The pH of a solution is a measure of the molar concentration of hydrogen ions in the solution and as such is a measure of the acidity or basicity of the solution. The letters pH stand for "power of hydrogen" and the numerical value is defined as the negative base 10 logarithm of the molar concentration of hydrogen ions.

The measurement of the pH of a sample can be done by measuring the cell potential of that sample in reference to a standard hydrogen electrode, as in the accepted procedure for measuring standard electrode potentials.

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For this expression, a base change from the natural log to the base 10 logarithm was made in the Nernst equation. In practice, the pH is not usually measured in this way because it requires hydrogen gas at standard pressure, and the platinum electrode used in the standard hydrogen electrode is easily fouled by the presence of other substances in the solution Ebbing.

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Laboratory pH meters are often made with a glass electrode consisting of a silver wire coated with silver chloride immersed in dilute hydrochloric acid.

The electrode solution is separated from the solution to be measured by a thin glass membrane. The potential which develops across that glass membrane can be shown to be proportional to the hydrogen ion concentrations on the two surfaces. In the measurement instrument, a cell is made with the other electrode commonly being a mercury-mercury chloride electrode. The cell potential is then linearly proportional to the pH and the meter can then be calibrated to read directly in pH.

The usual range of pH values encountered is between 0 and 14, with 0 being the value for concentrated hydrochloric acid 1 M HCl7 the value for pure water neutral pHand 14 being the value for concentrated sodium hydroxide 1 M NaOH. It is possible to get a pH of -1 with 10 M HCl, but that is about a practical limit of acidity. An important example of pH is that of the blood.

If the pH of the blood gets outside the range 7. If you measure the pH of tap water with a pH meter, you may be surprised at how far from a pH of 7 it is because of dissolved substances in the water. Distilled water is necessary to get a pH near 7.

Meters for pH measurement can give precise numerical values, but approximate values can be obtained with various indicators. Red and blue litmus paper has been one of the common indicators. Red litmus paper turns blue at a basic pH of about 8, and blue litmus paper turns red at an acid pH of about 5.

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The measured value for the color change can vary over a range of approximately 4. Neither changes color if the pH is nearly neutral. Litmus is an organic compound derived from lichens. Phenolpthalein is also a common indicator, being colorless in solution at pH below 8 and turning pink for pH above 8. Methods have been developed for the measurement of the concentrations of various kinds of ions using the cell potentials of special electrochemical cells.

If you form a voltaic cell with two different kinds of metal electrodes, you get a characteristic cell potential for standard conditions where the electrolytes are at the standard 1 Molar concentration. If the ion concentrations are different from 1 M, then there is a change in the cell potential described by the Nernst equation. Measurement of that change is in essence a measure of the ion concentration, and can be exploited for that purpose.

Ebbing cites an example of a specialized electrode designed for measuring the concentration of urea, an nonelectrolyte. Examples of pH Values The pH of a solution is a measure of the molar concentration of hydrogen ions in the solution and as such is a measure of the acidity or basicity of the solution. Numerical examples from Shipman, Wilson and Todd. Measurement of pH values. Ch Measurement of Ion Concentration Methods have been developed for the measurement of the concentrations of various kinds of ions using the cell potentials of special electrochemical cells.In water H 2 Oa small number of the molecules dissociate split up.

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In pure water, there are an equal number of hydrogen ions and hydroxide ions. The solution is neither acidic or basic. An acid is a substance that donates hydrogen ions. Because of this, when an acid is dissolved in water, the balance between hydrogen ions and hydroxide ions is shifted. Now there are more hydrogen ions than hydroxide ions in the solution. This kind of solution is acidic. A base is a substance that accepts hydrogen ions.

When a base is dissolved in water, the balance between hydrogen ions and hydroxide ions shifts the opposite way. Because the base "soaks up" hydrogen ions, the result is a solution with more hydroxide ions than hydrogen ions. This kind of solution is alkaline. Acidity and alkalinity are measured with a logarithmic scale called pH. Here is why: a strongly acidic solution can have one hundred million million, or one hundred trillion , times more hydrogen ions than a strongly basic solution!

The flip side, of course, is that a strongly basic solution can have , times more hydroxide ions than a strongly acidic solution.

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Moreover, the hydrogen ion and hydroxide ion concentrations in everyday solutions can vary over that entire range. In order to deal with these large numbers more easily, scientists use a logarithmic scale, the pH scale.

Here's How to Calculate pH Values

Each one-unit change in the pH scale corresponds to a ten-fold change in hydrogen ion concentration. It's a lot easier to use a logarithmic scale instead of always having to write down all those zeros!

By the way, notice how one hundred million million is a one with fourteen zeros after it? It is not coincidence, it is logarithms! What the equation means is just what we said before: for each 1-unit change in pH, the hydrogen ion concentration changes ten-fold. Pure water has a neutral pH of 7. The pH of a liquid or solution is often an important piece of information in science.

Measuring pH can be done simply and quickly using pH test paperpH indicator sticksor a pH meter. To measure pH, a piece of pH test paper or an indicator stick is dipped into the liquid. Each color on the key represents a different pH.

