideal behavior of hydrogen ion in solution.

Here are things that he actually wrote:

(1) "As you also know, there are 14 orders of magnitude that define the pH scale from zero to fourteen units as per this equation. "

This is incorrect. The pH scale is open-ended.

(2) So if we moved .1 units towards acidity from the alkaline 8.2 to 8.1 oceans and compared the change, we have [delta H+] = 8 E-9/6 E-9 = 1.33 or a 33% increase in the hydrogen ion concentration, not an increase of 33% in the pH. None the less, that is how the story was reported and it is wrong.

In fact, the story does not mention pH at all. Lori Tobias quotes Alan Barton (hatchery research manager at Whiskey Creek Shellfish Hatchery on Netarts Bay), ' "Over the last 100 years we've made the ocean a little more acidic than it used to be," he said. "There is a 30 percent increase in the acidity of the ocean. If you are an oyster lover, that little bit more acid is a big problem. Eventually it will put our hatchery out of business." '

No mention of pH.

Here is a list of every reference to acids, acidification, or acidity in the article:

"Rising levels of carbon dioxide -- which lead to increased acidity in the ocean water, or ocean acidification -- were making it difficult, if not impossible, for the larvae to grow."

"The shell [of oyster larvae] is easy to dissolve and sensitive to acidic conditions, Barton said, unlike the hard adult oyster shell that's made of calcite."

"Some say it is impossible for the larvae to grow the shell; some say they can, but it is really hard to do.They use a lot of energy and don't grow. Whether it is impossible or just difficult, they are still dead. We know that the ocean acidification is causing our problem."

"The impact of ocean acidification is heightened by upwelling, which occurs when winds blowing from the north push the ocean surface waters away from the coastline, allowing deeper waters to move to the top. The deeper waters are already higher in carbon dioxide from all the decomposition below. Water that upwells is always higher in acidity, but our use of fossil fuels has added to the carbon dioxide in the atmosphere, in turn increasing ocean acidity, Barton said. "

"Over the last 100 years we've made the ocean a little more acidic than it used to be," he said. "There is a 30 percent increase in the acidity of the ocean. If you are an oyster lover, that little bit more acid is a big problem. Eventually it will put our hatchery out of business."

That is it. No mention of pH.

(3) "You agreed with me in my premise that hydrogen ion concentration makes up the pH but it is not defined by that number because the number of ions in an aqueous solution of water are very large. "

This is incorrect. Chuck Weise is correct earlier when he says that pH is defined by the equation pH = - Log[H+], where [H+] is the hydrogen ion concentration. Now he says it isn't defined 'by that number.' This is just wrong. In fact, pOH and pK, and for that matter, pAnything are defined in a similar manner:
            pAnything = -Log(Anything)
with the corresponding unique inverse relationship
            Anything=10^(-pAnything) .

This is done in chemistry all the time because the scale of chemical quantities such as concentration varies by many orders of magnitude.

How do chemists describe how much of a given substance is present in a solution? There are a number of ways, but every single one of them can be related to the molar concentration of a substance, that is, moles of substance per liter of solution, which is usually written as [X], where X is the substance in solution; it links two quantities: (a) the moles of a given material - a specific way of counting the molecules or ions of a given type and (b) the volume of the solution measured in liters.

For example, hydrogen ion concentration, written [H+], represents the moles of hydrogen ion in a liter of solution. 0.00010 mole of hydrogen ion in each liter of solution is written as 0.00010 moles/Liter or 0.00010 M when abbreviated. The quantity 'moles per liter' is also called 'molar' for short; so a 0.50 molar NaCl solution has 0.50 moles of NaCl per liter of solution.

What does Chuck Weise mean by "the number of ions in an aqueous solution of water are very large" ? I have no idea and it is irrelevant.

Comments that argue that "It’s even more complicated than that, because there are buffers involved" or something like that are missing the whole point of the acid-base chemistry of a buffered solution. The presence of a buffer (another weak acid or base) in significant quantity makes it more difficult to change the pH of a solution, not easier. So a significant decrease in the pH of a buffered solution is a clear indication that a significant source of acid has been added.

