pH meter FAQ repost
Joe Sallustio
this.
Regards
Joe
The Selection, Use and Maintenance of pH Meters in Winemaking.
Revision 1
Overview: Accurate pH measurement of must and wine is an important
consideration to the winemaker. pH impacts the antimicrobial
properties of sulfur dioxide, the effectiveness of bentonite as a
fining agent, the stability of wine color, along with aging and
oxidation reactions (browning and changes in flavor). The range of
interest is approximately pH 2.5 - 4.5 for must and wine and pH 8.0 -
8.5 for the Titratable Acidity (TA) titration endpoint (8.2).
pH is related to the strength or concentration of the acids in wine,
while the TA relates to the amount of acid. Although interrelated,
they are not the same thing. A solution containing a specific
quantity of a relatively weaker acid such as malic acid will have a
different (higher) pH than a solution containing the same quantity of
a stronger acid such as tartaric.
The pH of a solution is defined as the -log of the hydrogen ion
concentration. It is typically measured on a scale from pH 0 to pH
14. The total quantity of hydrogen (H) and hydroxyl (OH) ions is
constant in a solution, as one increases the other decreases. Acidic
solutions (pH 0 - 7) contain more hydrogen ions than hydroxyl ions;
basic (pH 7 - 14) solutions contain more hydroxyl ions than hydrogen
ions. A pH of 7 is neutral (neither acid or base) as the
concentration of hydrogen ions and hydroxyl ions are equal at that
point. Given the measurement is logarithmic in nature and the
baseline is 7, a solution with a pH of 3.0 is ten times stronger than
a pH of 4.0. (For acids, smaller values of pH are stronger.)
SELECTION: pH measurement systems consist of a meter and probe or
probes. They come in many different configurations; a discussion of
the major features to consider follows. There are other methods to
measure pH such a using universal indicator (litmus) paper, but they
lack the accuracy required by the winemaker.
Self-contained meter and probe: Less expensive meters may include a
non-replaceable probe, when the probe becomes unserviceable the entire
system must be replaced. That does not mean this type of meter is
inferior, they can be very well made, accurate and very simple to use
and maintain.
Probes: The most common type in use today is the combination probe;
it combines a reference electrode with a measuring electrode in one
sheath. Some meters employ a separate reference and measuring probe.
Probes can be gel filled or contain a port to permit replacement of
the electrolyte filling solution. Gel filled combination probes may
be the best choice for a novice since they are relatively maintenance
free. Probe life is finite (typically 6 months to 1 year) and can be
extended by proper maintenance. The cost of a replacement probe(s)
when the original becomes unserviceable should be considered.
Accuracy / Resolution: A system accuracy (precision) of +/- 0.1 is
sufficient for home winemaking purposes. A higher resolution of 0.01
when digitally displayed is often mistaken for higher accuracy. A
recently calibrated pH meter with 0.1 resolution is much more useful
than an uncalibrated meter at 0.01. Knowing if the wine pH is 3.17 or
3.23 is somewhat academic for the home winemaker from a chemical and
sensory perspective.
Note: It is common practice to use the ‘toggling’ action
of a digital meter to advantage and extend the resolution by ½ digit.
In other words, if the meter reads 3.1, then 3.2, then back to 3.1 and
so on the reading can be considered 3.15. (Don’t get carried
away here. If it’s mostly 3.1, call it 3.1. When the meter
can’t decide, use that to your advantage and extend the
resolution.)
Temperature Compensation: The pH of a solution is temperature
dependent and must be compensated for. Automatic Temperature
Compensation (ATC) is very convenient, but is not required. A meter
without ATC requires a separate temperature measurement and subsequent
manual adjustment or correction tables.
Calibration: All quality meters will incorporate a means of
calibration utilizing either a set of electronic or manual
adjustments. At least two adjustments (usually at pH 7.0 and 4.0) are
required to set the offset and slope of the calibration curve. This
correlates the pH readings to the millivoltage output of the probe,
which is what the meter actually measures.
