PH
From Silvergrain Research
pH is the measure of acidity and is a negative logarithm of hydrogen ion concentration. It is an important parameter in photographic processing chemicals, emulsion making, and almost any chemical process. Although photographic processing procedures usually do not require the accuracy of analytical chemistry, measurement of pH is important in developing original processing chemicals, troubleshooting processing chemicals, and working on more involved chemical problems related to photography. This page collects information on how my experimental procedures are currently set up.
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pH measurement
For those who are not familiar with pH measurement systems, pH measurement is not as simple as measuring voltage or temperature. This is an electrochemical measurement subject to chemical compatibility, and the compatibility issue limits the selection of pH electrodes suitable for photographic solutions. Therefore, selecting proper electrode is already a project by itself. If you do not have a serious urge in measuring the pH accurately, I discourage you from using a pH meter, and recommend pH test strips or indicator dyes.
Criteria
A pH electrode to be used for photographic applications should meet the following criteria:
- double junction Ag/AgCl reference (or single junction platinum reference)
- refillable
This is a little tight requirement than most light applications, but remember that photographic chemicals are very though on the pH electrodes.
The problem is that, vast majority of pH electrodes use Ag/AgCl as the reference electrode junction, and photographic developers, fixers, bleach, toners and some silver image stabilizers react with this junction, making it non-functional and useless. Double junction electrodes relieve this problem by adding an outer fluid of KCl outside of inner fluid of KCl/AgCl, connected by a junction such as ceramic, in order to isolate AgCl solution from the test solution. This outer junction electrolyte (usually 2M or 3M KCl) should be frequently replaced with fresh one, so that any contamination is removed before they reach the inner junction. This replacement of electrolyte is easy with refillable electrodes, but it is impossible with gel-filled electrodes. Gel-filled electrodes are considered disposable, but photographic solutions tend to kill them way too quickly to be economical.
When selecting pH electrode, you should be looking for indication such as:
- compatible with tris buffer
- compatible with heavy metal ions
- compatible with reducing agents
- compatible wtih sulfides
- compatible with high salt concentration
- suitable for environmental monitoring
- suitable for protein research
Darkroom chemicals are tougher to the pH electrode than all of the above combined.
For everyday quality control, I use an epoxy body, glass membrane pH electrode with refillable double junction reference electrode. The outer reference junction is filled with 3.0M KCl solution, and the outer junction is made of ceramic.
Unnecessary features for pH electrode:
- temperature compensation
Temperature compensation is a convenience feature but it is totally unnecessary for photographic applications, because we use solutions at standardized temperatures. Temperature compensation comes with pH electrodes and meters of proprietary interface, so that you'll have to buy a meter and electrode made by the same manufacturer. The choice of temperature-compensated double junction electrodes is very limited, and it is not worth the trouble.
Recommended electrodes
* ROSS electrode of Thermo Orion. * pH electrode with platinum reference electrode, such as Denver Instruments Cat. No. 300731.1 directly available from Denver. * Double junction electrodes with refillable reference electrode.
Note that even newer pH probes using ion selective field effect transistors (ISFET's) do have reference electrodes in them, and they suffer from the same chemical compatibility issues. Therefore, at this moment, ISFET models are not recommended.
A few pocket pH testers advertise that they use double junction electrode. However, they use gel filled reference electrodes and they are particularly problematic in my experience. Oakton pH Testr 3+ appears very convenient, having automatic temperature compensation and temperature reading, but it was very problematic with photographic solutions in my recent experience with it. Although I do hope future pocket testers will improve their chemical compatibility issues, I recommend against any of these products for photographic processing, even if accuracy is not required.
Eastman Kodak Company's publication H-24 Module 3 explains measurement of pH for commercial motion picture processing. The practice explained therein is a bit of overkill for routine checking of pH in black and white still photography, but it is worth reading. They recommend calomel (mercury based instead of silver based) electrodes to avoid reaction with developers. However, calomel electrodes are phased out in the industry for environmental considerations. Thermo Orion's ROSS electrodes as well as other non-silver based reference junction system should be considered.
pH electrode calibration solution
pH electrodes are like a scale without graduation, and daily calibration is a necessity. I calibrate my measurement system with NBS standard buffer of pH 7.00 and 10.01 at 25°C at the beginning of the session. In the session, between uses, I rinse the electrode in tap water and then "sanity check" in NBS standard buffer of pH 9.18.
Buffers of pH 10.01 is a carbonate buffer, and it is extremely susceptible to gas exchange with air. Therefore, this buffer should be kept in well sealed bottle, and should be replaced fairly frequently. This buffer can be mixed from reagent grade 0.025M sodium carbonate and 0.025M sodium bicarbonate in deionized water.
Buffer of pH 9.18 is also susceptible to gas exchange. However, I only use it to check the normal operation of the measurement system instead of calibration. Therefore, I mix 0.01M photographic grade borax with tap water treated as above, and keep it in tightly sealed bottle. This solution is used in an open beaker, and discarded at the end of each session.
pH electrode maintenance solution
pH electrode is routinely washed in tap water. However, if the electrode becomes erratic, I use warm water with dishwasher detergent, as recommended by many manufacturers. When deposition is suspected, I soak it in dilute HCl solution. The rust removal solution sold as CLR, Calcium Lime and Rust Remover (Jelmar) is said to be a mix of 12-14% hydoxyacetic (glycolic) acid, 3-5% sulfamic acid, 3-5% citric acid, 2-4% alkyl alcohol ethoxylate-acetate (a nonionic surfactant), 2-4% proprietary detergent mixture, and can be diluted for pH electrode cleaning purpose. (Incidentally, this is not a particularly effective rust remover or lime remover, but it works and is reasonably nontoxic. The pH of this working solution is about 1.5.)
When silver deposition is suspected on the reference junction, a solution consisting of 10% thiourea and some HCl to make the solution acidic is used. Use this solution sparingly, and then replace the reference electrode filling solution.
When not in use, pH electrode should be kept in a mixture of 3M KCl solution whose pH is adjusted to 6 to 7 with a bit of phosphate buffer. Never keep the pH probe in deionized/distilled/otherwise purified water.
