Introduction and Working Principle
A pH meter is a device for measuring how acidic or basic a substance is on a scale of 0 to 14. Zero point is the most acidic and 14 is the most basic (alkaline). The pH scale is “logarithmic” which means that as the scale rises by one point, the concentration increases 10 times. So a pH level of 6 is 10 times more acidic than pH 7, and pH of 5 is 100 times more acidic than pH 6. The extreme effects of acids and bases can be cancelled out or neutralized by mixing them together.
Note: An acid or a base’s strength is not related to how corrosive it is.
How it Works
The meter measures the concentration of hydrogen ions in an aqueous (water-based) solution by calculating the voltage between two electrodes, hence the name pH which stands for power of Hydrogen. pH can only be measured if it is in an aqueous solution so non-aqueous liquids like oils do not have a pH. On the pH scale 7 is neutral which means it is neither acid or alkali.
Modern pH meters consist of a glass electrode probe that is dipped in the aqueous solution, connected to an electronic unit that measures the hydrogen ion concentration. The electrode is filled with a reference solution with a known pH value, and has a thin wire sensor running up through the centre of it. The tip of the probe is a bulb made of thin glass which allows hydrogen ions to pass through into the reference solution.
Because the bulb is necessarily made of very thin glass, the probe is extremely fragile and great care must be taken with it. You should not touch it with your hand and only very careful cleaning should be attempted.
The most inexpensive meters at the bottom of the range are small, portable pen style devices with digital readouts. Moving up through industrial handheld meters to fixed installations in process plants, food grade meters in factories producing food and drink, and highly accurate laboratory equipment with multiple inputs, sophisticated computer technology and printers.
A very important part of using your pH meter is regular calibration. This is carried out by using reference buffer solutions which have a known pH. It is best to use at least two solutions, one of them acid and the other base and for normal use where absolute accuracy is not a prime factor, a 4.01 calibration solution and a 10.0 solution are adequate. For greater accuracy some higher-end pH meters use three buffer solutions. For normal operation the meter can be calibrated daily but for best accuracy and precision the meter should be calibrated before each use. Temperature is the other factor that needs to be considered when calibrating but you should refer to the manufacturer’s manual for details of this. Better quality pH meters have built-in temperature probes and temperature compensation or correction within the unit with no calibration needed.
The pH meter should be cleaned after every single use. For this it is simply a matter of rinsing it with de-ionized or distilled water and dabbing with a scientific wipe to dry any water remaining on the probe. The probe should be given a proper monthly clean with a specific solution designed for cleaning electrodes – usually hydrochloric acid.
Because of the fragile nature of the electrode probe, from time to time it will break and need to be replaced. In general, the pH meter should be cleaned with a damp, soft paper towel after each time it is used, do not use solvents to clean the meter. The electrode filling solution should be replaced regularly in line with the manufacturer’s instructions and any servicing or repair must be carried out by a qualified service technician only.
Uses of pH Meters
Because all water-based aqueous solutions have a pH, there are a vast number of uses and applications that you need a pH meter for. Acidity effects most liquid solutoins and often needs to be controlled within strict limits.
Some of the more common uses of pH meters are:
– Food and drink manufacturing and production
pH meters are used extensively in the food and drink industry. Everything from food safety and hygiene to the actual taste and flavors of the food and dring are heavily reliant on accurate pH monitoring. Examples are food canning, dairy, yogurt, brewing, wine making and homebrew.
– Home and Garden
If you are a keen gardener you will probably already know the benefits of using a pH meter to keep your garden in prime condition. Especially important for monitoring soil pH for lawns, flower gardens, vegetable patches and orchards. You need to know that you have optimal soil for what you are growing – different plants thrive and achieve maximum health and productivity in different soils.
When to check soil pH – Ongoing checks will ensure that you keep your soil at its best but it in most important to check before planting a new garden, or designing and making vegetable plots. Also prior to planting fruit and on occasions where you are disappointed in growth and output or if you experience discoloring (yellowing) of foliage.
– Hydroponics and Home Gardening
Most hydroponic crops grow within the pH range of 5.5 and 6.5 and it is important to maintain these levels. The pH affects the plant growth by helping the soil to absorb the vital chemical elements nitrogen, phosphorus and potassium required for the plant to flourish. Each plant has a different optimal level of pH for best results and healthy growth. Some examples of common hydroponic fruit and vegetable crops and their pH are:
– Swimming pools
To prevent burning red-eye sensations when swimming it is important to maintain the pH in your swimming pool between 7.0 and 7.6, that is between neutral and very slightly alkaline. Human eyes have a pH of between 7.2 – 7.4 so the pH of the pool should be kept in the same range. To maintain an effective and accurate pH in the swimming pool, you need to make sure the volume of water in the pool is kept at a consistent level – more volume dilutes the pH and less will concentrate it and make the value higher.
The important point for fish owners to understand is that different fish survive and thrive at different levels of pH. The range of different pH levels can be quite broad so you must take great care to make sure that you match your fish properly and know what level of pH is correct for each fish you are going to keep.
As a general guide, freshwater fish live in water with a range of 5.5 – 7.5 (acid) while saltwater fish live in a pH of 8.0 or higher (alkaline). The actual value of pH that you need to maintain varies according to individual species of fish but it means that for normal aquarium fresh water fish your pH should be in the acidic range.