The mechanics of measuring pH are rather complex so calibration
verification is very important. (No one wants to make decisions based
on inaccurate information.)
Buffer solutions are used to calibrate the pH measurement system.
Buffers have a relatively constant pH and the ability to resist a
change in pH. pH probes can and do drift over time and with use;
buffers permit compensation for this. Buffer solutions accuracies of
+/- 0.02 @ 25 C are readily available and are the minimum accuracy
recommended. A third calibration point at pH 10.0 increases accuracy
around the TA endpoint, one at pH 3.0 increases accuracy when
measuring wine pH.
Calibration uncertainties are calculated using the root sum of the
squares of the linear accuracies. See the chart below for several
combinations of system/buffer accuracy and the resultant uncertainty
of the actual measurement.
System Buffer Measurement
Accuracy Accuracy Uncertainty
(pH) (pH) (pH)
+/- 0.01 +/- 0.01 +/- 0.014
+/- 0.01 +/- 0.02 +/- 0.022
+/- 0.02 +/- 0.02 +/- 0.029
+/- 0.05 +/- 0.02 +/- 0.054
+/- 0.10 +/- 0.02 +/- 0.103
For winemakers, buffer solutions of 3.00, 4.00, 7.00 & 10.00 are
recommended. A very useful intermediate point can be made in the
following manner. Place a small amount of potassium bitartrate (buy
cream of tartar at the market) in about 50 milliliters of distilled
water. Stir well; ensure the solution is saturated by adding enough
bitartrate so that some crystals remain visible. The pH of this
solution will be 3.55 +/- 0.01 from 20 - 35 C (68 – 95 F).
Cost: A 2-year supply of pH 3.00, 4.00, 7.00 and 10.00 buffer salts
with an accuracy +/- 0.02 can be purchased for $50 (US). (The salts
are added to 100 ml of distilled water along with a preservative.) A
self-contained pH meter and probe with ATC and a resolution and
accuracy of +/- 0.1 pH can be purchased for $50. Higher accuracy
self-contained meters cost $90. Higher accuracy meters with a
detachable probe can be purchased for $135 and up complete with probe.
Replacement probes can cost between $40 and $80. Buffers of higher
accuracy can also be purchased.
In order to measure pH accurately a minimum investment of
approximately $100 will be required unless access to chemicals to
create buffers is available. (This is beyond the scope of this FAQ.)
USE: Assemble the pH measuring system per the manufacturers
instructions and allow the appropriate warm up time, if any.
1. Prepare the buffer solutions for use. The meter manufacturer may
suggest specific calibration points (typically 7.00 and 4.00).
Buffers should be tightly capped when not in use. They require
replacement periodically.
2. Select additional buffer solutions that bracket the value(s) to be
measured. For winemakers, most pH measurements will occur in the pH
3.0 to 4.0 range for must and wine. The TA endpoint is reached by
convention at pH 8.2. Additional measurements within these ranges
will improve confidence in the accuracy of the measured value.
3. Calibrate the meter using the buffers recommended by the
manufacturer. It is good practice to gently stir the solutions when
measuring pH. Buffers should be used at room temperature, typically
specified as 25 C (77 F), follow the buffer manufacturers guidelines.
Always rinse the probe with distilled water prior to and after
measurements and between buffers.
4. Verify the accuracy of the meter using the 'bracketing buffers' and
any additional buffer(s) selected. Note discrepancies and compensate
accordingly.
The meter is now ready for use. Most winemakers combine the pH and TA
measurement; a simple procedure follows.
1. Select the sample; approximately 1 ounce (25 ml) is a convenient
sample size.
2. Dissolved gases such as carbon dioxide should be removed from the
sample prior to measurement. A simple test for dissolved gases is to
pull a partial vacuum on the sample (draw it into a syringe or
pipette); if the sample 'boils' or foams up, gas is present. A simple
way to remove the gas is to boil the sample for a few seconds. (A
microwave oven is very convenient; a setting of 15-20 seconds is
usually sufficient.) As soon as the sample begins to boil, remove the
heat source. Retest for dissolved gas after the sample cools to room
temperature.