An example of a used pH indicator stick and the corresponding color key is shown below in Figure 1. They consist of a probe that is dipped in a solution, and a digital readout. Table 2 below discusses what types of pH measuring devices are best for different science project applications, and offers a quick link to purchasing different pH test papers and indicator sticks.

Print Email. What does it mean for a solution to be acidic or basic alkaline? The pH Scale: Some Examples. Figure 1. For example, the fermenting of foods. Short range pH test paper 6. For example, a range suitable for aquatic organisms. Micro range pH test paper 5. We recommend checking if there is one available at your local high school chemistry laboratory before purchasing. Various models are available from Amazon. Suitable for more advanced research where the exact pH of a solution matters.PHquantitative measure of the acidity or basicity of aqueous or other liquid solutions.

A solution with a pH less than 7 is considered acidic ; a solution with a pH greater than 7 is considered basicor alkaline. The measurement was originally used by the Danish biochemist S. Because of uncertainty about the physical significance of the hydrogen ion concentration, the definition of the pH is an operational one; i. The U. National Bureau of Standards has defined pH values in terms of the electromotive force existing between certain standard electrodes in specified solutions.

The pH is usually measured with a pH meterwhich translates into pH readings the difference in electromotive force electrical potential or voltage between suitable electrodes placed in the solution to be tested. Fundamentally, a pH meter consists of a voltmeter attached to a pH-responsive electrode and a reference unvarying electrode. The pH-responsive electrode is usually glass, and the reference is usually a mercury-mercurous chloride calomel electrode, although a silver-silver chloride electrode is sometimes used.

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When the two electrodes are immersed in a solution, they act as a battery. The glass electrode develops an electric potential charge that is directly related to the hydrogen-ion activity in the solution, and the voltmeter measures the potential difference between the glass and reference electrodes. The meter may have either a digital or an analog scale and deflected needle readout.

Digital readouts have the advantage of exactness, while analog readouts give better indications of rates of change. Battery-powered portable pH meters are widely used for field tests of the pH of soils. Tests of pH may also be performed, less accurately, with litmus paper or by mixing indicator dyes in liquid suspensions and matching the resulting colours against a colour chart calibrated in pH. In agriculture, the pH is probably the most important single property of the moisture associated with a soilsince that indication reveals what crops will grow readily in the soil and what adjustments must be made to adapt it for growing any other crops.

Acidic soils are often considered infertile, and so they are for most conventional agricultural crops, although conifers and many members of the family Ericaceaesuch as blueberrieswill not thrive in alkaline soil. As soil acidity increases so does the solubility of aluminum and manganese in the soil, and many plants including agricultural crops will tolerate only slight quantities of those metals. Acid content of soil is heightened by the decomposition of organic material by microbial action, by fertilizer salts that hydrolyze or nitrify, by oxidation of sulfur compounds when salt marshes are drained for use as farmland, and by other causes.

Article Media. Info Print Cite. Submit Feedback. Thank you for your feedback. PH chemistry. See Article History. Alternative Titles: hydrogen ion concentration, potential of hydrogen.

Read More on This Topic. The relative acidity or alkalinity of a solution is reported by the pH scale, which is a measure of the concentration of hydrogen ions in…. Get exclusive access to content from our First Edition with your subscription. Subscribe today. Learn More in these related Britannica articles:.

The relative acidity or alkalinity of a solution is reported by the pH scale, which is a measure of the concentration of hydrogen ions in solution. Neutral solutions have a pH of 7. A pH of less than 7 denotes acidity an increased hydrogen….

Most bacteria grow in the range of neutral pH values between 5 and 8although some species have adapted to life at more acidic or alkaline extremes.

An example of an acidophilic bacterium is A. When coal seams are exposed to air through….The pH scale runs from 0 to 14—a value of seven is considered neutral, less than seven acidic, and greater than seven basic.

To calculate it, take the log of a given hydrogen ion concentration and reverse the sign. See more information about the pH formula below. Here's a more in-depth review of how to calculate pH and what pH means with respect to hydrogen ion concentration, acids, and bases.

There are several ways to define acids and bases, but pH specifically only refers to hydrogen ion concentration and is applied to aqueous water-based solutions. When water dissociates, it yields a hydrogen ion and a hydroxide. See this chemical equation below. When calculating pH, remember that [ ] refers to molarityM. Molarity is expressed in units of moles of solute per liter of solution. If you are given concentration in any other unit than moles mass percent, molality, etc.

In other words, pH is the negative log of the molar hydrogen ion concentration or the molar hydrogen ion concentration equals 10 to the power of the negative pH value.

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It's easy to do this calculation on any scientific calculator because more often than not, these have a "log" button. This is not the same as the "ln" button, which refers to the natural logarithm.

You can easily use a pH value to calculate pOH if you recall:. This is particularly useful if you're asked to find the pH of a base since you'll usually solve for pOH rather than pH. Here it helps to rewrite the concentration as 1.

Or, you could just use a calculator to take the log. This gives you:. Usually, you aren't given the hydrogen ion concentration in a problem but have to find it from a chemical reaction or acid concentration.

