The pH value on a pH meter’s display describes the character of an aqueous solution with respect to an acidic or basic alignment. The latter, either acidic or basic, is dependent on hydrogen ion activity. The pH is an important factor in our ecosystem. Here are some examples:
In the plant world, the pH of the soil has an impact on the intake of nutrients by the roots and therefore on the overall plant growth.
The human blood usually has a pH between 7.35 and 7.45. Should the pH of the blood drop, the hemoglobin will absorb less oxygen.
In the area of the underwater world some fish species need a rather acidic and the others – a rather alkaline environment.
In the field of chemistry, the pH value is an indicator of the strength of an acid or base.
Various methods or a measuring device / ph meter is used to determine the pH value.
PH-photometers also use the effect of the color change. One water sample should be taken for the measurement, and then measured by the pH meter in two steps. During the measurement step 1, a blank test is carried out (only the sample taken). Before the second measurement step, a reagent is added to the sample which causes a color change of the sample. Here the color change is measured with an optical measuring system and converted directly into the pH value. The maintenance expenses for a pH meter based on photometric procedure are relatively low, but the reagents are required for each measurement.
A fast and simple measurement is carried out with a pH meter, which has got an electrode. These measuring devices are available as a table or hand-held measuring device, as well as a stationary measuring unit. Measurement with one electrode is the most commonly used measuring method for the pH value determination.
For applications in laboratories, tabletop devices with glass electrodes are generally used. These very thin-walled glass electrodes require very careful handling. For this reason, in laboratory operation, glass electrodes are used with an electrode holder, which holds the electrodes and is balanced by the springs. A tabletop pH meter has, in most cases, an internal memory or an interface via which the recorded or currently measured data can be transferred to the PC. As for power supply, in addition to the pH meter devices with normal mains operation, there are some pH meters models available that also provide operation via a battery or a battery pack. This kind of pH meter is very flexible and can also be used for on-site measurements.
Stationary pH measuring systems are used where the pH value must be measured or adjusted constantly. This pH meter is mostly used as transmitters and send the measured values as analogue signals further. The analog signals, e.g. 4 ... 20 mA or 0 ... 10 V, are processed in electronic controls and possibly passed on to displays, dosing devices or pumps and valves.
The largest proportion of the pH measuring devices on the market are the mobile handheld instruments.
A mobile pH meter is offered in many different versions and with different equipment. The pH measuring devices, which form a solid unit with the electrode, are generally referred to as pH testers. These pH testers are the most cost-effective devices and are used in applications where a few measurements are required. Since all the pH electrodes are subject to drift and therefore also to wear, it should be considered when purchasing a pH meter that the electrode can be replaced. Mobile pH measuring instruments with a replaceable/ removeable electrode can be used in almost all areas, due to the possibility to rapidly change the electrode to a special one. A mobile pH meter often has measured value memories, which can record the current value or if the logger function is available, the whole measurement series. The measured value memories are designed as an internal memory or as a SD card recorder. From these pH-meters, the values can always be transferred to a PC and processed there. For the application in humid environments, the pH tester and handheld device should be equipped with an appropriate IP protection.
The selection of the pH electrode should always be based on the application. Thus, in the most common cases, for the mobile pH meter, the electrodes with a plastic housing and a gel filling is selected. These electrodes are mechanically relatively stable and are not directly destroyed when the container walls and edges are hit. For the area of foodstuffs, the electrodes with a conical tip or electrodes embedded in a stainless-steel blade are available. To avoid shortening the lifetime of the electrodes, they should always be cleaned after use. Depending on the application, special cleaning solutions are required. In case of applications in clean waters or comparable media, cleaning with tap water and rinsing with distilled water is sufficient. The storage of the pH electrodes should always be carried out in a storage solution for pH electrodes. In order to compensate for the above-mentioned drift of the pH electrodes, a calibration of the pH meter with the electrode is required from time to time. For this purpose, for example, calibration solutions with pH values 4, 7 or 10 can be used. If pH measurements are carried out in media at the temperatures above 25 ° C, pH meters with automatic temperature compensation (ATC) are recommended. For the pH measuring instruments with ATC, the measured value shift caused by the temperature is voided / removed.
DIN Guideline for pH value measurement in hard soil
The soil pH is a distinctive mark of its acid or base content. The pH of a soil, as in all the other aqueous systems, is affected by natural buffer systems. The determination takes place either in a suspension (extraction procedure) of soil in a neutral salt solution (0.01 N calcium chloride or potassium chloride) - lab measurement method. Here, since the absorbed H + ions on the ground exchanger are replaced by the CaCl2-ions, the pH measured in such a way is about 0.3-1.0 units below the pH, which is measured in purely aqueous suspensions. However, not all the data in the literature are corrected accordingly.
