A Rapid Method for the Determination of Sodium and
Potassium
Health Protection Branch Laboratories
Bureau of Nutritional Sciences
Ottawa
Laboratory Procedure LPFC-125
June 1983
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Definition:
This method is applicable to the determination of sodium and potassium
in processed foods.
Scope:
The rapid method has been evaluated by comparison with a dry ashing
sample preparation. The sodium content of seven foodstuffs and the
NBS-1577 Liver powder standard was measured in two laboratories. A
comparison of sodium results (rapid method ÷ dry ash method x 100)
showed good agreement in both laboratories with an average, based on the
analyses of eight items, of 100.5 and 100.7 (Table 2). The potassium content
of the same eight items was measured in one laboratory. A comparison of
results for potassium (rapid method ÷ dry ash method x 100) is
97.9. The sodium and potassium content of the NBS-1577 Liver powder standard
was measured in the two laboratories using both the rapid and dry ash
methods which generated results which were within the certified limits
for both sodium and potassium.
Apparatus:
- Flame atomic absorption/emission spectrophotometer.
- Homogenizing mill with stainless steel shaft and cutters, capable
of shearing, cutting, impact, cavitation and a speed of 45,000 rpm.
- Centrifuge with head to hold 250 mL centrifuge battles and be capable
of centrifugation at 1000 x g.
- Food blender with rheostat to control speed.
- Top loading balance of 1000 g capacity, capable of + 0.01 g accuracy.
- Polypropylene (PP) ware:
- straight side wide mouth jars (950 mL) with screw caps.
- wide mouth centrifuge bottles (250 mL) with screw caps.
- bottles with screw caps, 60, 120 and 250 ML.
- funnels (for 11.0 cm filter paper).
- stirring rod.
- Filter paper, Whatman No. 41 (11.0 cm, ashless) or equivalent.
- Plastic gloves.
- Pyrex beaker.
- Plastic wash bottles (one for deionized water, the other for acetone).
Reagents:
- Deionized water (resistivity of 1 megohm or better), referred to as
H2O.
- Sodium atomic absorption standard solution, 1000 ppm (commercially
available).
- Potassium atomic absorption standard solution, 1000 ppm
(commercially available).
- Reagent grade concentrated hydrochloric acid (HCl), 37%
- Dilute HCl (approximately 0.37% HCl) prepared from reagent grade HCl
by diluting concentrated HCl (37%) 1.2 g to 100.00 g with H2O.
- Reagent grade acetone.
Standards:
Standards are prepared on a weight for weight basis using the appropriate
size of PP bottle.
- (a)Prepare a 100 ppm Na solution by diluting 10.00 g of 1000 ppm
Na to 100.00 g using H2O.
- (b)Prepare a 100 ppm K solution as in 1.a).
Prepare an intermediate solution of 25 ppm Na/K solution by diluting
50.00 g of the 100 ppm Na (1.a)) plus 50.00 g of the 100 ppm K (1.b))
plus 2.40 g concentrated HCl to 200.00 g, using H2O.
- Prepare working standards of 0.5, 1.0, 2.0, 3.0 and 4.0 ppm Na/K
by diluting 2.00, 4.00, 8.00, 12.00 and 16.00 g respectively to 100.00
g, using dilute HCl.
Preparation:
Wearing plastic gloves is mandatory throughout the sample preparation
procedure (Na/K from skin can be a source of contamination). The glass
and plastic ware must be rinsed in H2O, dilute HCl and then
H2O, then dried in an oven at 100oC.
Procedure:
A. Preparation of the sample.
- Prepare a homogeneous, finely divided sample of 500 g or more using
the blender with rheostatic control. Use the PP stirring rod to facilitate
mixing.
- Transfer the blended sample to a straight side, wide mouth jar and
refrigerate until analysis.
B. Extraction
- Allow the refrigerated samples to come to room temperature (about
30 minutes).
- Into a previously tared centrifuge bottle (250 mL), weigh about 10
g of the blended sample, with the use of a PP stirring rod. Record sample
weight.
- Add sufficient H2O (usually 100 mL to cover the cutters
of the homogenizing mill and homogenize at maximum speed (about 90
seconds for meat and tune, and about 60 seconds for vegetables).
- Rinse the shaft and cutters of the mill with H2O, allowing
the rinse water to flow into the centrifuge bottle. Make up to about 200 g. Record
the solution weight.
- Cap the centrifuge bottle and shake vigorously to disperse the
homogenate.
- After every sample.
- rinse the shaft and cutters with acetone.
- Immerse the cutters in a beaker of acetone and run at half
speed.
- repeat 6.a) and b) with H2O.
- Prepare a blank with each series of samples in the same manner as
the samples, starting at step 8.3.
- Centrifuge the bottles for 20 minutes at 1000 x g to settle fibrous
material.
- Filter a portion of the supernate (about 50 mL) into a PP bottle,
discarding the first 5 to 10 mL. Dilute and determine Na/K in the filtrate
within 3 days.
C. Dilution
Dilute the sample (to obtain a concentration of about 1.5 ppm for Na/K)
and the blank weight for weight, using dilute HCl and PP bottles of the
appropriate size.
D. Determination
- Set up the spectrophatometer for flame emission according to the
manufacturer's instructions.
- Determine Na at a wavelength of 589.6 nm, with a spectral band width
of 0.3 nm and K at a wavelength of 769.9 nm, with a spectral band width
of 1.0 nm.
- Use the highest standard (4 ppm) to adjust the photo multiplier voltage
to give a reading of about 1 abosrbance unit.
- Run standards and samples.
Reference: Spitzer, M.E., Ritchey, C., Glennon, J.M., Villarreal, Y,
and Mason, Jr., A.D. A rapid method of preparing food for sodium and potassium
analyses. J Am Dietetic Assoc 62:44-46, 1973
Method Evaluation
The rapid method has been evaluated by comparison with a dry ashing
sample preparation. Homogenates of weiner, bologna, canned tuna, canned
peas, canned corn, canned carrots and canned green beans were prepared
in one laboratory and an aliquot of each shipped to the second laboratory.
A sample of NBS-1577 bovine liver powder standard was also analyzed in
both laboratories.
The results obtained by the two laboratories are given In Tables 1, 2
and 3. The results of the sodium analyses in Table I show that the coefficient
of
variation for both methods in two laboratories were generally low and
did not exceed 6.3%. As indicated in Tables 2 and 3, comparable results
for the sodium content of eight items were achieved using both methods
in two laboratories.
Table 4 shows the results of the potassium analyses using both the rapid
method and the dry ash method in one laboratory.
The certified 95% limit of sodium and potassium content of the MS-1577
liver powder standard was 230-256 mg/100 g and 910-1030 mg/100 g respectively.
Analyses of this standard for both sodium and potassium, using both methods
in two laboratories generated results which fell within the certified
limits for sodium and potassium (Table 1, Table 4).
Table 1 - Sodium Content of Some Foods Determined by Two
Laboratories
View
Sodium
Content of Some Foods Determined by Two Laboratories Table
Table 2 - Comparison of Sodium Results from Two Methods
View
Comparison
of Sodium Results from Two Methods Table
Table 3 - Comparison of Sodium Results from Two Methods
(Lab A ÷ Lab B) x 100
View
Comparison
of Sodium Results from Two Methods Table
Table 4 - Potassium Content of Some Foods Determined in
One Laboratory
View
Potassium
Content of Some Foods Determined in One Laboratory Table
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