How to determine pH value, total titratable acid, ash alkalinity, formol number, total sulfurous acid, chloride, sulfate, calcium and magnesium with Metrohm instruments.
This Metrohm Application Bulletin AB-085 describes analysis methods for determining the following parameters: pH value, total titratable acid, ash alkalinity, formol number, total sulfurous acid, chloride, sulfate, calcium, and magnesium.
These titrimetric methods are suitable for the analysis of jams, fruit and vegetable juices, and their concentrates.
1. pH Value
Calibrate the combined pH glass electrode with the buffer solutions pH = 4.0 and pH = 7.0. Immerse the electrode in the jam and read off the pH value at drift constancy. In the case of fruit and vegetable juices carry out the measurement in the undiluted sample with stirring. The pH value is given with one decimal place.
2. Total titratable acid
Add 90 mL dist. water to 10 g sample and heat just to boiling point. After cooling down, titrate with c(NaOH) = 0.1 mol/L to pH = 8.5 using the SET mode.
b) Fruit and vegetable juices
Add 40 mL dist. water to 10 mL sample and heat just to boiling point. After cooling down, titrate with c(NaOH) = 1 mol/L to pH = 8.5 using the SET mode.
3. Ash alkalinity
Boil down 25 mL juice in a porcelain evaporating dish at 110 °C in a drying cabinet.
After addition of 1 drop paraffin oil, pre-incineration is induced over the flame, then the sample is completely reduced to ash in a muffle furnace at 520 ± 25 °C. (If black carbon particles are still present allow the ash to cool down, crush with a pestle, moisten with dist. water and repeat the incineration in the muffle furnace. If necessary, repeat this process again until only pure white ash remains.) After cooling down, rinse the ash first with 20 mL c(HCI) = 0.1 mol/L, then with a little dist. water into a glass beaker and heat for 15 min in a boiling water bath.
After cooling down again, titrate back to pH = 4.5 with c(NaOH) = 0.1 mol/L using the SET mode.
4. Formol number
Pipet 25 mL juice into a glass beaker and titrate with c(NaOH) = 1 mol/L to pH = 8.5 using the SET mode. While stirring, add 15 mL formaldehyde solution and wait for 1 min, then titrate the acid thus liberated in a second SET titration with c(NaOH) = 0.1 mol/L again to pH = 8.5 (EP1).
5. Total sulfurous acid
Mix 25 mL juice (a) or 10 g homogenized jam plus 50 mL dist. water (b) in a glass beaker with 25 mL c(NaOH) = 1 mol/L and allow to stand for 15 min. Then add 10 mL H2SO4 as well as 10 mL KI solution and titrate with the iodide/iodate solution using the MET Ipol mode (Ipol = 1 μA) and the double Pt sheet electrode.
6. Chloride content
Pipet 10 mL juice, 15 mL dist. water and 2 mL H2SO4 into a glass beaker. Then titrate the chloride with c(AgNO3) = 0.1 mol/L using the Ag Titrode.
7. Sulfate content
Add 30 mL dist. water to 20 mL juice and, if necessary, adjust the pH value to 3.5 with HCl. Then add 0.5 mL Ca standard and 5.00 mL BaCl2 solution and allow to react for 3 min with stirring. Add 10 mL buffer solution pH = 10 and titrate with c(EGTA) = 0.05 mol/L using the MET mode and the Ca ISE.
Two equivalence points are obtained: EP1 corresponds to Ca, the difference between EP1 and EP2 to the Ba excess.
First, the EGTA consumption for the BaCl2 has to be determined. This is done by means of a blank sample (without sulfate), which is prepared and titrated in the same way as the actual sample. This blank consumption is stored as common variable C31 in the titrator.
8. Calcium and magnesium
Ca and Mg are present only partially as free ions in these foodstuffs. The rest is bound in a complex man- ner to organic components. In order to determine the total contents, it is therefore necessary to incinerate the samples.
Pipet 25 mL juice into a porcelain evaporating dish and boil down at 120 °C in a drying cabinet. Afterwards, heat to redness in a muffle furnace at 600 °C until only pure white ash remains. After cooling down, add 2 mL HCI, heat in order to dissolve the ash and rinse into a glass beaker with distilled water. Adjust the pH value of this solution to 8.5 with NaOH. Add 20 mL auxiliary complexing solution and titrate with c(Na2EDTA) = 0.05 mol/L using the Ca ISE.
Two equivalence points are obtained: EP1 corresponds to Ca, the difference between EP1 and EP2 to Mg.
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