Practical example: pH measurement in soil with a soil extraction solution - Laboratory measurement method according to DIN 19684 Part 1
Range: between pH 2 to pH 9
pH electrodes: pH electrode with spherical membrane, refillable or gel-filled, fabric or ceramic diaphragm
Required equipment: 1 electrode stand, 1 large-volume beaker (> 500 ml), 1 magnetic or glass rod for stirring
Carrying out a pH measurement - experimental method:
Make the soil mix sample (air-dry clay) taken from various representative sampling points. Remove foreign bodies (inter alia stones, roots). Slightly loosen the soil with hands.
Suspension with distilled water as the solvent
This method detects the normal acidity of the soil. The existing bound acid ions present in the soil matrix are not detected. Put 260 ml of distilled water to 100 g of a soil mix sample and stir / shake this suspension for 5 minutes. After this, the sample is left still for minimum 2 hours, but not longer than 24 hours.
Suspension with 0.01 M CaCl2 solution
Take 75 ml of soil and mix it with 300 ml of 0.01 M CaCl2 solution (0.01 M = 1.11 g of anhydrous calcium chloride or 1.47 g CaCl2 * 2 H2O for 1 liter distilled water), stir very intensely and let it stand without stirring for at least 60 minutes. Stir the sample shortly before you use the pH meter the measurement and immerse the pH electrode and temperature into the fluid to be measured.
The suspension with 0.01 M CaCl2 solution the easily replaceable hydrogen ions are also incidentally detected, as these are exchanged from the bound sample matrix by the cations. This determination method is very close to the longer-term process in the soil. That is why, the pH of this suspension is 0.5 to 1.0 pH units lower than the pH of the aqueous soil suspensions.
Take a record of the measurement value with 0.01 pH accuracy when the measured value is stable.
Measured value indication in suspension:
- With distilled water: pH (distilled water) x.xx (y °C) pH (distilled water) 7.22 (23 °C)
- With CaCl2: pH (CaCl2) x.xx (y ° C) pH (CaCl2) 7.22 (23 °C)
Please note that during the use of a pH meter any pH measurement the temperature constancy between the liquid under measurement and the electrode can take place. Finally, after the measurement clean the pH electrode of contaminants by rinsing with distilled water. pH sensor should not be affected mechanically. Then keep the electrode in (STORAGE-Solution).
pH Measurement in solvent besides than water
Application of a pH meter in Non-Aquaeous Solvents
We recommend to consider the following when using a pH tester or test kit:
- The results will be most accurate if you keep the sensor of the main electrode immersed in the liquid to be tested for 24 hours prior to the pH measurement.
- It is very important to use the following three electrodes: the main (glass) electrode of your pH meter, a reference electrode and an electrode for temperature compensation.
- Allow 5 minutes for stabilisation of the value before you read it out from the display of your pH tester.
- Change the unit of measurement of your pH meter if the pH value is below pH 0 or above pH 14 and repeat the measurement.
- When you have taken your measurement, immerse the sensor of the pH meter into hydro-chloric acid (0.1 mol/L HCl) for approx. 10 minutes before washing the sensor as well as the external cylinder of the reference electrode carefully with pure water.
One of the most important facts to know about pH meters is that pH values of non-aquaeous solutions, in contrast to those of aquaeous solutions, are more or less only an indication of the hydrogen ion activity in the sample.
The potassium chloride (KCl) that leaks from the liquid junction of the pH tester’s reference electrode containing the KCl solution does not normally dissolve in most non-aqueous solutions, which means that the liquid junction will be blocked. Therefore, lithium acetate (CH3COOLi) is used in some electrodes of a pH tester as this will dissolve in organic solutions.
pH Meter with ISFET Electrode
There is a wide range of the pH meter models in the market, starting from portable small inexpensive testers to complicated and pricy laboratory versions. The most well-known design of the pH meter is a device with the glass electrode. Not depending on the model, it is important to note that this kind of pH meter requires very careful handling and regular calibration. Glass is very fragile and once the electrode is broken or cracked there is no chance for getting accurate pH value measurement. The electrodes should be cleaned regularly, kept in a special solution under favorable ambient conditions (no high temperatures and humidity!), free from contaminants and fat (that is why it is recommended never to touch the device with the hands).
