Wikipedia:Reference desk/Archives/Science/2012 September 11

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

September 11

Auric reactivity

What is the reactivity (measured in mol m^-2 s^-1) of gold towards hydrochloric acid, as a function of acid concentration; system pressure, and temperature?

I am planning to run an experiment using thin-film, gold coated glass electrodes. I need to establish the relationship between the film thickness (measured in tens of nanometres) and the usefull lifetime. Plasmic Physics (talk) 10:25, 11 September 2012 (UTC)

Gold should be completely unreactive towards HCl. It will react with aqua regia to produce Chloroauric acid, which can also be formed via electrolysis in the presence of just HCl. But you should be able to safely keep gold in a solution of HCl unperturbed, and you could wait centuries and not notice any change. So, if your gold does react, it will do so because of electrolysis. You can calculate how fast the gold reacts via electolysis using a form of the Nernst equation known as Faraday's law. In that equation, Q is the total electric charge imparted to the gold, which can be calculated if you know the amperage of your input current (amperage = coulombs per second). So there is a direct relationship between amperage and rate of reaction. However, if you just want to know how long a piece of gold will last in an inert solution of HCl, the answer is "about forever". --Jayron32 12:21, 11 September 2012 (UTC)

The soluble equilibrium of gold in water at room temperature/pressure is around 10^-22 kg of gold per kg of H₂O, which is entirely negligible for nearly all purposes. But since you asked about pressure and temperature, it is worth noting that this increases to as much as 10^-7 kg of gold per kg of H₂O at 400 C and 1000 bar. Not sure about the reaction rate. I don't know how to quantify the effect of HCl, though I would expect the solubility to still be negligible near room temperature / pressure. The caveat here, as Jayron32 suggests, is the potential for electrolysis. Au⁺³ will readily react with HCl, and so a current carrying electrode is likely to erode quickly if the current is high enough. Using Faraday's law, referenced above, this comes to roughly 10^-6 mol of gold per second per amp. The compound AuCl₃ remains soluble in solutions up to several percent, so there shouldn't be any problem with the current completely dissolving the gold. Of course, if you are working at low currents it may not matter. Dragons flight (talk) 17:19, 11 September 2012 (UTC)

Good, I will use it only for the cathode then? What should I use for an inert anode? Plasmic Physics (talk) 23:29, 11 September 2012 (UTC)

Before you suggest graphite, it doesn't work very well, the electrode disintegrates rapidly, and poses an explosion hazard at high current. Plasmic Physics (talk) 01:45, 12 September 2012 (UTC)

It might help if you specify the system more completely. In other words, what currents / voltages, what temperature / pressure, and whether any compounds beside HCl and H2O are present and their concentrations. Dragons flight (talk) 01:52, 12 September 2012 (UTC)

Indeed. I should note that the "classic" inert electrode is usually platinum; it is used as the inert electrode for the Standard hydrogen electrode. --Jayron32 03:49, 12 September 2012 (UTC)

Will I not have the same problem with platinum as I will have with gold? Plasmic Physics (talk) 05:56, 12 September 2012 (UTC)

The system involves current and voltage on the dA and dV scale, respectively. Temperature will vary between 25 and 145 degrees Celsius, pressure will remain at ambient levels. Other compounds present are varied metal chlorides, and hydroxides. Plasmic Physics (talk) 05:56, 12 September 2012 (UTC)

Socialization and digestion

Hi. This will be largely based on anecdotal evidence, but is there any actual research to support the idea that socializing in a group actually aids individual digestive ability, say over lunch? I ask this because I'm a slower eater during periods of social withdrawal and potentially a faster eater when socially active. Furthermore, there is also the observation that people are less talkative when hungry - does this mean that eating and social behaviour are intertwined, and if so, could it potentially explain the rise of eating disorders such as anorexia and bulimia across the Western world? Thanks. ~AH1 ^(discuss!) 17:51, 11 September 2012 (UTC)

I think this would be a function of how stressful the social interactions are, since activation of the sympathetic nervous system inhibits digestion whereas activation of the parasympathetic nervous system promotes it. Looie496 (talk) 18:16, 11 September 2012 (UTC)

On the other hand, stress often leads to overeating. StuRat (talk) 18:19, 11 September 2012 (UTC)

Note that eating more quickly in a group may be a psychological leftover of having had to eat before the food was gone, especially if you grew up in a large family, where the slow-pokes often get the less desirable food.

