WCL (Wet Chemistry Lab) sample Options

[bcory]

Apparently we're all going to be eating asparagus on Mars.

Stinky urine, anyone?

Edit: that article has some ... interesting units. A cubic metre of soil? Blimey!

That article left out half his sentence though...

"Preliminary results showed the soil had a pH between 8 and 9, researchers said. A pH less than 7 means the solution is acidic, while a pH over 7 means it is alkaline. Phoenix also detected the presence of magnesium, sodium, potassium and chloride in the mixture.

"It's very typical of the soil here on Earth minus the organics," Kounaves said during a teleconference from Tucson, Ariz.

On Earth, asparagus, green beans and turnips could be planted in such an environment and chemical-loving bacteria would thrive there, he said.

Planetary scientist David Paige of the University of California, Los Angeles, said it is too early to tell whether the minerals found in the soil could support life. Paige, who had no role in the mission, said the find was not surprising because rocks weather over time and bits of minerals mix with the soil."

http://news.yahoo.com/s/ap/20080626/ap_on_...3uBRCTV7nL0kPUI

[siravan]

What does a pH of 8-9 do to the "missing carbonates" paradox? If there was liquid water on the surface of the Mars at some point of time and CO2 in the atmosphere, then pH 8-9 (in contrast to an acidic pH most of us expected) is ideal for precipitation of carbonates (e.g. limestone). Where are all those carbonates?

[Juramike]

Apparently we're all going to be eating asparagus on Mars.

From the article:
"When told the pH levels, one colleague 'jumped up and down as if he had the winning lottery ticket,' mission soil analysis specialist Michael Hecht told a telephone news conference."

Evidently he was in the UMSF poll.

[Stephen]

Non-metric has to go. It just acts as a fog.

Careful! It's an American mission. If they get antsy it'll more likely be the metric measures which will get scrapped, in which case you guys are going to have to learn to live with feet and miles as well as inches.

Some people -- you give 'em an inch, and they'll take a hectare.

Mixing measures of length with measures of area in your metaphors serves only to confuse rather than to clarify.

======
Stephen

[ngunn]

Note for admin - the top quote in post 34 there is mis-attributed, not sure how that can happen.

Strange - somebody must have got there units mixed up - I've fixed it - James

[ngunn]

Back on the pH, I have a question for the chemists. On Earth clays in soil act as pH buffers. Is this only because of their associated organics, or would organic-free martian clays be expected to do the same?

[TheChemist]

Just to clarify things I wrote earlier in this thread: the fact that the pH of the first soil sample was 8-9, i.e. slightly basic, does not mean that the main salts present are carbonates, phosphates or whatever other "weak acid" anions. The first results from the team, also mentioned in the nice sum up in Emily's blog, show they found Na+, Ca+, K+, and Cl-, and are currently measuring the levels of sulfates, SO4--
Combinations of the above ions form salts where both ions are very soluble in water, and thus, do not alter the original pH of the water brought from earth, which was 7, although they might be the main salts in the soil concentration-wise. Basic salts could be just a very small proportion, or even absent if martian basaltic silicate minerals in sand have a slight alkaline pH, as mentioned by D. Burt.

[Juramike]

Back on the pH, I have a question for the chemists. On Earth clays in soil act as pH buffers. Is this only because of their associated organics, or would organic-free martian clays be expected to do the same?

Buffers can be either inorganic and organic.

So an inorganic salt could be a buffer (example: sodium bicarbonate solution).

Hydrate complexes formed by cations (K+, Mg++, Ca++, etc.) dissolved in water also have their own pKa's (and pKb's) and could also act as buffers.

-Mike

[nprev]

Hmm. What does this tell us (if anything at all) about the existence of superoxides at the site? Straining my memory of ancient high-school chemistry here, I would expect such compounds to be aggressive electron donors & therefore acidic.

(Okay, now tell me I got that backwards. Plus, I know it's a subsurface sample, but you'd expect this stuff to get circulated down at least a couple of cm due to wind action over time.)

[Juramike]

Hmm. What does this tell us (if anything at all) about the existence of superoxides at the site? Straining my memory of ancient high-school chemistry here, I would expect such compounds to be aggressive electron donors & therefore acidic.

Yup. The pKa of hydroperoxy anion (HOO-) is 4.88 (acidic like vinegar). So at any pH above 5, the predominant form will be the superoxide anion (O2--)

At pH 9, the ratio of superoxide anion to hydroperoxy anion will be about 10,000 : 1.

(Source: Wikipedia/superoxide + others to confirm)

-Mike

[dburt]

Hmm. What does this tell us (if anything at all) about the existence of superoxides at the site? Straining my memory of ancient high-school chemistry here, I would expect such compounds to be aggressive electron donors & therefore acidic...

As you feared, you actually have it a little reversed. An oxidizing agent like oxygen gas, peroxide ion, or superoxide ion wants to gain electrons (be an electron acceptor) - all want to turn into the oxide ion, O2-. A reducing agent like iron metal wants to lose electrons (be an electron donor), to either the ferrous ion Fe2+ or the ferric ion, Fe3+. Combine them and you get either FeO (wustite) or Fe203 (hematite). Oxidizing and reducing are clearly relative terms, because an ion like Fe2+ can either be oxidized to Fe3+ or reduced to Fe metal.

