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salcedam wrote:

AlphaTauri wrote:

Phenylethylamine wrote:I was familiar with Benedict's solution from doing Food Science years ago, and I know that if you have a food sample that contains a relatively small amount of a reducing sugar, it can take up to ten minutes for the qualitative test to show anything. With pure glucose, it only took a minute or so, but still, you had to wait. Are there some instructions out there that are leading people to believe it's an instantaneous test?

Perhaps the fact that NH4Cl turns deep blue immediately with Benedict's is leading people to think that glucose will turn orange immediately as well?

Also, does KCl turn green with Benedict's + heat? I've seen it written in our old forensics notes, but I haven't been able to find confirmation nor get the lab time to test it out.

Edit: Although Wikipedia does say that Benedict's can form a green precipitate...

With respect to the confusion about Benedict's and glucose, I think people just don't know how long to wait because the reaction time can vary and they may have done previous tests where the orange precipitate formed almost instantaneously (perhaps 30 seconds or so). The most I had to wait for a solution of glucose and Benedict's to react was about 4 minutes. Not sure why it took that long...perhaps the water wasn't hot enough...

True; and given that almost all the reactions involved in this event occur more or less immediately, it's not a ridiculous assumption to make. I just would have expected more people to do some minimal reading about the reagents in the rules before the event

salcedam wrote:According to the dichotomous key the Woz has, KCl does turn Benedict's green WITH heat.

Thanks. I didn't see this information anywhere. Also, "the Woz"?

Phenylethylamine wrote:

salcedam wrote:According to the dichotomous key the Woz has, KCl does turn Benedict's green WITH heat.

Thanks. I didn't see this information anywhere. Also, "the Woz"?

Linda Wozniewski, the National Director for Forensics. xD

If you go to the soinc.org website, her website has useful information regarding chem events such as Crime Busters, Forensics, etc. One of the links in the Forensics section of the SO Inc. website is to her website. ^.^

I redid the reaction between ammonium chloride and Benedict's today, and there is indeed a slight, but noticeable color change. A basis for comparison IS needed, though.

salcedam wrote:According to the dichotomous key the Woz has, KCl does turn Benedict's green WITH heat.

I'll have to try this soon and see what happens.

Also, does anyone know what the general difference(s) between dog and horse hair are when looking at them in a microscope (light and/or electron)? I asked this question awhile back, but to little avail.

salcedam wrote:

Phenylethylamine wrote:

salcedam wrote:According to the dichotomous key the Woz has, KCl does turn Benedict's green WITH heat.

Thanks. I didn't see this information anywhere. Also, "the Woz"?

Linda Wozniewski, the National Director for Forensics. xD

If you go to the soinc.org website, her website has useful information regarding chem events such as Crime Busters, Forensics, etc. One of the links in the Forensics section of the SO Inc. website is to her website. ^.^

Ah yes, I think I've used some of her resources before, but I was not on a nickname basis with her xD (and also apparently missed her dichotomous key... maybe because I already had my own made up by the time I found that site).

Dragonshark wrote:I redid the reaction between ammonium chloride and Benedict's today, and there is indeed a slight, but noticeable color change. A basis for comparison IS needed, though.

Thank you, this is good to know.

I did my own tests with Benedict's today: ammonium chloride, potassium chloride, and lithium chloride (because it occurred to me that lithium chloride would probably react the same way with copper(II) sulfate as potassium chloride would).

With ammonium chloride, upon mixing it with Benedict's, I saw an almost immediate and quite noticeable color change – an almost indigo-y dark blue. I didn't need a basis of comparison to see the change; I recommend using more of the powder (and rather than making a solution of the powder and then adding Benedict's, try just dissolving the powder directly in Benedict's).

With lithium chloride, upon mixing it with Benedict's and placing the test tube in a hot water bath, I saw a color change at the end of the test tube after about thirty seconds (lime green). It was pretty easy to see, but definitely localized at the bottom of the tube.

With potassium chloride, same procedure as lithium chloride, I got nothing, even after about ten, fifteen minutes. I assume that under the right conditions (more potent Benedict's? More powder? Higher heat? Longer time?) you could get some green in there, and it would presumably look quite similar to what I got for lithium chloride, but it'll definitely be a slower/less noticeable reaction than the LiCl.

Also, not really related but another thing I noticed today: always make sure you're comparing your pH paper against one dipped in plain water as a control. On a practice event today, I had some weak acid/weak base powders that I misidentified as neutral and therefore ended up going down the wrong path of my key. I was using pretty lousy pH paper, not the stuff I use in competition, but still.

I'm almost finished with my flowchart for qualitative analysis. On average, how long should it take you to identify 8 powders? I'm going to time myself with this soon, but I'd like to know what a 'good' time would be beforehand.

How do you tell the difference between NaHCO3 and Na2CO3? Likewise, what's the difference between NaCl and NaC2H3O2?

>.> All the sodium ones are confusing me

4Dv wrote:How do you tell the difference between NaHCO3 and Na2CO3? Likewise, what's the difference between NaCl and NaC2H3O2?

>.> All the sodium ones are confusing me

You can tell those apart through pH tests. Sodium carbonate will be quite basic while sodium acetate and sodium bicarbonate will be slightly basic. I use pH paper that turns blue with sodium carbonate and green with sodium acetate/sodium bicarbonate then to tell the difference between sodium acetate and bicarbonate, I just react them with HCl. Whichever solution fizzes would be sodium bicarbonate. NaCl forms a neutral solution so that should give you no change in color with pH paper.

Thanks! Also, how do you tell the difference between the sodium compounds (NaCl, NaC2H3O2, etc.) and the calcium compounds (CaCO3, CaSO4)? Whenever I perform the flame tests, both flames look orange to me...

4Dv wrote:Thanks! Also, how do you tell the difference between the sodium compounds (NaCl, NaC2H3O2, etc.) and the calcium compounds (CaCO3, CaSO4)? Whenever I perform the flame tests, both flames look orange to me...

Apparently calcium nitrate sparks in addition to its orange flame (although, disclaimer, I can't say I've ever observed this myself), and calcium carbonate and calcium sulfate are both insoluble, which separates them from all the sodium compounds.