I've also developed the habit of eating quickly, and asking that my food be brought out as soon as possible, as opposed to waiting for everyone else's meals to be ready. This is because I order salads, which take much longer to eat (due to the low calorie-to-volume ratio) than, say, a burger, and I hate being the one holding everyone up while they stare at me trying to finish. StuRat (talk) 18:22, 11 September 2012 (UTC)

Also note that the rate at which you eat isn't necessarily correlated with the rate at which you digest, both because our digestive system can store food for later digestion, and since we can also digest food to a variable degree. StuRat (talk) 18:32, 11 September 2012 (UTC)

There are genetic factors too, some rats have a particular gene eats all the time and become fat while other given the same amount of food but not fat. --RexRowan (talk) 18:54, 11 September 2012 (UTC)

Don't know about digestive ability, but social facilitation claims that people eat more when in company: Furthermore, males ate 36% more food when with other people than when alone, and females ate 40% more food when with other people than when alone. Guess I'm an exception to the rule, as usual. Psychology ... Ssscienccce (talk) 21:16, 11 September 2012 (UTC)

Well, if you wanted to be sssscientific about it, you would have to do a little more then simply "reflect" on your past behaviour.. It's quite plausible that the people who eat more when in company don't actually realize they're doing it, they might even think they're eating less since they are distracted, maybe that's what leads them to eat more. Vespine (talk) 22:14, 11 September 2012 (UTC)

The way to a man's heart is through his stomach. μηδείς (talk) 02:39, 12 September 2012 (UTC)

Does the 'lensmaker's equation apply to non-Gaussian / fast / large-aperture lenses?

According to Gaussian optics, the paraxial approximation is used to derive simple explicit formulate for focal length, magnification and brightness. Is the paraxial approximation used in the lensmaker's equation? 71.207.151.227 (talk) 22:39, 11 September 2012 (UTC)

Any equation that uses focal length is paraxial. Focal length is a paraxial optics property. In real optics, you have to trace the path of each ray separately, and they will come to a focus at different distances depending on where they strike the lens.--Srleffler (talk) 17:15, 12 September 2012 (UTC)

why isn't MSG produced in neutral form?

I need to dissolve MSG in oil, but in order to do that I need to make it neutral by adding vinegar, which is a pain because I then have to decant the oil from the vinegar, and I suspect the oil/water partition coefficient for MSG might favour the vinegar. Which makes me think -- why is MSG produced as a sodium salt anyway? Is it because if its synergistic taste reaction with sodium? What does MSG taste like neutral? 71.207.151.227 (talk) 22:54, 11 September 2012 (UTC)

It's not just MSG, many spices come with sodium/salt, often far more sodium than the actual spice. I suspect that this is because sodium/salt tastes good, and is cheap. Thus, if you sell your glutamate as MSG (with sodium), you will make more money than if you sell it alone (as glutamic acid (flavor)). There's also (self-)deception involved, as people are trying to avoid sodium, but don't think of those other things they are shaking into their food as containing sodium, or at least not much, when they really do have a lot. StuRat (talk) 22:59, 11 September 2012 (UTC)

Curious how you're going to make monosodium glutamate without sodium. He's not saying he doesn't want the sodium. He's saying he wants something acidic included, to balance the alkalinity of the MSG. At least I think that's what he's saying -- original poster, is that right? --Trovatore (talk) 23:02, 11 September 2012 (UTC)

Oh, looks like I didn't read the whole thing. "What does MSG taste like neutral" doesn't really make sense — what you seem to be asking is "what does glutamic acid" taste like. I don't know the answer to that. --Trovatore (talk) 23:07, 11 September 2012 (UTC)

According to the article I linked to above, both give you the glutamate taste, "However, crystalline glutamic acid salts such as monosodium glutamate dissolve much better and faster than crystalline glutamic acid, a property important for use as a flavor enhancer." Presumably this refers to the ability to dissolve it in water. I'm not sure about dissolving it in oil or alcohol. StuRat (talk) 23:10, 11 September 2012 (UTC)

(edit conflict)How is vinegar supposed to neutralise monosodium glutamate which is also a weak acid? Last time I checked you need a base for that. MSG without sodium is just G, or glutamic acid. MSG is not equivalent to G. Plasmic Physics (talk) 23:09, 11 September 2012 (UTC)