One of the options to the glass electrode, which as mentioned above, really needs an operator to be very concentrated, another type of electrode – based on ISFET technology – has been created. The main advantage of a pH meter equipped with an ISFET (on-sensitive field-effect transistor) electrode is that it is robust enough and has a short response time, at least not more that the glass electrode and can carry out measurements under low temperatures without being damaged. The ISFET pH meter finds its application especially in industrial field, where the operating conditions are quite rough sometimes and a certain resistance of the meters is absolutely advantageous. As against to the majority of pH meters, there is no glass element in the construction. Such a pH meter is widely applied in food / beverages and pharmaceutical industry, various branches where high hygienic requirement should be met. It ensures the safety of the products under testing, not only due to its resistance to contamination and compliance with all the hygienic norms, but also the risk of shards of glass getting into the material in case of the breakage of the glass electrode is brought to “0”. A pH meter with an ISFET electrode is resistant to shocks, provides very good, stable and accurate measurement, even under low temperatures, the pH meter is less sensitive to storage conditions and its electrode being out of solution does not influence the operation. The ISFET electrode can be sterilized, calibration intervals are longer, as well as the service life time.
The desirable pH range for optimal plant growth varies according to the type of crop. Some crops grow better with a pH in the range of 6.0 to 7.0, while other crops – in slightly acidic conditions. The measurement of the pH serves to know the acidity of the soil, i.e. it quantifies the concentration of the hydrogen ions. The more hydrogen ions there are, the more acidic the soil is. The pH of the soil is measured on a logarithmic scale, in such a way that when a solution becomes one hundred times more acidic, the pH will decrease by two units. When a solution becomes a thousand times more acidic, the pH will decrease by three units.
The measurement of the pH value is of great importance for the agriculture. For the vast majority of crops, the soil pH values below 5.5 influence the yield significantly. If the value drops further, the consequences will be even worse. However, some crops are particularly acidophilic, and provide good yields even at pH values close to 5.
Different elements that soil consists of must be taken into account. One element that stands out is aluminum. When the soil has a pH value below 5.0, toxic amounts of aluminum and manganese can be released into the soil solution. The more acidic the soil is, the more soluble the aluminum is, which can have a very harmful effect. Soluble aluminum is toxic to the roots of many plants and therefore limits their access to soil, water and nutrients.
In the soil with low pH, microbial activity decreases and nutrients such as phosphorus, magnesium, calcium and molybdenum become less available. The agricultural production can decrease considerably and fertilizers become less effective. The more acidic the soil becomes the less agricultural plants will grow. There are different types of plantations that are very sensitive to soil acidity, such as rapeseed, alfalfa, and beans. Acidification of the underground soil (10-20 cm), the layer below the normal depth of cultivation, is a growing problem.
The facts mentioned above make clear the importance of raising the pH level to a nearly neutral. This can be achieved by applying agricultural additives to the soil, e.g. by means of calcium oxide, calcium hydroxide or agricultural lime. This process is also known as soil whitewashing. Depending on the type of soil, a higher or lower dose of limestone is applied.
The Importance of Soil pH in Agriculture
Soil pH is undoubtedly one of the most important factors influencing the yield of any crop, along with other more obvious and well-known factors, such as water availability or plant nutrient content. It is not in vain that the pH value of a soil can cause certain nutrients and trace elements that are essential for a given crop to be in a form that can be assimilated by the plant, or vice versa: an unsuitable or inadequately controlled pH of the soil can cause nutrients that were originally available in the soil, or that the farmer has subsequently added through fertilization, to be in the form of a chemical compound that cannot be absorbed by the plant, and consequently the plant will not achieve its optimal development. For this reason, and because of the inefficient use of expensive fertilizers, the farmer's profitability can be seriously compromised if he does not monitor the pH level of the soil with an appropriate pH meter. The latter will allow him to make the appropriate corrective decisions in each case. There are several known agronomic techniques that allow correcting the pH of the soil and adapting it to the optimal conditions of the crop. It must be taken into account that each crop has adequate intervals of the soil pH in which it is appropriate to work for an optimum performance. Although as a general rule most crops perform better in the soil in which the pH level is close to neutral or slightly acid, there are crops adapted and especially suitable for more acidic or more basic soils, away from neutral pH.
Without a doubt, one of the key factors providing a healthy and successful growth of the plants in a garden (without any nutritional deficiencies or phytosanitary problems) is the optimal for the plants, or at least acceptable pH value of the soil. However, when problems begin to appear, such as loss of the leaf color, or weak growth, one of the first factors to check is the pH level of the soil. This is why it is so important to pay a lot of attention to the pH of the garden soil. Unless it goes about extreme values, just an inadequate pH level alone is generally not the only reason that negatively influences the life of the plants, but it can affect their growth, depending on the degree of sensitivity of the plant. In fact, many plants are capable of adapting to a relatively wide range of pH levels. Hydrangeas, for example, produce flowers of different colors depending on whether they are grown in acid or alkaline soil.
