The Determination of Nitrite in Meat Products

The sample of cooked or uncooked meat is homogenized and the nitrate and nitrite extracted in hot water. After the addition of an ISAB, the nitrate is determined with a nitrate ISE. The nitrite in the sample is then oxidized to nitrate which is again estimated. The difference between the two values giving the nitrite concentration.

 

Equipment Required

1. pH/ION analyzer or pH meter with millivolt scale
2. Nitrate combination ISE
3. Blender/macerator/homogenizer
4. Acurate balance

 

Reagents

Nitrate stock standard (0.1M)

Potassium permanganate (0.1M)

ISAB: Sodium dihydrogen orthophosphate (1M)

 

Standard Preparation

 

Dissolve 15.601g NaH₂PO₄.2H₂O in de-ionized water in a 100-ml volumetric flask. Dilute to the mark.

 

From this stock standard prepare 10^-2 and 10^-4M standards by serial dilution. If a pH meter with a millivolt scale is being used, prepare 2 further standards, ie. 10^-3M and one other to enable a suitable standard curve to be constructed.

 

ISAB Preparation

 

Dissolve 15.601g NaH₂PO₄.2H₂O in de-ionized water in a 100-ml volumetric flask and dilute to the mark.

 

Sample Preparation

 

1. Weigh accurately 25g of sample and completely homogenize using, eg. A blender.
2. Add 100 ml of hot water (80°C) and mix well.
3. Transfer to a 500-ml beaker and make up to 300 ml with de-ionized water. Place on a boiling water bath with frequent stirring for 2 hours.
4. Filter the solution and collect the filtrate and washings in a 500-ml volumetric flask.
5. Dilute to the mark with de-ionized water.

 

 

 

Method

 

1. To 25 ml of each standard and sample add 3 ml of ISAB and 1 ml of 2M H₃PO₄.
2. Immerse the electrodes in the lowest concentration standard and record the stable reading.
3. Rinse the electrodes and read each standard in turn in increasing concentrations.
4. Plot a graph of mV reading vs. concentration on semi-log paper (unnecessary with pH analyzers).
5. Immerse the electrodes in the sample solution and record the stable reading.
6. With a pH/ION analyzer this will be displayed in direct concentration units. If a pH meter with a mV scale is being used interpolate the sample concentration from the standard plot.

 

This is the nitrate concentration of the sample (C₁). This doesn’t account for NO₂- interference.

 

7. Place 40 ml of the sample solution into a 50-ml volumetric flask and add 5 ml ISAB and 2 ml H₃PO₄ Stir well and add dropwise 0.1M KMNO₄ solution until a permanent purple color develops. Dilute to the mark with de-ionized water.
8. Repeat step 7 for all of the standard solutions. N.B. one drop of KMNO₄ should be sufficient for all standards.
9. Repeat stages 2 through 6 to obtain the total nitrate concentration of the sample (C₃).

 

Calculations

 

The nitrate ISE also senses to a certain extent nitrite ions, therefore C₁ must be adjusted to take this into account, ie. The NO₃- concentration is adjusted using the selectivity ratio of the NO₃- electrode to interfering NO₂-.

 

                        (C₂)  [NO₃-]  =  C₁-(C₁ x 3 x 10^-2)

 

Final nitrite concentration:  C₃ – C₂

 

N.B. Take into account the dilution factor.