Calcium
Ca²⁺The ELIT calcium electrode provides reliable divalent cation determination across a wide range of matrices, from drinking water to food products and soil extracts.
The Selectivity Coefficient (SC) represents the approximate apparent increase in the measured concentration caused by 1 unit of the interferent. Effect (% error) = (interferent concentration × SC / analyte concentration) × 100.
| Interfering Ion | Selectivity Coefficient | Notes |
|---|---|---|
| Aluminium (Al³⁺) | 5 | Strong interference — more sensitive to Al³⁺ than Ca²⁺. Only tolerable at very low concentrations. |
| Iron (Fe²⁺) | 0.02 | — |
| Strontium (Sr²⁺) | 0.008 | — |
| Barium (Ba²⁺) | 0.005 | — |
| Copper (Cu²⁺) | 0.002 | — |
| Magnesium (Mg²⁺) | 0.0006 | — |
| Sodium (Na⁺) | 0.0005 | — |
| Potassium (K⁺) | 0.00005 | — |
| Ammonium (NH₄⁺) | 0.00003 | — |
| Lithium (Li⁺) | 0.00001 | — |
Apparatus Required
- Ion-Selective Electrode: ELIT 8041
- Reference electrode: Single junction (ELIT 001)
- Dual electrode head (ELIT 201)
- ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter
- 150 ml polypropylene beakers, 100 ml volumetric flask, 1, 2, 5, 10 ml pipettes
Calibration
Calibrate with 1000, 100, 10, 1, 0.1 ppm Ca solutions. Add 2 ml of 4M KCl to each 100 ml standard for high ionic strength samples (> 0.001 M).
Sample Preparation & Measurement
Low ionic strength: immerse in 50–100 ml sample. High ionic strength: add 2 ml ISAB to 100 ml sample and stir well.
Results
Results are displayed as ppm and mol/l. If buffer solution has been added equally to standards and samples, figures will not need adjusting as all are affected by the same dilution factor. Allow 2–3 minutes stabilisation after electrode immersion. Wash and dry electrodes between samples to avoid cross-contamination.
Animal Feed
Ash 1g finely ground sample in a muffle furnace at 320°C for 3 hours. Dissolve ash in 50 ml 1M HCl + 5 drops conc. HNO₃ and boil gently for 30 mins. Adjust pH to ~7 with 4M KOH (monitor with pH electrode). Transfer to 250 ml volumetric flask, add 2 ml ISAB. Result: ppm × 250 ÷ sample weight = mg/kg Ca.
Beer and Fruit Juice
Use the Standard Addition method due to complex matrix and high ionic strength. Calibrate to find accurate slope, then enter slope and expected sample concentration into the Standard Addition software.
Meat Products
Liquidise ~1g meat with 100 ml distilled water. Centrifuge, take 25 ml supernatant and mix with 25 ml ISAB. Result: ppm × 100 ÷ sample weight = mg/kg Ca.
Skimmed Milk
Use the Sample Addition method (add small volume of sample to large volume of standard) due to high Ca content and complex matrix.
Soil
Shake ~4g ground air-dried soil with 50 ml 0.5M sodium acetate for 2 hours. Centrifuge. Pipette 10 ml supernatant into a 100 ml flask, dilute to mark. Add 2 ml KCl ISAB. Standards must also be mixed 1:1 with 0.1M sodium acetate before adding ISAB. Result: ppm × 250 ÷ sample weight = mg/kg.
Sea Water
Direct potentiometry is difficult due to high ionic strength. Use Sample Addition method, or for high precision use Ca ISE as end-point indicator in an EGTA titration (Xie et al., CHIN. J. OCEANOL. LIMNOL. 1997). The Ag/AgCl reference (ELIT 001) replaces the older Calomel reference.
- Divalent cation electrode — slope is approximately half that of monovalent electrodes (~26 mV/decade vs ~54 mV/decade).
- Aluminium interference is severe and must be absent for accurate results.