It is very useful for the gardener to know whether the soil in his garden is alkaline or acidic, because certain nutrients can only be absorbed by the plants when the soil pH is in an acceptable range. In fact, no matter how much fertilizer can be added to the garden soil, it will not improve the health of the plants until the pH value has been adjusted. Most plants prefer a neutral pH, between 6.2 and 7.0. However, there are many plants that have more specific pH needs, such as blueberries and azaleas, which like a more acidic soil, and lilacs and clematis, which prefer a more alkaline soil. There are even some, such as hydrangeas, which change the color of the flower depending on the pH of the soil, as mentioned above.
Measuring the soil pH value is important because it influences several soil factors that affect plant growth in a garden, such as soil bacteria, nutrient leaching, nutrient availability, toxic elements and soil structure. Bacterial activity, that releases nitrogen from organic matter and certain fertilizers, is particularly affected by the pH of the garden soil, as bacteria perform best in the pH range of 5.5 to 7.0.
When you measure the pH of your garden soil and notice that it is beyond the desired range, you will need to take some steps to adjust it. In this way you will be able to adapt it to the particular needs of the plants in your garden. The soil that is too acidic is counteracted by applying e.g. limestone or finely ground wood ash, and alkaline soil can be treated with additives such as ground sulphur, ammonium sulphate or other acidifying fertilizers.
ph Value in Drinking Water and Waste Water
The pH value, a parameter defined by the concentration of hydrogen ions in an aqueous solution, has a determining influence on any chemical reaction taking place in that solution, to the extent that, depending on its value, it allows or does not allow a certain chemical reaction to take place. From this it can easily be deduced that both, the control and the handling of the pH value will be crucial, both in the control of the drinking water for human consumption and in the treatment of the waste water itself.
During the treatment of wastewater, the problem is essentially reduced to the elimination of all types of organic compounds and inorganic toxic elements (heavy metals, etc.) that may be present in the water as a result of urban use or industrial discharges.
In general, depending on the origin of the wastewater, the wastewater treatment process usually starts with purely physical processes to separate the larger elements (waste, organic or inorganic elements of the appropriate size, etc.). In these preparatory physical processes, such as sieving, filtering or decanting of the most easily separable fractions, the pH value does not play such a decisive role. However, from that moment on and in the following phases, the pH value (together with other basic parameters such as the presence of oxygen and temperature) will be decisive for the completion of the waste water treatment process.
The simple fact that the pH of the water is too low or, on the contrary, too high is in itself a decisive factor in ensuring that the water is not suitable for being discharged into the environment (lakes, rivers, etc.), let alone for human consumption. For this reason alone, it is necessary to control the pH of the water and to correct it if necessary. But its effects go much further in the purification processes. For example, in the removal of heavy metals and other toxic inorganic elements that could not be removed by the initial screening with physical means. The pH value or the concentration of hydrogen ions or hydroxyl groups in the aqueous medium, depending on whether it is an acidic or alkaline medium, has a more than direct influence, for example, that a certain metal compound remains dissolved in the water, or on the contrary, that it reacts with the hydroxyl groups to form more or less insoluble salts and can thus finally be easily separated from the other components of the water by applying one of the physical methods originally described.
Each compound has certain pH values at which we can find it in one chemical form or another, so that the wise management and control of the pH value of the water by means of the suitable measuring instruments will mark the success or failure of the purification of certain elements.
Finally, the decisive influence of the pH value on the elimination of all types of pathogenic microorganisms is well known. These microorganisms, such as the enterobacterium Escherichia coli, can be present in the wastewater, e.g. through the discharge of the sewage or waste. A treatment that consists of bringing the pH below a certain level (depending on the pathogens we want to focus on) can be enormously effective in achieving our goals. It is obvious that after this treatment, it is essential to return the pH to the values that are closer to neutral and compatible with the rest of the living organisms before releasing it into the environment.
Especially when chlorinating water to make it suitable for the human consumption, to make it drinkable, the pH value is again crucial to whether the process is really effective or not. For reasons similar to those already mentioned for other inorganic compounds present in water, the pH-value will also be decisive in the case of chlorine for the formation of one or another type of compound and will make the difference as to whether chlorine actually develops its disinfecting effect or not. Besides other factors that can have an influence, such as water hardness, pH values below 6.5 - 6 or above 8 prevent the effectiveness of the process.
