Well, Randy has already covered alkalinity very well in
this article, and there is further discussion about it in
this thread.
I've looked into PO4 and NO3 a little bit. PO4 is frequently done using the molybdate method. Google "phosphate molybdate seawater". NO3 is typically done by converting nitrate to nitrite, and then measuring the resulting nitrite. For some examples, try googling "nitrate reductase seawater". One interesting example (you may want to look at this website for more about the "
IORodeo Colorimeter") can be found
here.
For Magnesium, a similar technique as that I describe for Calcium is done, also using EDTA. Sometimes the Calcium is precipitated as the sulfate to make it unavailable to the reaction, and the Magnesium is then titrated just like I described for Calcium above. For my DIY Magnesium test, I first test for Calcium, and then I test for "Total Hardness" using an EDTA titration that measures both the Magnesium plus the Calcium. I then subtract the Calcium value from the Total Hardness value to derive the Magnesium. For my DIY Total Hardness test, I have found an indicator dye that gives me such an extraordinarily sharp endpoint that I can determine Total Hardness with an uncertainty of less than 0.5%, and more typically less than 0.3%, which means I can know my Mg to within +/- 5 PPM or so. I have also developed a "trick" to get similar precision out of my DIY Calcium test. Hach describes a combined Total Hardness and Calcium Hardness test, where both results are determined that you can read about
here.
