electronic measurment of calcium

[jez]

Hi
I have recently noted a number of US dealers selling probes to retro fit to orp meters that allege to read calcium in saltwater.

I am aware that ion specific probes exist but have they reached the point where they can come out of the lab and be of benefit to the reefkeeper.

regards Jez

[Randy Holmes-Farley]

Jez:

Calcium specific electrodes would attach to an ORP meter if the connection were right, or to a pH/mv meter.

I've not seen any specific uses of a calcium selective electrode in seawater (though I've never looked either) but I expect it would work, especially if you had the right standards that could zero out the effect of magnesium on the electrode.

The big problem with these has typically been cost. The ones that I see for sale for lab use are in the $500 range (plus a meter).

What prices have you seen?

[jez]

I actually clicked a link through from one of the sponsers on this board which I do not remember nor did I bookmark it but I remember it being around 350-375 dollars, I will see if I can refind it an post the link.

regards jez

[jez]

We ll I can at least thank microsoft for a history button
the link is http://www.customaquatic.com./customaquatic/index.asp

the normal price is 329 sale price is less

regards jez

[Randy Holmes-Farley]

Thanks:)

[simonh]

Randy,

I looked at these a while back in a scientific catalogue but saw all the interferences suchas Mg++ etc... At the time someone asked I suggested they would be pretty useless in seawater with all the interferences listed.

However, I would be interested in how would you zero out the interferences to make them useful?

[Randy Holmes-Farley]

Simon:

I'd want to read up on how much interference comes from magnesium for these electrodes to know how much of an issue it is. Different Ca++ ISE's may have different levels of interference of Ca++, depending on how specific they are made.

There seem to me 2 different ways to go about using such a probe. The second is better, but both can work for us.

1. Measure the calcium in your tank somehow. Put in the calcium ISE and get a mv reading. Then you can either let the level drop, or spike it with more Ca++ (outside the tank if you prefer) and get a second reading for a different mv reading. You then have 2 values that can be used to construct a standard curve. Assuming that the interfering ions like magnesium stayed approximately steady, you can use the ISE to measure future concentrations.

2. Make standards that include NSW (or tank) levels of Na+, Cl-, SO4--, and Mg++ (and any other interfering ions of high enough concentration, there may be none; Sr++ will interfere, but may be low enough to ignore) and known levels of calcium, from zero to about 500 ppm. This sounds more difficult, but it shouldn't be that hard. Use these to construct a standard curve, and use the electrode to get an appropriate reading. An enterprising manufacturer could make such standards.

[Randy Holmes-Farley]

Here are some references for those interested:

Test and evaluation of ion-selective electrodes for insitu marine monitoring applications. Ward, Gary K. Test Eval. Lab., NOAA, USA. Proc., Annu. Tech. Meet. - Inst. Environ. Sci. (1979), 25 436-9.

Abstract:
Ion-selective electrodes ae available for the direct measurement of Ca, Mg, K, Na, and F in seawater. Drift values and response times of these electrodes are given.



Use of ion-selective electrodes for oceanographic studies. Kosov, A. E.; Konnov, V. A. Inst. Okeanol. im. Shirshova, Moscow, USSR. Okeanologiya (Moscow) (1980), 20(5), 943-6.

Abstract:
Electrodes selective towards Ca2+, Mg2+, Na+, K+, or F- were obtained and used to det. the concns. of ionic forms and their assocn. in seawater. The 5 electrolytic cells used were: (1) Ca2+-selective: Ag; AgCl/KCl satd.//Ca2+ soln./liq. membrane/CaCl2 (0.01M)/AgCl; Ag; (2) Mg2+-selective: Ag; AgCl/KCl satd.//Mg2+ soln./liq. membrane/MgCl2 (0.01M)/AgCl; Ag; (3) Na+-selective: Ag; AgCl/KCl satd.//Na+ soln./glass membrane/NaCl (0.1M)/AgCl; Ag; (4) K+-selective: Ag; AgCl/KCl satd.//K+ soln./liq. membrane/KCl (0.01M)/AgCl; Ag; (5) F--selective: Ag; AgCl/KCl satd.//F+ soln./LaF3/NaF(10-4M), KCl (0.1 M)/AgCl; Ag. The cells exhibited considerable sensitivity to temp. changes and had to be thermostated. They also had to be protected against loss of electrolytes by evapn. The anal. values of Ca2+, Mg2+, Na+, K+, and F- concns. and their assocn. consts. with anions were used to obtain a chem. model of seawater at 25°.