Some comments represent serious scientific ignorance:
"It is being assumed by these people that the ocean obeys the second law of thermodynamics, it does not because of surface tension."
"The oceans are infinitely buffered."
"A pH change of .1 corresponds to a change in hydrogen ion concentration of ..007%."
"A pH change from 8.2 to 8.1 cannot be truthfully described as ‘becoming ‘more acidic’. "
"the acidity isn’t increasing; the alkalinity is decreasing."
"pH is (negative) log because that is how organisms perceive acidity. They don’t perceive it in a linear scale, so it shouldn’t be quoted in one. No-one, but no-one talks about acidity other than via pH."
"The reason the change in pH is meaningless in this situation is that in a pure system where only H+ is considered to be driving pH, the change is one gram of hydrogen ions in one billion liters of water."
"Weren’t they lucky to have chosen the H+ ion concentration as reference, and not the OH- (OH minus) ion concentration?"
"Chem Prof seems to have misunderstood the nature of the complaint. The ion concentration is not the acidity (or alkalinity) as munderstood in pH. It is not used to derive the pH from -log10 of H+ ions...The measurements of the ocean are of pH...The ion concentration is derived from the pH. Not the other way round."
"As a Chemical Professor – cough! – you will be aware that there is a p[OH] measure too. So why not quote the alarming story that – oh noes! – hydroxide concentration in the oceans has fallen by 80%. I would hope you would see how stupid it was to use it, regardless of being technically true."
NOTE: That isn't true; a 0.1 decrease in pH corresponds to a 0.1 increase in pOH, with a corresponding decrease in [OH-] of 26.0%. If you are mystified by this, please note this arithemetic: 1.26 x 0.794 = 1.00 . 

On additional note: "I’ve copied a discussion from Wikipedia..." is not the strongest line of argument one might take.

Quite a few comments indicate there is confusion between the ideas of variability in a measurement and an error in a measurement. There is a large variability in the height of a human, varying from 0.55 m tall Chandra Bahadur Dangi of Nepal to 2.45 m tall Zeng Jinlian of the People's Republic of China. This natural variation does not preclude us from measuring individual height to a precision of 0.001 m or better. And variations in human height are worthy of study:

Chao-Qiang Lai, of the Jean Mayer Institute wrote
' ..."How much variation (difference between individuals) in height is attributable to genetic effects and how much to nutritional effects?" The short answer to this question is that about 60 to 80 percent of the difference in height between individuals is determined by genetic factors, whereas 20 to 40 percent can be attributed to environmental effects, mainly nutrition.'

He adds,
“Heritability allows us to examine how genetics directly impact an individual's height. For example, a population of white men has a heritability of 80 percent and an average height of 178 centimeters (roughly five feet, 10 inches). If we meet a white man in the street who is 183 cm (six feet) tall, the heritability tells us what fraction of his extra height is caused by genetic variants and what fraction is due to his environment (dietary habit and lifestyle). The man is five centimeters taller than the average. Thus, 80 percent of the extra five centimeters, or four centimeters, is due to genetic variants, whereas one centimeter is due to environmental effects, such as nutrition.”

Notice that one is still able to make a strong scientific statement about a 1 cm height difference in spite of a 200 cm wide variation in human height.

Some comments were made by those hanging their hats on the high ionic strength of sea water (it is significant, about 0.73 mole/liter) or the difference between the activity of hydrogen ions and [H+], but forgetting entirely that the activity of a 10^(-8) molar solution is ---- 10^(-8) molar ! Debye-Hückel theory plays no role in this whatsoever.

Chuck Weise adds, "In all of the work- ups and computations I have seen NOAA and others do when talking about ocean acidity, the units have always been referenced in units of pH. I have never seen work- ups in ion concentration" and misses the point again.

Some comments are rich with irony:
"Does anyone know how much CO2 it would take to change 1.3 billion cubic kilometres of [buffered - my clarification] salt water from pH 8.2 to 8.1?"
"Because even though the numbers do calculate to roughly 30%, it’s incredibly inappropriate to apply percentages with pH."
"To many non-scientific readers this implies a hugely significant increase in environmental impacts – 30% more acidic ! NOAA compound the misleading nature of this statement by not making it clear that this is a reference to a percentage change in hydrogen ion concentrations."
Willis Eschenbach: ..."applying percentages to logarithmic scales doesn’t work."



Onymousguy said...

Thanks a bizillion, Chem Prof.

Anonymous said...

Thanks for that detailed explanation. Now I won't have to go back and re-learn Chem 103 just to have this discussion. -- Greg