Tools for environmental health.

Monitoring pH

Probably the most frequently asked questions we get at conferences and through our e-mail are those about pH field-testing methodologies. Since we started this column five years ago, a good answer has continued to elude us. Measuring food pH or using pH as an inspection monitoring tool is not always easy and straightforward. The more you work with pH, however, the greater the familiarity you will have with the various kits and instruments at your disposal and, in particular, with their proper application. At any rate, we have procrastinated enough. It is time to put pen to paper and share with you some of the trials, tribulations, successes, and failures we have had with pH monitoring over the years.

When we know that we will he monitoring pH in the field, we try to anticipate the type and numbers of samples we will have to take. We have two basic technologies at our disposal and a few variations of each type. The first technology is the portable electronic pen-type pH meter that ranges in cost between $50 and $200, depending on the degree of complexity or sophistication required. We have found that those selling for under $100 serve our purpose quite well. The second technology is colorimetry. While not as accurate as the pen-type meters, colorimeters are a rapid and inexpensive screening tool. We use two variations of colorimetric technology: pH test strips and liquid-dye indicators. The choice depends on the type of material we want to test. Here is how and why we make our choice when we go into the field.

Pen-Type pH Meters

Our pen-type pH meters are quite good; each of us owns one or two of them. At times, though, they can be exasperating--acting like spoiled children at a high-mass wedding. We find them particularly useful when measuring larger liquid volumes and liquids that are colored and/or opaque. In fact, the technology of the pen-type pH meter is the only way to measure the acidity or alkalinity of gravies, colored soft drinks, some juices, and any liquid with high levels of suspended solids. On the obverse side, we find at times that they are awkward to use--more time consuming than our other pH-testing methods. For the most part, the pen-type pH meters require "field" calibration in the truest sense of the term. This means carrying National Institute of Technology (NIST)--traceable buffer solutions or pH buffer pouches, distilled water, and sometimes an inert container to the job site. While that requirement is no problem at a spa or swimming-pool job site, it can prove cumbersome at some retail-food evaluations, insp ections or audits. In spite of these minor drawbacks, we must admit that once the pen-type pH meters are calibrated, they are fast and easy to use. They also demonstrate good accuracy, of [+ or -]0.1 pH unit, which nicely meets our needs for the most critical inspections. The less expensive units have an accuracy of [+ or -]0.2 pH units, which may be too broad for our use.

In comparison with our electronic thermometers, the pH meters are not as indestructible or as forgiving. On occasion we have found that our pH meters simply did not work properly; the instrument's response was slow or the readings were erratic and non-reproducible. Although we have not been able to isolate the causes of the malfunctions, we have taken several precautions to prevent repeated poor performance. We replace the 1.5-volt button batteries before each major use; most units rate their batteries for 24 hours. We carry the instrument in a padded case, avoiding jarring and temperature extremes. After each use, we ensure that the instrument is clean and dry before returning it to storage. We have even started removing the button batteries after each job and storing them separately to avoid current leakage. These measures seem to help. While the problems are not altogether eliminated, they are minimized, and we can certainly live with that.

For occasions when we need our electronic instruments, we have learned a few things that will aid in their use:

Any food can be liquefied; cooking or boiling does not change the pH. Therefore, we always take with us at least 500 milliliters (1 pint) of distilled or deionized water; a box of 4-by-4-inch weighing papers (they are inexpensive, dust-free, and inert) to provide a surface for macerating food; a clean stainless-steel pocket knife (melon testers are the most convenient); and a stainless-steel spoon (long-handled iced-tea spoons are the handiest). We use the spoon to take most samples, and the knife and the back of the spoon are used to chop and smash the food, which makes it easier to liquefy. Then we transfer the macerated food to a Styrofoam, plastic, or waxed cup and add enough water to immerse the instrument's electrodes for the test. The food-water mixture is stirred and the pH measurement taken. After each test, the knife and spoon are washed, rinsed with the distilled water, and air-dried before the next sample is prepared.

Before we forget, the best instruments are temperature-compensated from 0 to 50[degrees]C and require two-point calibration; acid/neutral or alkaline/neutral. The best, although not the least expensive, way to calibrate these instruments is with a calibration kit that consists of pouches containing 4.0, 7.0, and 10.0 pH buffer and a rinse solution. The handheld pH testers and calibration kits are available through most scientific catalogs.

Co1orimeters

For most of our inspections, we favor speed, portability and versatility in taking pH measurements, particularly when we carry more than a clipboard, pencil, flashlight, and dial thermometer. For this reason, we justify sacrificing accuracy with the hope that our rapid screening techniques will provide the answers we want. To this end, we turn to our colorimetric measuring tools. Because most colorimetric tests measure pH in whole numbers, we always have, when in doubt, our PawKit for testing water activity and a salinity/brix refractometer handy to recheck and verify our findings about food safety. The colorimetric tests we use fall into two categories: liquid dye indicators and dye-impregnated indicator papers.

Solids and semisolids such as sushi rice are easily measured with a colorimetric soil-pH testing kit, Our favorite is an old LaMotte soil-pH kit, which contains a white ceramic well tile. A sample of food is placed in the larger indentation of the tile, to which we add the pH solution (purchased fresh every year) drop by drop until the food is saturated. The excess liquid indicator fluid flows into a smaller indentation via a channel. The color of the indicator liquid indicates the pH when compared with a color chart that comes with the kit. The color change between a pH of 4.0 and one of 5.0 is sharp and easy to distinguish. We also use this method when measuring the acidity of fermented meats, vegetables, and grains. Soil-pH testing kits are available from most garden supply and discount stores for under $5.

For liquids such as water, some juices, soft drinks and pickling brines, we have found that pHydnion[R] Micro Brilliant dipsticks, Whatman[R], or EM ColorpHast[R] pH indicator papers are an ideal tool. For versatility we prefer the strips that measure the entire pH range of 0 to 14. These colorimetric papers are actually plastic strips onto which four differently colored dyes are covalently bound to a paper carrier and affixed to the strip. Each dye reacts to a different pH range, with significant overlapping. The color pattern of the four dye squares on the strip indicates the pH when compared with the color indicator chart contained in or on the container. The plastic strips are easier to handle than paper indicators. (Needless to say, this convenience comes with a price: A hundred strips cost about $12, versus twice that number for an order of magnitude less money.) To perform the test, simply immerse the colored bands on the strip in the liquid for a few seconds; if the solution is buffered, wait until th e color change is stable. These colorimetric pH papers are available through most scientific supply catalogues.

Epilogue

If any manufacturer of the pen-type pH instruments is reading this, please consider manufacturing an instrument that operates on standard AA or AAA batteries. We would be first in line to buy one.

Inspection Tip of the Mouth

Include an extendable inspection mirror in your everyday tool kit. The inspection mirror makes it easy to get behind refrigeration coils, to see under and around fixed equipment and palletized storage, and particularly to find those hidden cross-connections in bars and soda fountains. The mirrors are inexpensive and surely beat getting down on your hands and knees or standing on a milk crate.

This article is provided by NEHA for informational purposes only. It is designed to better inform our members about topical and relevant goods and services available to the environmental health professional. Opinions expressed about any product or service in this column either expressly or implied are solely and completely those of the author/s and do not necessarily represent the views or opinions of the National Environmental Health Association.