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The Selection, Use And Maintenance Of Ph Meters In Winemaking.


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#1 Joe_Sallo

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Posted 15 January 2010 - 11:18 AM

I wrote this a while back, some may find it useful when trying to decide what to buy or how to get the most out of what they have. I can post it in text too if desired.

I did one on calibration of hydrometers too if anyone is interested in that.

#2 Joe_Sallo

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Posted 15 January 2010 - 11:31 AM

No idea what happened to the attachment, if it happens again i will cut and paste...

#3 Joe_Sallo

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Posted 15 January 2010 - 11:45 AM

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 ˝ 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.



  • 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.

  • 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.

  • 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.

  • 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.



  • Select the sample; approximately 1 ounce (25 ml) is a convenient sample size.

  • 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.

  • 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.

  • Measure the pH of the sample; apply any necessary corrections to the reading.

  • 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).



  • Measure the buffer; note the reading.

  • Measure the sample; note the reading.

  • 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.eutechins...s/tech-tips.htm



http://www.greenair....oaktonindex.htm



http://www.hannainst...ro/elecguid.htm



http://www.laborator.../foodphbook.tam



http://www.omega.com/techref/ph.html



http://www.orionres.com/faq.html





Joe Sallustio

10/06/2000





#4 Hammered

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Posted 15 January 2010 - 11:55 AM

Nice writeup Joe. Was this ever published anywhere? If not, you might submit it to Winemaker Magazine. They might even pay you for it!

Steve, Garagiste

Manufacturer of IntelliTanks at
www.Catalyst-Manufacturing.com

Author of The Homebuilt Winery @ www.HomebuiltWinery.com

 

 

#5 DesertDance

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Posted 15 January 2010 - 12:06 PM

I agree with Steve! Very valuable information there, and you communicated so we can all understand. This is a topic I have wondered about. I do have those PH strips which are pretty much useless. Thanks for your post!
Suzi

#6 Michael A

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Posted 15 January 2010 - 12:28 PM

Thanks Joe! Now we can link to this whenever we get the "just how do I use my meter?" question.

Cheers,
MRA
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#7 NorthernWiner

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Posted 15 January 2010 - 02:36 PM

I agree. Good information, Joe.

I took the liberty of "pinning" this thread to the top of the "Testing PH, Acids and Adjustments" section. So anytime you guys want to refer back to it, it will be easy to find.

Steve Kroll
President, Purple Foot Winemaking Club
"41 Years of Fine Winemaking"
www.purplefoot.org


Wine a little... and you'll feel much better!


#8 Joe_Sallo

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Posted 15 January 2010 - 05:35 PM

Nice writeup Joe. Was this ever published anywhere? If not, you might submit it to Winemaker Magazine. They might even pay you for it!


You are too kind. It's been in the public domain for about 10 years on the old usenet group, rec.crafts winemaking so I doubt they would be interested. :)

#9 Crazy Run Ranch

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Posted 15 January 2010 - 07:51 PM

It looks like you put a great deal of time into this guideline. Nice job.

"A system accuracy (precision) of +/- 0.1 is sufficient for home winemaking purposes"

This has been debated at length on multiple threads. Don't wish to start it up again but might consider softening the absolute tone of this statement. Not all would agree. There is also an alternative technology using a stainless probe with a chip to measure pH. The probe stores dry and they make very good and durable field testers with the accuracy to also function in the lab.

#10 Joe_Sallo

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Posted 16 January 2010 - 05:26 AM

It looks like you put a great deal of time into this guideline. Nice job.

"A system accuracy (precision) of +/- 0.1 is sufficient for home winemaking purposes"

This has been debated at length on multiple threads. Don't wish to start it up again but might consider softening the absolute tone of this statement. Not all would agree. There is also an alternative technology using a stainless probe with a chip to measure pH. The probe stores dry and they make very good and durable field testers with the accuracy to also function in the lab.



Thanks, when I wrote that what I ended up doing was a sort of peer review among some of the more knowledgeable winemakers in the group; then I sent it to a winemaking professor at UC Davis; here is the actual edit he suggested:



* Accuracy: A system accuracy (includes meter and probe) of +/-0.01 to 0.02

<S>is ideal for winemaking. If budget constraints are tight, a home winemaker </S>

<S>may consider an accuracy of +/- 0.1 acceptable if special measurement </S>

<S>techniques discussed later are employed.</S>

A system accuracy and precision of +/- 0.1 is sufficient for homewinemaking purposes. A higher precision (+/-0.01), digitally displayed, is often mistaken for higher accuracy. A freshly calibrated pH meter at 0.1 resoltion is much more useful, than a poorly maintained at 0.01.

Besides, knowing if the wine pH is 3.16 or 3.20 is completely irrelavant for the homewinemaker from a chemical or sensory perspective.



<S>* Resolution: 0.01 is recommended. 0.1 is the minimum required if the </S>

<S>budget is tight.</S>



I pretty much accepted that edit as is. I do explain how to arrive at the actual system accuracy in the body of the paper but do understand and appreciate your comment.

I can add the solid state probe if you have one you are happy with.

#11 Crazy Run Ranch

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Posted 16 January 2010 - 12:56 PM

I liked your text better than the Prof's. I understand the piece about calibration, difference between resolution and accuracy, and the importance of maintenance. But in actual use, you can basically double the tolerance of any meter. Here's what I mean:
Say you have a sample that is a true 3.60 pH. A +.1/-.1 meter will read from 3.5 to 3.7. That's a pretty good range in itself. But the real kicker is that you don't know which way the meter is going. So when the display says the sample is 3.5, you don't know that it is reading -.1. You must consider that a reading of 3.5 might be 3.4 because for all you know, the meter is reading +.1 at 3.5. So in actual use with a .1 accuracy meter, your range becomes 3.4 - 3.8. I would say with this kind of uncertainty, the meter is not giving you useful data. This example is extreme because its not likely the meter is going to swing to its max. tolerance. But it would make me uncertain of the information.




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