[marauder]

I'm trying to figure out a way to analyse my aquarium water for Ca content in an automated, continuous way.  First concept is using Spectrophotometry.  Its been years since I studied chem, so I was hoping someone could help me out.

Would it be possible to determine the concentration of these species by shining light through a sample taken directly from the aquarium, without adding indicators, or using a flame?  I guess what I really want to know is if Ca++ ions in solution would absorb light in the same way as ions that are passed through a flame.

I am looking for a Ca concentration in the vicinity of 450 ppm +- 50 ppm.

[Arkcon]

No, Ca2+ doesn't absorb in the visible or UV wavelengths usually used in a spectrophotometer.  You'll have to Google around for the indicators you might want to use.  You'll also find out if you can have the sensitivity and precision you need, but your numbers look a little tough for spectroscopy to me.

[marauder]

Wow... fast response.  Thanks for the input.

Back to the drawing board.

[ARGOS++]

Dear Marauder;

From  <200 nm till the IR it should not absorb at all.
The only Reagent for Ca²⁺ in the analytical sense I remember on the fly is :  "Murexide”.

But of course, it is not a real solution for continuously automated monitoring.
Maybe something could be possible with measuring a potential or so.

Good Luck!
                    ARGOS⁺⁺

[Arkcon]

Maybe something could be possible with measuring a potential or so.

A very good idea here, there are calcium selective electrodes for determining potentials.  In fact, they're pretty commonly sold:
{Google search}

[marauder]

I have looked into selective electrodes, but they are very expensive.  I am a bit of a tinkerer, so I am hoping I can use my skills in electronics to make an analyser using a broad spectrum light source, diffraction grading for wavelength selection and a light detector.

This link is what lead me to believe I may be able to analyse visible light for Ca:
http://jersey.uoregon.edu/vlab/elements/Elements.html

From my web searching I have identified EDTA as a common reagent in titrating Mg and Ca.  An indicator  (EBT) is used in conjunction with a buffer to maintain suitable pH.  Only problem is that ALL of these are toxic for various reasons (EDTA - C Ca depletion in biological systems, EBT - just plain nasty, pH buffer - upset natural pH in water).  So... not a continuous testing option.

If I cant find a method of continuous testing, I at least would like a means of automating the test procedure.  Currently I use commercial test kits, which require tedious drop-wise addition of reagents.

As a first stab I think I will try an EBT indicator, then measure the absorption of light by the colored complexes with Mg and Ca.  Hopefully there will be differences in the absorption spectra for the two complexes.

Any thoughts and comments would be appreciated.

[Borek]

I wonder if the final solution will be cheaper that just an electrode... Start with library - something like Vogel's Quantitative Chemical Analysis or Kolthoff Treatise on Analytical Chemistry could be a good place to look for ideas.

Calcium can be determined spectroscopically in the form of complex with GBHA in 0.04M NaOH. GBHA is glyoxal bis(2-hydroxyanil).

[Arkcon]

This link is what lead me to believe I may be able to analyse visible light for Ca:
http://jersey.uoregon.edu/vlab/elements/Elements.html

The page linked has no descriptive headers, and connects back to no page, and uses scripts (Flash).  I generally hate it when people hotlink those, they could be bot servers, or other malicious sites (of course, that's pretty unlikely from an .edu domain, just a pet peeve of mine, really.) 

At any rate, the abundance of lines suggests to me that they mean flame or ICP spectroscopy.  It seems to resemble the posters given away by elemental spectroscopy accessory vendors. 

At any rate, with no descriptive text on the page, my "looks like something I've seen, somewhere" is the best explanation we have so far -- and that's a pretty useless data source.

*[EDIT]*

I viewed the source for the page, even there, there is only a little, invisible on the page, hint:

Code: [Select]

<html>

<HEAD$SET(script=)>
</HEAD>

<BODY background = "../marb18.jpg">
<center>
<table border = 10><tr><td><center>
<APPLET CODE="elements.classes.Elements"
codebase=".."
WIDTH=765 HEIGHT=500>
<PARAM name = "ionized" value="true">
</APPLET>
</td></tr>
</table>