Potentiometric titration of calcium in seawater. Xie, Shi-nan; Ji, Hong; Wu, Ai-qin; Chen, Guo-hua. Department of Marine Chemistry, Qingdao Ocean University, Tsingtao, Peop. Rep. China. Chin. J. Oceanol. Limnol. (1997), 15(1), 32-35.

Abstract:
Using Ca ion selective electrode (Ca ISE) as indicating electrode, SCE as ref. electrode and EGTA as titrant, Ca in seawater was detd. by potentiometric titrn. in borate buffer solns. This method can reduce observation errors in the detn. of the endpoint, and thus provide better anal. precision (<0.08%) than the complexometric titrn.


Computer-processed potentiometric titration for the determination of calcium and magnesium in seawater. Kanamori, Satoru; Ikegami, Hisashi. Water Res. Inst., Nagoya Univ., Nagoya, Japan. J. Oceanogr. Soc. Jpn. (1980), 36(4), 177-84.

Abstract:
In an improved potentiometric titrn. method for the detn. of Ca and Mg in seawater with a precision of 0.1%, a mini-computer is used for the automation of titrns. and ion-selective electrodes are used as end-point detectors. Ca is detd. by titrn. with EGTA and total alk. earth metals are detd. by titrn. with EDTA. Mg can be detd. by difference after Sr has been detd. by a suitable method, e.g., at. absorption spectrophotometry, flame emission spectrophotometry, or calcn. from the Sr/chlorinity ratio.


Evaluation of calcium sensors in fresh and saltwater. Ward, Gary K. Test Evaluation Lab., Natl. Ocean Surv., Rockville, MD, USA. Avail. NTIS. Report (1979), (EPA/600/7-79/058; Order No. PB-299332), 39 pp. From: Gov. Rep. Announce. Index (U. S.) 1979, 79(25), 204.

Abstract:
The Orion Calcium Ion Electrode No. 93-20 was evaluated for suitability as a Ca sensor for monitoring or in-situ marine applications by testing for the following parameters: accuracy, precision, temp. dependence, short- and long-term stability, durability, sensitivity to fluctuations in light intensity and flow conditions, response time as a function of temp. and concn. and variability between modules. The tests were performed at 10° and 25° in freshwater, synthetic seawater, and natural waters (35-, 20- and 5-ppt salinity). A description of the sensor, theory of operation, and a summary of the test results are included.

[Randy Holmes-Farley]

Also interesting:

ThermoOrion, a high end manufacturer of ion selective electrodes only lists sodium, chloride, nitrate, fluoride, and ammonia in seawater under their lists of applications in seawater:


http://www.thermo.com/eThermo/CMA/PDFs/Articles/articlesFile_13458.pdf

[smiller]

Randy,

I talked to Custom Aquatic last month about calcium probes. He told me that they had not received one yet but should have some in January and would be able to give me the results of their testing at that point. I will follow up and let you know.


Steve

[Randy Holmes-Farley]

Thanks, Steve

[smiller]

Custom Aquatic has sold several of these but never tested one.:rolleyes: He said he would try one this weekend...I will check back with them next week and let you know something when I know more.


Steve

[smiller]

Randy,

I talked to Custom Aquatic again this week. They tell me that the calcium probe was junk. It would have to be calibrated several times a day to stay close to an accurate reading. They refunded money on all sold.


Steve

[Randy Holmes-Farley]

Thanks for the update, even thugh it turned out poorly. :(