AOAC Official Method
pH of Acidified Foods
First Action 1981
Final Action 1982
A. Principle
pH is measurement of H ion activity and indicates acidity. It may
be measured by determining electric potential between glass and ref-
erence elctrodes, using commercial apparatus standardized against
NIST primary standard pH buffers.
B. Apparatus and Reagents
(a) pH meter.—Commercial instrument with scale graduated in
£ pH unit and reproducibility of £ unit. Some instruments
permit expansion of any 2 pH unit range to cover entire scale and
have accuracy of ca – pH unit and reproducibility of – pH
unit. Other instruments have digital read-outs with similar capabili-
ties. Operate meter in accordance with manufacturer’s instructions.
In this method, several procedures for standardization and operation
of pH meters and electrodes are outlined. When these procedures
differ from manufacturer’s instruction, the latter should prevail, ex-
cept that NIST standard buffers must be used as primary reference.
Working buffer standards should be checked at least daily against
NIST reference buffers.
(b) Standard buffer solutions.—See and Table (see
).
(c) Electrodes.—Glass membrane indicator electrode and calo-
mel reference electrode (single or combination). Keep calomel elec-
trodes filled with saturated KCl solution because they may be
damaged if allowed to dry out. Maintain uniform temperature of ca
25° C for electrodes, standard buffer solutions, and samples. Soak
new electrodes several hours in distilled or deionized H2O before
use. Store glass electrode in pH 4 buffer. Store reference electrodes
in their own electrolyte filling solution. Store combination electrode
in pH 4 buffer with a few drops of saturated KCl solution added.
Store electrodes in manner consistent with manufacturer’s recom-
mendations if they differ from above. Store electrodes so that junc-
tion and hole are covered. Rinse electrodes with next solution to be
measured. If test sample material is insufficient, rinse electrodes
with distilled or deionized H2O. Lag in meter response may indicate
aging effects or fouling of electrodes, and cleaning and rejuvenation
of electrodes may be necessary. Clean electrodes by placing in
NaOH solution 1 min and then transferring to HCl solution
1 min. Repeat twice, ending with electrodes in acid solution. Rinse
electrodes thoroughly with H2O before proceeding with standard-
ization. Oil and grease from samples may coat electrodes; therefore,
clean electrodes with ethyl ether and restandardize instrument fre-
quently, usually after 3 determinations.
C. Standardization and Operation of pH Meter
Switch instrument on and let electronic components warm up and
stabilize before proceeding.
Standardize specific instrument according to manufacturer’s in-
structions, using NIST SRM buffers. Equilibrate electrodes, buffers,
and samples at same temperature (ca 25 ° C) before pH measure-
ments. Set temperature compensator control of instrument at ob-
served temperature. When determining pH of either unknown
sample or buffer, gently stir solution before testing.
D. Standardization of Analog pH Meter
Note temperature of buffer solution and set temperature compen-
sator control of instrument at observed temperature (ca 25° C). Stan-
dardize instrument and electrodes with acid potassium
phthalate buffer solution, (c) (see ).
Rinse electrodes with distilled or deionized H2O and blot—do not
wipe—with soft tissue.
Immerse electrode tips in buffer solution and read pH, letting me-
ter stabilize 1 min. Adjust standardization control so that meter read-
ing corresponds to known pH of buffer (ca ) for ambient
temperature. Rinse electrodes with distilled or deionized H2O and
blot with soft tissue.
Check expanded scale pH meters with pH or standard buff-
ers. Buffers and instruments can be further checked by comparison
with values obtained using another properly standardized instrument.
Check indicating electrodes for proper span by using 2 separate
buffers. For example, first standardize electrodes by using pH
buffer at ca 25° C. Adjust standardization control so that meter reads
exactly . Rinse electrodes with H2O, blot, and immerse in pH
buffer. If the electrode fails span test, rejuvenation or electrode re-
placement may be necessary.
