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 B-751 (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 B-751 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 B-751 and with ICP-OES (HORIBA Jobin Yvon. Model: ULTIMA2).
  8. Perform this measurement with 5 different samples.

*Although the measurement range of B-751 is between 40 mg/L to 4000 mg/L Ca2+, B-751 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 B-751.

Fig.1 shows the relation between measured values of Ca2+(mg/L) by ICP-OES and by B-751. The B-751 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 B-751 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 B-751 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 and B-751


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.