Hi Mike, Response in your text.
James Osbourne Holmes [email protected] -----Original Message----- From: M. G. Devour [SMTP:[email protected]] Sent: Friday, November 12, 1999 4:33 AM To: [email protected] Subject: RE: CS>wet chem > why not shop the net for a used > spectrophotometer? I have seen several which will do the trick for > about 400. You can get excellent really high-end but obsolete stuff > for a bit more. Just curious: what kind of capability does this kind of spec have? JOH: Good enough to measure Ag to a tenth of a mg/L. The Spectronics 20 is the workhorse of routine lab and educational work. The more expensive instruments have more bells and whistles, such as built in timers for specific procedures and alpha prompts and instructions, but the basic unit is solid and precise. I chose one with an analog (scale and needle) rather than digital because the digital displays are much more fragile except for resistance to shock. The one innovation I wish I had known about is a solid state photomultiplier instead of the tube I have. I can make a dilution of Hach reference standard AgNO3 to a calculated 4 PPM and hit it every time so far at 4.0 PPM. On the button. What range of wavelengths, UV, visible? Is wavelength fixed, manually JOH: Look in a VWR catalog for that sort of stuff, or mfg's sites. I am not knowledgeable about the field generally. The Hach reagent set uses 550 nm. This meter can work in IR and UV, but you need more stuff; different tubes and filters. Might be useful for some sort of particle size analysis, but not needed for PPM. This one and most later ones have a computer interface. You need software and a cable. More bucks. Scanned I don't understand. The 20 is set by turning a solid aluminum disk about 3' dia. with etched calibrations. I think it changes the angle of a ruled glass. You just turn a knob. I never change it. selected, scanned? Computer interface? What other parameters are important in evaluating the quality of such an instrument? Sensitivity? Resolution? JOH: I think if it will work in the 550 nm range, and reads " Percent Absorbance" it will be good enough. A tenth of a PPM/Mg/L is good enough for me, and probably good enough for medical research. Since optical alignment is an important factor in repeatability, I think one that weighs a lot may be better. The 20 has a cast aluminum chassis. If I understand correctly, you use the hach reagents to digest JOH: Digestion, using Hach's equiptment and reagents did not work for silver sol. A tech rep actually came to Marx's lab. Went away weeping. They decided that digestion was not necessary. That is more appropriate with stuff like sewage analysis. We just do the Hach procedure. Berger, Marx, and myself have gotten very colose results with the same sample. I think mine are good because I can hit the test solution so accurately. and mark the silver, then read the absorption on the spec. This is at a fixed set of wavelengths, one wavelength, ... or what? JOH: 550 nm. Essentially colorimitry? JOH: Yes, instead of eyeballing it the meter reads the density of that color. For the theoretical basis, look up "Beer's Law". I'm looking for an overview, just so I understand how the pieces fit a little better. If an investment of a grand would give me the ability to do what I want to do, it'd be just worth thinking about. JOH: It will, if you just want a reasonable measure of the PPM you are making. A college text on Quantitative Analysis will have a section devoted to it, even though much is being done today with AA and Flame. If there is a used instrument repair place near you, try to strike up a friendship ....it is a field where experience counts. Like most. > I have just begun recording mS of my batches to plot against > analyzed mg/L, more out of curiosity than anything else. Bruce Marx > told me a couple of years ago that he had very poor correlation. Yeah, who knows with the hvac product? You could have such dependency on water quality or other seemingly minor variables that it might not work *because of the nature of the product*. But for a narrow range of process conditions, Ivan is showing us a decent correlation between ppm and conductivity in at least his lvdc process. . JOH: It seems that way to me too. He may be making a very uniform particle size. Then it would probably work. > And, knowing PPM, even with precision, is not very helpful in terms > of effectiveness it you don't know anything about particle size. Which is one thing Marshall, I think, told us: there is a correlation between absorption vs. wavelength vs. particle size. Which is why it would be useful to have a scanning, or scannable, spectrophotometer sensitive enough to directly detect the absoption characteristics of samples of CS. JOH: I think that is probable. However, you must have some particle size analysis made by a reliable method against which to standardize. I don't think it needs to be scanned in that technique It will be based on how much light gets through, under very strictly specified conditions. The problem is that some of the light is absorbed and some is re-emitted at different frequencies and at a varying angles from the incident beam. I think the technique is a variation of turbidity measurement. I would be interested for quality control just to make sure nothing really grossly off occurred. According to Marx, Scanning Tran. Electron scope is where it is at. I don't know. At the U of Mont. they have one set up with a cryogenic stage. Freeze one drop in the stage. The machine counts each and every particle, sizes each and every particle, and give you a bell curve printout. $150/hour. Can do several in an hour, but it may cost a couple of hours initial setup fee. Once I get a repeatable process down pat I will send them some. I can now make what I think is excellent sol own use, and do it off-grid too, but I want to certify any I try to sell to a larger market than a few friends. > We are also into the realm of how many Angels can dance on the head > of a pin: how many silver bits does it take to destroy [Can't kill > 'em; my microbiology text says they aren't alive] one herpes > simplex viron? Will a huge clump with a big charge do it, or is a > single atom best? Well, if we can at least know the physical characteristics of what we're making, we can then attempt to evaluate performance on the basis of effectiveness, absorption, excretion, etc. JOH: Yes, and that is probably being done quietly in a few very well funded labs. One step at a time. JOH: Yes. Good talking with you my friend. Take care and have fun. there is lots more, search. Be well, Mike D. [Mike Devour, Citizen, Patriot, Libertarian] [[email protected] ] [Speaking only for myself... ] -- The silver-list is a moderated forum for discussion of colloidal silver. To join or quit silver-list or silver-digest send an e-mail message to: [email protected] -or- [email protected] with the word subscribe or unsubscribe in the SUBJECT line. To post, address your message to: [email protected] List maintainer: Mike Devour <[email protected]>
The Used Lab Equipment Fixer [labeqpfix.com].url
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