pH Meters, Specifications and Spheres of Use Jan. 29, 2015

What is pH and why do I need to measure it? pH measures the amount of acidity or alkalinity in a food, soil or solution using a numerical scale between 1 and 14. A pH value of 1 is most acidic, a pH value of 7 is neutral, and values above 7 are referred to as basic or alkaline. Acidified foods have a pH value less than or equal to 4.6. The proper pH of a canned food product can be critical to ensuring the safety of the product. It is very important that pH testing be done correctly and accurately.

Who Uses a pH Meter?

pH Meters are used by chemists, biologist and scientists working in laboratories, on manufacturing plants or out in the field. A pH Meter can be used to test the acidity or alkalinity in a wide range of substances such as chemicals, water, foods, beverages, washing powders, soaps, paper etc. Some pH Meters are large pieces of equipment whilst others are small hand-held devices. pH papers can also be used where appropriate.

pH Meters can also be used in a range of applications where acidity/alkalinity is crucial such as:

  • In agriculture, horticulture and commercial and domestic gardening a pH Meter can be used to test the alkalinity or acidity of a soil. Some plants grow well in acidic soils whilst others thrive in alkaline soils
  • In the maintenance of swimming pools and aquariums where an accurate pH level is essential in maintaining the right balance of the water so that the Chlorine used to sanitize the pool water does not cause a problem to swimmers; and as for aquariums - rapid changes in pH do cause stress to fish and should be avoided (also see additional tables-prompts for maintaining pH ranges for specific fish breeds and families below the article)
  • In the manufacture of cheese as cheeses only ripen and develop flavour and texture within a fairly narrow pH range
  • In the manufacture of wine where pH level is critical to produce a wine which is balanced. Acidity and alkalinity can affect the oxidisation, colour and stability of a wine. Some acidity is required to give a wine a tart, crisp taste but too much acidity makes it undrinkable whilst too little acidity results in a wine lacking in body and flavour. The ideal pH of a drinking wine is between 3.3 and 3.7 pH

The features to look for:

1. Resolution and a sufficient level of accuracy. The cost of a pH meters may range up to $2,000 or more. A major factor in determining cost is the accuracy and resolution of the meter. Smaller numbers indicate better resolution and accuracy. The cheapest meters typically feature a resolution of 0.1 pH units. Federal agencies typically require that pH readings be reported to the nearest tenth (0.1) unit. Most units therefore technically offer sufficient resolution to meet government standards. However, it is important to also consider the accuracy range of the meter.

Some of the least expensive meters may have an accuracy range of plus or minus 0.2 units. In other words, if the meter reads 4.3, the actual pH of the product could be anywhere from 4.1 to 4.5. This might present a problem if the pH of your product approaches the legal limit of 4.6. It is generally advisable to invest in a pH meter/electrode combination that offers resolution and accuracy of 0.1 pH units or better.

2. A pH meter with auto calibration and temperature compensation. Many pH meters now come with automatic sample temperature compensation and/or automatic calibration buffer sensing. These are conveniences that make it easier to calibrate the meter and to test the pH of samples. Users of meters without these features will need to manually adjust the meter either during calibration, sample measurement or both. In these case the best choice is a pH meter with at least 2-point calibration. There are 3 standard solutions: pH 4, pH 7, and pH 10. If you are buying a pH meter to test acidic foods, you need to have 2 points of calibration, pH 4 and pH 7. If you are buying a pH meter for scientific research, then you'll also need the pH 10 calibration standard solution.

Calibration solutions are color-coded for easy identification. The pH 4 solution is usually red, the pH 7 solution is usually yellow or green, and the pH 10 solution is usually blue.

Temperature. pH readings are affected by temperature. In order to get an accurate reading, the pH meter must be calibrated at the same temperature as the samples being tested. More expensive meters will compensate for variations in sample temperature (too warm or too cold). If you take care of calibrate your pH meter just before you monitor product pH, and test the pH of room-temperature samples (after equilibrium pH has been reached), you do not necessarily need to purchase a meter with temperature compensation. If you can afford a meter with this feature, it’s nice to have...

If you use a pH meter without temperature compensation, you can either test samples only at room temperature or use a temperature compensation chart to determine the correct pH.

3. Detachable or all-in-one Probe. Meters may come either with detachable, replaceable probes or they may be an all in one unit with an integral probe. Both types may work equally well. The units with detachable probes typically cost a bit more. The all-in-one units are more convenient and may require less maintenance. The important consideration is that all pH probes have a finite lifespan. The useful life of a probe is strongly influenced by the use, or abuse, it receives. But even in the best case, one may expect a probe to have a useful lifespan of about one to three years. Units with a detachable probe allow a user to replace only the probe as needed. All in one type units will need to be completely replaced.

4. Electrode type. Meters with detachable probes typically offer a choice of pH sensing units, called electrodes. Most probes feature a glass bulb type electrode located at the tip of the probe. These may be sealed or refillable. Sealed electrodes require less maintenance and are easier to set up and use. Refillable types may have a longer life since their design allows them to be cleaned and “rejuvenated” when performance begins to suffer. Other electrode specifications, such as reference type and junction type, are typically not important in food testing. It is always important, however, to be sure that the electrode type matches the meter with which it is to be used. Always consult the meter documentation when choosing a new probe. New meters are now available with probes featuring solid state electrodes. These systems offer the advantage of being easily cleanable and requiring very little liquid to sense pH. This makes them especially suitable for oily or semi-solid substance/food testing. Currently, however, these probes are much more expensive than a comparable probe with a glass bulb electrode. They may also be sensitive to static charges and electromagnetic fields such as those generated by magnetic stirring units. Solid-state electrodes may also have a relatively small sensing pore that can easily be obstructed by various small particles.

Additional Tables-Prompts for fish maintenance.

Fish and Their Respective pH Ranges:

Angel fish and discus

5.0 - 7.0 pH

Most tropical fish

6.5 - 8.5 pH

Tanganyikan cichlids

7.0 - 9.5 pH

Malawian cichlids

7.0 - 9.2 pH

Central American cichlids

6.0 - 8.5 pH

South American cichlids

5.0 - 8.0 pH

West African cichlids

5.0 - 7.8 pH

Fish Family pH Ranges:

Rasboras, barbs, clown loach, discus, angelfish, other South American cichlids

6.5 - 7.0 pH

Most tetras, silver dollars, most loaches, plecostomus, algae eaters

7.0 - 7.4 pH, low GH

Danios, gouramis, many cichlids, most catfish, rainbow sharks, redtail sharks, guppies, platies, and swordtails

7.0 - 7.4 pH, high GH

Mollies, African cichlids from Lake Malawi and Tangynika

7.6 - 8.6 pH, high GH

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