E. For Digital pH Meters with Slope Control
Select 2 standard buffer solutions, preferably such that difference in
pH levels does not exceed 3 units and such that expected pH of sample
to be tested falls within their range, ., standard buffer solutions of
pH and . For most accurate results, one standard buffer should
be chosen with pH at or near pH of solution to be evaluated. Standard-
ize meter first in one pH buffer (., pH buffer) with standardized
control, and then use slope control to standardize meter in second pH
buffer, ., pH buffer. This procedure establishes the proper in-
strument response (slope) for particular pH electrode used, and results
in more accurate pH reading.
Sometimes difficulty is encountered with drifting of combination
electrode. When this occurs, identify and correct source of trouble.
Very often, reference electrode junction is responsible.
In case of faulty meter operation, refer to manufacturer’s
operating manual for proper trouble-shooting techniques.
F. Process pH Determination
Obtain test sample portions of material for pH determination as
follows:
For process test liquids, let temperature equilibrate to ca 25° C,
and determine pH by immersing electrodes in liquid.
Drain solid materials on No. 8 sieve (ss preferred) and blend to
workable paste. Let temperature of prepared paste equilibrate to ca
25° C, and determine pH.
Where appropriate, mix representative aliquots of liquid and solid
materials at same liquid-to-solid ratio as original sample, and blend
to workable paste. Let temperature of prepared paste equilibrate to
ca 25° C, and determine pH.
If pH meter is equipped with temperature compensator, then it
may be used in lieu of equilibrating samples to specified tempera-
ture, provided it is – 15° of 25° C standard temperature.
G. Preparation of Test Samples
(a) For estimating degree of pH equilibrium or uniformity.—Use
for foods which have not come to pH equilibrium, ., production
line samples, warehouse samples.
(1) Liquid and solid component mixtures.—Drain contents of
container 2 min on No. 8 ss sieve inclined at 17–20° angle. Record
weights for liquid and solid portions and retain separately. If liquid
contains sufficient oil to cause electrode fouling, separate layers in
separator and retain aqueous layer. Determine pH of aqueous layer
at ca 25° C. Remove drained solids from sieve, blend to uniform
paste, adjust temperature to ca 25° C, and determine pH. Mix
aliquots of solid and liquid fractions in same ratio as found in origi-
nal container, and blend to uniform consistency. Adjust temperature
to ca 25° C, and determine pH.
(2) Marinated oil products.—Separate oil from solid, and
blend solid to paste in blender. Add small amount ( £ 20 mL/100 g
product) of CO2-free H2O if necessary. Determine pH by immers-
ing electrodes in prepared paste after adjusting temperature to ca
25 ° C.
(3) Semi-solid products (puddings, potato salad, etc.).—Blend to
paste, adding 10–20 mL CO2-free H2O/100 g product if necessary.
Adjust temperature of prepared paste to ca 25° C, and determine pH.
(4) Special product mixtures (., antipasto).—Pour off all oil,
blend remaining product to paste, add 10–20 mL CO2-free
H2O/100 g product if necessary, and blend. Adjust temperature of
prepared paste to ca 25° C, and determine pH.
(b) For confirming pH equilibrium.—If product has been stored
long enough to attain pH equilibrium, then determine pH on normal
containers as follows:
(1) Determine pH on container mixture only, by opening con-
tainer, inserting electrode(s), and measuring pH.
(2) For products in oil, follow procedures outlined in (a)(2) above
to remove oil and obtain accurate pH reading.
H. Determination
Adjust test sample temperature to ca 25° C, and set temperature
compensator control to observed temperature. With some expanded
scale instruments, test sample temperature must be same as tempera-
ture of buffer solution used for standardization.
Rinse and blot electrodes. Immerse electrodes in sample and read
pH, letting meter stabilize 1 min. Rinse and blot electrodes and re-
peat on fresh portion of test sample.
Determine 2 pH values on each test sample. Readings in close
agreement indicate that test sample is homogeneous. Report values
to 2 decimal places, ., .
Reference: JAOAC 64, 332(1981).
