Measurement of calcium in various soils with LAQUAtwin

Generally, Atomic Absorption Instrument (AA) or Inductivity Coupled Plasma-Optical Emission Spectrometry Instrument (ICP-OES) is used to measure calcium ion, first by extracting the calcium ion from sample soils by 1 mol/L ammonium acetate CH₃COONH₄. These are the methods performed in laboratories.

We have developed a method for a rapid measurement of calcium ion in various soils by the use of small and affordable instrument LAQUAtwin calcium ion meter Ca-11 (Ion Selective Electrode Method). The extraction method is the same as the lab method, which the high concentration existence of CH₃COONH₄ will affect the measurement result as the ionic strength difference is too large. The following procedure explains how this effect can be cancelled to obtain a correlative result as the lab method.

【Measurement procedure】

1. Dry the test soils in a room about a week, and sift them by the use of 2mm diameter net sieve.
2. Place 1g each of test soils in 100mL glass beakers.
3. Add the 20 mL of 1mol/L CH₃COONH₄ to the beakers.
4. Shake the beakers (amplitude 40m/min, speed 250 rpm) around 1 hour to extract  Ca²⁺ from the soil by the use of Recipro Shaker SR-ⅡW made by Taiyo Kagaku Kogyo Company of Japan.
5. Filtrate the liquid of the beakers through JIS No.6 filter paper.
6. Calibrate LAQUAtwin Ca-11 with 150mg/L and 2000mg/L Ca²⁺ standard solutions which contain the same concentration of CH₃COONH₄ as in the filtrated samples.
   (Do not use the standard solutions packed with the instrument)
7. Measure calcium ion concentration of the filtrated solution with above calibrated Ca-11 and with ICP-OES (HORIBA Jobin Yvon. Model: ULTIMA2).
8. Perform this measurement with 5 different samples.

*Although the measurement range of Ca-11 is between 40 mg/L to 4000 mg/L Ca²⁺, Ca-11 can perform theoretically down to 4mg/L Ca²⁺.

【Results】

Calcium ion concentrations are shown at Table 1 in the order of;
Measured values (mg/L) by the use of ICP-OES. 
Measured values (mg/L) by the use of compact ion meter Ca-11.

Fig.1 shows the relation between measured values of Ca²⁺(mg/L) by ICP-OES and by Ca-11. The Ca-11 was pre-calibrated by the use of 150mg/L and 2000mg/L Ca²⁺ standard solutions containing the same concentration of CH₃COONH₄ as the filtered samples.

Measured values of Ca²⁺ concentration (mg/L) by Ca-11 accorded with the results from ICP-OES (Table 1).
We obtained the correlation coefficient R=0.998(n=5) between results of the two methods (Fig.1). 
As described above, we come to the conclusion that Ca-11 is very useful to measure the exchangeable Ca²⁺ in various soils with good correlation with the lab method by calibrating with calcium ion solutions with the same concentration of CH₃COONH₄  included.

Table 1

Fig.1：Relation between measured values of Ca2+(mg/L) by ICP-OES

For the soil diagnosis, measured Ca²⁺ concentration (mg/L) can be converted to CaO mg/100g of air-dried soil according to the next equation.
 

CaO mg/100g of air-dried soil = {Ca²⁺(mg/L)} × {a /1000} × {100/ b} × 1.399   
            a ; CH₃COONH₄ liquid (mL) 
            b ; air-dried soil ( g )

CaO converted values are also shown in Table 1.

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