3. Measure out 7.5 ml of the degassed sample using a burette, pipette
or 10 cc syringe. It is common practice to use separate instruments
for this sample and the NaOH.
4. Measure the pH of the sample; apply any necessary corrections to
the reading.
5. Select a burette, pipette or syringe to dispense a measured amount
of 0.1 N NaOH (1/10 normal Sodium Hydroxide). Slowly add (titrate)
the NaOH to the sample, stirring and observing the pH measurement. As
the pH approaches 7 the rate of change will increase; slow the
addition of NaOH. By convention, the endpoint occurs at pH 8.2. (The
official European method for analysis uses an endpoint of pH 7.)
The amount of NaOH added to the sample (in ml), equals the Titratable
Acid (expressed as Tartaric Acid) in grams/liter.
Any known quantity of the sample can be used as long as the ratio of
0.75 part sample to 1 part 0.1 N NaOH is used to calculate the TA.
Note: If you would like to calibrate the instruments used, 1 cc or 1
ml (they are the same thing) of distilled water weighs 1 gram at 60 F;
draw a measured sample of distilled water at 60 F and weigh it.
This procedure calculates TA expressed as Tartaric, to express TA as
another acid multiply the results by the following:
Sulfuric = Tartaric x 0.65
Acetic = Tartaric x 0.80
The triple substitution method can be useful when using pH meters with
a resolution of +/- 0.1 or when extra confidence in the accuracy of
the measurement is desired. Select the buffer closest in value to the
measured value (3.00, 3.56 or 4.00).
1. Measure the buffer; note the reading.
2. Measure the sample; note the reading.
3. Measure the buffer again and note the reading.
If the buffer measured values remain the same, the measured value of
the sample can be corrected by extrapolating any difference in the
buffers actual value versus the measured value.
Example: The buffer measured values are 3.1 and 3.1, the wine measures
3.2. Since the meter reads 0.1 high at 3.0, subtract 0.1 from the
wines value for a measured value of 3.1.
If the buffer values are not the same, repeat the test 3 to 5 times;
average the readings of steps 1 2 and 3 (and consider maintenance or
replacement of the probe). If the averaged buffer values of step 1
and 3 are not within +/- 0.05, the measurement is suspect.
MAINTENANCE: A well-maintained pH electrode used for wine analysis
can last several years; a poorly maintained electrode may last only
months. Signs of problems include slowed response and drift. A few
tips follow.
Always install the probe cover when not in use. A small damp patch
placed in the cover can help keep the probe moist. Do not let the
probe dry out, always store the probe wet.
A better option is to store the probe in soaking solution or a buffer
solution of pH 4.00 or 7.00. NEVER store the probe in distilled
water; it will shorten the probe's life dramatically.
Never wipe or touch the probe tip, it is very sensitive and easily
damaged.
Every few months or when the response time slows, soak the electrode
in pH 4.00 or less for a few hours to clean it.
To remove protein deposits, use a commercially available contact lens
soaking solution, following the manufacturers instructions.
To determine how accurate your own pH measurements are, have several
fellow home winemakers analyze samples from the same wine. In
professional winemaking, this is called a proficiency test.
Several vendors provide useful maintenance and use information on the
Web. This is by no means an exhaustive list or a product endorsement,
just a compilation of several sites known and used by the author from
time to time.
http://www.coleparmer.com/techinfo
http://www.eutechinst.com/techtips/tech-tips.htm
http://www.greenair.com/oaktoncat/oaktonindex.htm
http://www.hannainst.com/products/electro/elecguid.htm
http://www.laboratorymart.com/cgi-bin/nph-tame/lab/info/foodphbook.tam
http://www.omega.com/techref/ph.html
http://www.orionres.com/faq.html
Joe Sallustio
10/06/2000