– Home Medicine
Rudimentary checks on blood and urine can be done by testing the pH but we will not be covering this in depth, for advice on this please refer to a qualified medical practicioner.
Acids and Bases
Acids and bases are typically classified as strong or weak, a strong acid or base is one whose ions fully break down in water but a weak acid or base doesn’t completely dissociate. When an acid reacts with a base it causes what is called a neutralization reaction, and the two substances combine to produce salt and water. Acids and bases taste different with an acid being sour and having a drying effect and a base having a soapy taste.
They are both used by both humans and animals in their bodies, for example in digestion acid is used to break down food while bicarbonate neutralizes the effect of stomach acid.
pH of Common Liquids
All water based solutions have a pH value somewhere between 0 and 14 depending on whether they are classified as acid, neutral or base. On the pH scale acids have a value of less than 7 while alkalis (bases) have a value of more than 7. The actual scientific definition of pH is a bit technical but if you want to read about it here is a good explanation definition of pH. A lot of common substances are classified as either acidic or basic, some examples are:
Acids: vinegar (acetic acid), lemon juice, citric acid (as found in citrous fruit), vitamin C (ascorbic acid), carbonic acid, sulfuric acid, hydrochloric acid, nitric acid
Bases: bleach, soap, ammonia, sodium hydroxide, detergents, lye
Other ways to Determine pH
Although using a pH meter is by far the best and most accurate method of determining the pH value, there are other methods that can be used for a quick indication. The first and most common way is to use the Litmus test. This works by simply immersing a strip of litmus paper in the solution and reading off the pH. Litmus paper is a paper that has been treated by immersing in a lichen that changes colour to red to indicate the liquid it is dipped in is acidic and blue to indicate an alkali. It is a traditional method of determining the value of pH and is reasonably accurate if you don’t need a precise value.
– DIY ph meter
A home-made testing kit can be easily made by using red cabbage juice. The juice is perfect for this application as it is chemically neutral with a pH of 7 and has a distinctive red colour that changes depending on what is mixed with it. To make the red cabbage juice chop some cabbage into a glass bowl and cover with boiling water and leave to stand for 10 – 15 minutes until the juice comes out of the cabbage. When you separate the solid cabbage from the juice you now have a basic pH indicator that will react and change colour depending on what is mixed with it.
– Cabbage Meter scale
The colour of your cabbage water solution will change to give an approximate indication of pH value as per the table below.
Obviously this will only be a rough guide to pH value as the scale only goes up in increments of 2 but it can be used for educational purposes to give school pupils and students an insight into the pH values of liquids and for basic imprecise applications like gardening.
pH meter selection guide – How to decide which pH meter you need
What is the best pH meter type for your needs? The answer to this question lies in various determining factors, but as with most things in life it will primarily come down to cost. If you only have $50 to spend you will not be looking at a meter in the $500 bracket! So determine your budget then find out what suits your needs best in your price range. Next up is the accuracy you require for your application, there is no point spending over the odds on a high end sophisticated piece of equipment if you only need your meter for a basic task. Other factors you will want to consider is how rugged and robust you need your meter to be, reliability, ease of use and how much maintenance is required.
Comparisons with Alternatives
ph meter vs paper indicator strips
- – The meter gives an accurate reading of the actual ph value whilst the indicator strip is an approximation.
- – The indicator strip probably wins this one. If you lose your power supply for the meter and do not have a charger or replacement battery then the pH meter won’t work, the indicator strip will always work however.
- – pH meters are generally of fairly robust construction and if you look after them properly they will last you well, you will however need to replace electrodes from time to time. Indicator strips can deteriorate over time and with them being so flimsy are easy to lose so this is a hard one to judge – we’ll call it a tie.
- – Another win for the indicator strip which is cheaper than even the most inexpensive pH meter.
- – You can’t get much more portable than a strip of paper so indicator strips can be taken anywhere.
- – There are pH meters available for any type of use and circumstance whilst indicator strips only do one job.
Ease of use
- – Both are very easy to use, all you do is dip it into the solution and take a reading so we will declare this one a tie.
Cleaning and Maintenance
- – Hands down it’s the pH paper indicator strips that win this one, just use once and throw it away. The meter however requires cleaning after every use and replacement of consumable parts.
- pH Meter vs pH Test Kit
Accuracy – The test kit works by using a solution which changes colour when some of the solution to be tested is added. It is a rough guide to pH and the meter’s accurate digital reading is obviously massively superior.
Reliability – Most modern meters are reliable in use, however with anything electronic they require a power supply which can go wrong and also they have a shelf life. The test kit is dependant on you being able to source the components to make it and is more suited to use confined to the home.
Durability – Modern plastics make pH meters strong but the probe is fragile and will generally need replaced. As the kit is made new each time there isn’t really a question of durability so this is a difficult one to assess.
Price – Although you can get some fairly cheap meters, there is only one winner in this category.
Portability – Again with the test kit being for home use only due to its cumbersome nature, the ph meter takes this one.
Versatility – pH test kits are not versatile or adaptable in any way – another win for the meter.
Ease of use – The meter is simplicity itself to use but the kit takes some sourcing and setting up so the meter wins this one.
Cleaning and Maintenance – The meter needs to be rinsed and cleaned regularly but the kit can be poured down the drain after use so there is no maintenance whatsoever other than putting a few items in the dishwasher!
These are our top five recommended pH meters