As is generally known, the pH is nothing more than a measure of the activity of hydrogen ions in a particular solution, and its value ultimately determines how acidic or alkaline the medium in which it is measured is. It is only when this first definition is taken into account that the first consequence of this parameter on the organoleptic quality of both food and fruit and vegetables is obvious: the acidity will ultimately be a determining factor in defining the taste of any food.
It is known that the acidity directly determines the taste of many raw materials, such as certain fruits. In fact, most foods, especially fruit, are acidic, i.e. with pH values below 7, which indicates neutrality. Only few foods are alkaline, i.e. with a pH value above 7.
But the pH not only makes the taste more or less acidic in the primary sense of the word, in many raw materials, but especially in other processed foods, which for example, in their production with fermentation processes (we would speak in this case of products such as wine or cheese), we find that the pH (almost always together with the temperature factor, which is usually closely related), this in turn will have a direct influence on such crucial parameters of organoleptic quality and taste of a food, such as e.g. the consistency of the food or the flavours of one kind or another that result from these fermentations. The control of the pH value and therefore of the type of the microbial fauna that the fermentation process involves can clearly make the difference between a product that is clearly recognizable and appreciated by the customer and for which he is prepared to pay a high price, and a product that is ready for direct disposal.
In addition to the mentioned above, there is one last factor where pH plays a crucial role in the agricultural and food sector, and that is food preservation. For example, it has been known for a long time that the use of vinegar is one of the most effective methods of preserving certain foods. This is because the high acidity preserves the food. As has been indicated earlier, almost all micro-organisms have an important sensitivity to the pH-value, and this includes in particular those micro-organisms (bacteria, fungi, etc.) that can be harmful to the food product and thus, spoil it for its marketing and consumption. A sufficiently low pH can, if not completely inhibit the attack of harmful micro-organisms, at least minimize or delay this negative impact by extending the shelf life of the food, which can be done by the correct use of the other important parameters such as temperature (sterilization or freezing processes).
As a conclusion, it is clear that in the food and agricultural sector, and especially during the production process of more elaborate foods, it is essential to control the pH, either in a specific way at some critical points of the process using portable pH-meters or even with pH-meters with electrodes for continuous measurement, which allow a much more intensive control of the production if the process requires it.
The electrodes are generally designed for the measurement by direct immersion in an aqueous solution. However, for the measurement in certain solid foods, such as fruit, cheese or meat, pH-meters with special electrodes (with blades) are already available on the market, with which they can be inserted directly into the food itself and measure its pH value.
pH value Measurement in Wastewater
An essential contribution to life in water is that it has the right pH value. In fact, for many aquatic organisms life is only possible if the pH value is in the range of 5-9, for some organisms even in the narrower range of 6-8. This applies to both fauna and flora as well as aquatic plants. If the pH is too high or too low, wastewater can have very harmful effects on the environment into which it is discharged. In addition, several organisms are very sensitive to changes in the pH-value. Wastewater must therefore be properly treated before it enters the rivers or lakes.
It should not be forgotten that certain bacteria and other organisms play an active role in wastewater treatment. In fact, it is very important for wastewater treatment plants to constantly monitor the pH value of the wastewater. The purpose of a wastewater treatment plant is to purify the wastewater. This is done through mechanical, biological and chemical treatments. The process of water purification requires all three types of treatment to be applied in sequence. However, only a pH value close to neutral or pH 7-value allows the microorganisms to act correctly and efficiently eliminate the nitrogen compounds and other impurities during the microbiological purification phase.
Measuring the pH value of the wastewater is also important for another reason. A too high or too low pH value can damage the pipes and technical equipment. Controlling the pH value also protects cement and metal from corrosion. For the industrial sector, it is essential to monitor the pH value of the wastewater. This prevents from having to deal with expensive repairs over some time.
There are many factors that influence the pH value of wastewater. In the private sector, the use of the cleaning products such as detergents, fabric softeners and other products must of course be considered. Many of these cleaning products are usually alkaline, with a pH value between 8 and 10, although there are also more acidic cleaning products. Depending on the type of dirtiness, one or the other type of product is used.
On the other hand there is the industrial sector with its industrial discharges. Many industrial processes require the use of millions of liters of water per year. For example, bleach plants or fabric bleaching factories mainly produce rather acidic waste water. Metallurgy also discharges acidic water. All this shows that wastewater rarely has a neutral pH value. For this reason it is necessary to neutralize the waste water. The use of a pH meter, whether it is a fixed or portable installation, makes it possible to determine the most suitable water treatment.