The calcium content of commercial animal foodstuffs is determined by ashing a known weight of sample and dissolving the ash in dilute HCl. The pH of samples and standards is kept to between 5.5 and 6.0.
- pH/Ion analyzer
- detectION 3041calcium Ion Selective Electrode
- Muffle Furnace
- Accurate balance
- pH combination electrode and lead
- Calcium chloride (analar) (CaCl2)
- 1 Molar hydrochloric acid (HCl)
- 4 Molar potassium hydroxide (KOH)
- Concentrated nitric acid (HNO3)
- 2 Molar KNO3 (ISAB)
Accurately weigh 5.55g CaCl2 (analar) and dissolve in a 500ml volumetric flask, dilute to the mark with demonized water.
This is 0.1M CaCl2 stock solution.
10-4 Molar CaCl2 Preparation:
Accurately pipette 250ml of the stock solution into a 150ml pyrex beaker and add 40ml 1 Molar HCl and 6 drops of concentrated HNO3.
Boil the mixture for 30 minutes, allow to cool.
Adjust the pH of the standard using 4M KOH to pH 5.7.
Transfer the solution to a 250ml volumetric flask. Dilute to the mark with demonized water.
Prepare a 10 - 2 Molar CaClg solution in the same way except using 25ml of the stock standard in place of 0.25ml.
Note. Cone. HNO3 and 4M KOH are hazardous substances - handle with care.
Only two standards are required when using a Corning pH/Ion analyzer, however if a pH meter with millivolt scale is being used a 10- 3 Molar CaCI2 standard should also be prepared ideally along with a fourth standard, e.g. 5 x 10 -2 Molar CaCl2.
Weigh a suitable quantity of dried finely ground sample to give an expected concentration of calcium within the limits of the highest and lowest standards. If the approx. calcium concentration is not known, Ig of sample is a typical weight to use.
Transfer the sample to a crucible or ashing dish and ignite in a muffle furnace at 320°C for 3 hours.
Cool in a dessicator.
Carefully transfer the ash into a 150ml beaker and dissolve in 40ml 1M HCl and 6 drops HNO3 (conc.). Boil the sample for 30 minutes, cool, adjust the pH to 5.7 with 4M KOH and transfer to a 250ml volumetric flask and dilute to the mark with deionized water.
- To each 100ml of sample and standard add 10ml 2 Molar KNO3 as an Ionic strength adjustment buffer.
- Immerse the electrodes in the least concentrated standard and record the stable potential.
- Repeat 2 for further standards in increasing concentrations ensuring to rinse the electrodes between measurements.
- If a pH/Ion analyzer is being used instead of basic pH meter, only two standards are measured and no calibration curve need be constructed. If a pH meter with millivolt scale is being used plot the standard concentration against mV response on semilog graph paper to attain the calibration curve.
- Immerse the electrodes in the unknown solution and record the stable potential.
- Interpolate the calcium concentration from the standard curve or direct from the display if a pH/Ion analyzer is being used.
To attain the % calcium w/w in the foodstuff multiply the calcium molarity attained by 1002 and divide by the weight of sample taken.