It has a free amino group which is just begging to be acidified. Vinegar won't protonate the COO- group, it will protonate the RNH2 group. 71.207.151.227 (talk) 00:03, 12 September 2012 (UTC)

Oh yes, I forgot about that. Plasmic Physics (talk) 01:40, 12 September 2012 (UTC)

• The distinctive flavor of glutamate is called umami -- quite a number of scientists believe that it is a fifth basic taste, in addition to sweet, sour, salt, and bitter. Looie496 (talk) 23:16, 11 September 2012 (UTC)

How is neutralising MSG going to solubilise it? Plasmic Physics (talk) 01:42, 12 September 2012 (UTC)

Fat itself is now believed to be the sixth 'taste', richness. Our articles on these matters are not very comprehensive. μηδείς (talk) 02:38, 12 September 2012 (UTC)

I don't think fat is desired so much for it's flavor as it's smooth texture. StuRat (talk) 04:06, 12 September 2012 (UTC)

I think you left out a comma: "I don't think fat is desired so much, for it is flavor as it is smooth texture." μηδείς (talk) 05:05, 12 September 2012 (UTC)

We have fat receptors. It's not just texture. Anyway, neutral glutamic acid will probably dissolve in oil (just like sugar); but charged salts won't. I'd like to store it oil form because I'm going to prepare an oily condiment mixed with other fat-soluble ingredients. 71.207.151.227 (talk) 04:21, 12 September 2012 (UTC)

Except that neutral glutamic acid isn't neutral, it is zwitterionic. --Jayron32 04:51, 12 September 2012 (UTC)

Doesn't that depend on the conditions such as the nature of the solvent? In either case, both zwitterionic and non-zwitterionic glutamic acid should be poorly soluble in oil on account of its polarity being very much stronger than oil. Plasmic Physics (talk) 04:56, 12 September 2012 (UTC)

Sugar, which is polar, dissolves easily in hot oil. 71.207.151.227 (talk) 04:57, 12 September 2012 (UTC)

It would be considered neutral in the HPLC. For example in my lab, we have an HPLC optimised to detect charged neurotransmitters like dopamine. The non-decarboxylated L-DOPA washes out with the acetonitrile solvent front. The charged amines are retained by the alkylsulfonate solid phase. L-DOPA, which is zwitterionic, is sufficiently neutral to the blood-brain barrier and passes through easily, whereas dopamine doesn't. 71.207.151.227 (talk) 04:57, 12 September 2012 (UTC)

• Alrighty. Lets end this once and for all. This paper is 80 years old, but is highly relevent, being titled "THE SOLUBILITY OF GLUTAMIC ACID IN WATER AND CERTAIN ORGANIC SOLVENTS". Solubility clearly decreases as the size of the hydrophobic chain increases; it is most soluble in water, less soluble in methanol, even less soluble in ethanol, and essentially insoluble in acetone. So no, neutral glutamic acid is highly unlikely to be very soluble in oil, given those trends. --Jayron32 05:04, 12 September 2012 (UTC)

I assume Jayron's source is authoritative, but at least according to this website acetic acid will cause glutamic acid to precipitate from a saturated solution of monosodium glutamate. http://www.microscopy-uk.org.uk/mag/artsep08/rp-glutamic.html μηδείς (talk) 05:16, 12 September 2012 (UTC)

Well, it's all relative. MSG is more soluble in water than glutamic acid, so acidifying a saturated solution of MSG should cause glutamic acid to precipitate. The article gives the solubility figures in grams/50 cc of solvent, but converting to standard units for solubility, which is usually grams/liter, gives values of about 8.8 grams/liter for gram solubility from that article, a figure confirmed by Wikipedia's own article Glutamic acid. Monosodium glutamate is about 9 times as soluble, at 74 grams/liter. --Jayron32 05:36, 12 September 2012 (UTC)

As someone said right off, glutamic acid (flavor) is available. I can't make sense of this site's prices, which are all over the map, but here's an example of lots of people selling it in up to ton quantities. [1] Our article says that stuff like this can be labelled as simply "natural flavor" when it doesn't have the monosodium bit, which would explain how they sweep that one under. It is true that a salt is best suited for a polar solvent i.e. water, and the neutral acid better suited to dissolve in oil. Wnt (talk) 04:27, 13 September 2012 (UTC)