This solution diluted 1 in 2 with deionised water is the blank solution.
Sample Preparation:
To 5g of dried accurately weighed sample in a sealable glass container add 250 ml of extractant solution. Seal and shake vigorously for 15 minutes (intermittently), stand the solution for 2 hours (or overnight if convenient).
Filter the solution through a Whatman No. 6 filter paper into a 500 ml volumetric flask and dilute to the mark with deionised water.
Standard Preparation:
From the Sherwood 1000 ppm Ca2+ standard prepare a 100 ppm Ca2+ by adding 10 ml of the stock standard to 50 ml of extractant solution in a 100 ml volumetric flask and diluting to the mark with deionised water.
Prepare also a 75, 50 and 25 ppm Ca2+ solution ensuring each standard contains a 1 in 2 dilution of the extractant solution.
Method
Set up the flame photometer as outlined in its instruction manual.
Select the calcium filter position.
Aspirate the blank solution (1 in 2 dilution of extractant solution) and set to zero.
Aspirate the calcium standards and record their stable readings.
Plot on linear graph paper meter readings against calcium concentration.
Aspirate the sample and note its reading. The calcium concentration can now be obtained from the graph. Multiply by dilution factor. 7. Repeat with further samples.
N.B. If the calcium concentration of the sample is outside the standard range then dilute with the blank solution.
The method of extraction is suitable for most samples, however if a more vigorous technique is required operate the extraction using an HCI/CH3COOH solution followed by autoclaving for 20 minutes at 20 psi and adjusting the pH to 4.5 with NH3 solution.