Regarding acids, by the electronic (Lewis) definition of acids and bases, an acid is electron pair acceptor, generally with a large charge to radius ratio (the ultimate acid is the tiny naked proton H+), whereas a base is an electron pair donor (e.g., the oxide ion, O2-, which has a pair of electrons to donate). Combine them and you get neutral H2O or water. In aqueous solution, these two ions are unstable - the proton combines with a water molecule to form H3O+ (hydronium) and the oxide ion takes on a proton to form OH- (hydroxide). By common convention (acidic and basic, like reducing and oxidizing, being relative terms in a continuum), an aqueous solution with an excess of hydronium is called acidic and one with an excess of hydroxide is called basic.

As you correctly recalled, there is some overlap between the definitions of oxidized and acidic, and between reduced and basic, inasmuch as both definitions involve electrons and ionic charge. This is accounted for in the so-called Usavovich definition, only rarely used by geochemists, which does not differentiate between oxidizing agents and acids, or reducing agents and bases. A familiar example of its importance is to compare the acid strength of reduced hydrogen sulfide H2S (very weak) with that of oxidized sulfuric acid H2SO4 (very strong). That is, if it's oxidized, its generally more acidic, as you correctly you pointed out.

Relevance to Mars? That's a longer story, and this post is already too long.

-- HDP Don

[dburt]

What does a pH of 8-9 do to the "missing carbonates" paradox? If there was liquid water on the surface of the Mars at some point of time and CO2 in the atmosphere, then pH 8-9 (in contrast to an acidic pH most of us expected) is ideal for precipitation of carbonates (e.g. limestone). Where are all those carbonates?

No one else has dared to tackle this, one of the biggest unanswered questions of Mars, so I'll foolishly try. One common hypothesis is that the ancient martian atmosphere was very different from today's, owing to a very high content of volcanic sulfur dioxide, itself caused by extremely active volcanism in the past. By this hypothesis, the acidification of surface waters by the atmosphere rendered deposition of carbonate rocks impossible. This would imply that the rate of absorption of acid species from the atmosphere always greatly exceeded the rate of neutralization of those acid species by the pulverized basic rocks of the martian crust. One chemical problem with this hypothesis is that seasonal or permanent ice cover on Mars would completely cut surface waters off from contact with the acidic atmosphere, allowing neutralization by reaction with rocks, at least seasonally. Another one is that surface waters on Earth (I know, terrestrial analogs are dangerous for Mars) are only rarely acidic, despite all the acids (carbonic, sulfuric, and nitric) constantly being tossed into the atmosphere by our civilization, not to mention our extremely active volcanism (compared to Mars). The implication is that surface rocks (on Earth at least) generally buffer the pH of surface waters to neutral or basic, even without an ice cover, and with active volcanism, coal-burning power plants, smelters, and so on.

Another common hypothesis, a far simpler one, is that, owing to low pressures and low temperatures, liquid water on Mars was always somewhat transient. That is, if present, it never stuck around long enough to reach chemical equilibrium with either surface rocks or the atmosphere, and thus to precipitate appreciable quantities of carbonates (other than the minor quantities found along fractures in some meteorites from Mars). Offhand, I'm not aware of any chemical problems associated with this hypothesis, but you may be. Also, keep in mind that the two hypotheses need not be mutually exclusive (that is, you could logically have a sulfur dioxide-rich ancient atmosphere, or any other ancient atmosphere, and transient or no liquid water at the same time).

Anyone else have any thoughts?

-- HDP Don

[Bill Harris]

pH is a quirky parameter and we may be putting too much stock in this value. I do wish that the media would stop proclaiming "conditions favorable for life" when the conditions are simply "not hostile to life". I'll be wanting to see an anion-cation balance and Piper-Stiff diagrams (et al) when the full chemistry comes in. It's been centuries since I've done one manually, I'll need to unearth my textbooks...

--Bill

[elakdawalla]

It's not really the media's fault. The Phoenix panel came out of the gate with a story from Hecht about how he called a friend working in Antarctica and told her that they now knew what they needed to know about how to grow stuff in Martian soil, because they knew what "nutrients" were there. And then they seemed confused when the reporters asked, okay, what kind of life are you talking about? Viruses? Bacteria? Multicellular stuff? Plants? And they weren't prepared to answer. Finally they said the pH was compatible with asparagus, turnips, and green beans, and there was nothing incompatible about the soil. But they'd started talking about life, so the reporters were asking, what about the trace elements? What about organics? And of course they couldn't answer those questions, because they couldn't do those tests.

--Emily

[TheChemist]

I really wish the panelists were a bit more conservative and better prepared in this last press conference.
After all, we know that the popular media will exaggerate whatever they hear to infinite proportions.
The press here today is full of comments about how easily "you could grow asparagus on Mars that is as tasty as in our own garden".