Most problems asking for pH are for strong acids because they completely dissociate into their ions in water. Weak acids, on the other hand, only partially dissociate, so at equilibrium, a solution contains both the weak acid and the ions into which it dissociates. Remember, Hydrochloric acid is a strong acid that dissociates according to a molar ratio into hydrogen cations and chloride anions. So, the concentration of hydrogen ions is exactly the same as the concentration of the acid solution.

When you're performing pH calculations, always make sure your answers make sense. An acid should have a pH much less than seven usually one to three and a base should have a high pH value usually around 11 to While it's theoretically possible to calculate a negative pHpH values should be between 0 and 14 in practice.

This means that a pH higher than 14 indicates an error either in setting up the calculation or the calculation itself. Share Flipboard Email. Anne Marie Helmenstine, Ph. Chemistry Expert. Helmenstine holds a Ph.


She has taught science courses at the high school, college, and graduate levels. Facebook Facebook Twitter Twitter. The relationship between pH and molarity can be expressed as:.

The equilibrium equation yields the following formula for pH:. Try these sample problems to test your knowledge of pH. Find the pH of a 0.To save this word, you'll need to log in. Accessed 19 Apr. Keep scrolling for more More Definitions for pH pH. Water has a pH of 7. Entry 1 of 2 1 pharmacopoeia 2 phosphor 3 phot pH. Please tell us where you read or heard it including the quote, if possible.

What does capricious mean? Test Your Knowledge - and learn some interesting things along the way. Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free! We have a hard decision to make. Or 'unessential'?

And who put it there, anyway? Literally How to use a word that literally drives some people nuts.


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PH abbreviation. First Known Use of pH Nounin the meaning defined above. Keep scrolling for more. Learn More about pH. Statistics for pH Look-up Popularity. More Definitions for pH.The scale of pH values is from 1. Since the pH is logarithmic, each change of one pH unit means a tenfold change in the number of hydrogen ions. The more acidic a solution the greater the hydrogen ion concentration and the lower the pH; a pH of 7. The pH is used as a measure of whether the body is maintaining a normal acid-base balance.

A favorable pH is essential to the functioning of enzymes and other biochemical systems. The body's fluids are normally somewhat alkaline, the pH being between 7. A pH above 7. See also acidosis and alkalosis. From Applegate, Compare: dissociation constant of water. Adverse physiologic effects of stored blood is negligible in the absence of a previous compromise of the Pt's—recipient's status.

In chemistry, a measure of the hydrogen ion concentration of a solution. The degree of acidity or alkalinity of a substance is expressed in a pH value. A solution that is neither acid nor alkaline has a pH of 7.

Increasing acidity is expressed as a number less than 7, and increasing alkalinity as a number greater than 7. Maximum acidity is pH 0 and maximum alkalinity is pH Because the pH scale is logarithmic, there is a fold difference between each unit. For example, pH 5 is 10 times as acid as pH 6 and pH 4 is times as acid as pH 6.

The pH of a solution may be determined electrically by a pH meter or colorimetrically by the use of indicators. A list of indicators and the pH range registered by each is given under the indicator. A measurement of the acidity or alkalinity of a solution based on the amount of hydrogen ions available.

Based on a scale of 14, a pH of 7. A pH below 7. Blood pH is slightly alkaline basic with a normal range of 7. Symbol for the logarithm to base 10 of the reciprocal of the hydrogen H ion concentration measured in gram molecular weight in an aqueous solution.Science Explorer.

Acids, Bases, & the pH Scale

Frequently Asked Questions. Multimedia Gallery. Park Passes. Technical Announcements. Employees in the News. Emergency Management. The range goes from 0 to 14, with 7 being neutral. The pH of water is a very important measurement concerning water quality.

No, you don't often hear your local news broadcaster say "Folks, today's pH value of Dryville Creek is 6. Maybe for a science project in school you took the pH of water samples in a chemistry class Geological Survey we take a pH measurement whenever water is studied. Not only does the pH of a stream affect organisms living in the water, a changing pH in a stream can be an indicator of increasing pollution or some other environmental factor.

By the way Thus, you can't have a pH of vegetable oil or alcohol. Water that has more free hydrogen ions is acidic, whereas water that has more free hydroxyl ions is basic. Since pH can be affected by chemicals in the water, pH is an important indicator of water that is changing chemically. Water with a pH of five is ten times more acidic than water having a pH of six.


The pH of water determines the solubility amount that can be dissolved in the water and biological availability amount that can be utilized by aquatic life of chemical constituents such as nutrients phosphorus, nitrogen, and carbon and heavy metals lead, copper, cadmium, etc.

For example, in addition to affecting how much and what form of phosphorus is most abundant in the water, pH also determines whether aquatic life can use it. In the case of heavy metals, the degree to which they are soluble determines their toxicity.

Metals tend to be more toxic at lower pH because they are more soluble. As this diagram shows, pH ranges from 0 to 14, with 7 being neutral. Normal rainfall has a pH of about 5. You can see that acid rain can be very acidic, and it can affect the environment in a negative way. The U. Geological Survey analyzes hundreds of thousands of water samples every year.

Many measurements are made right at the field site, and many more are made on water samples back at the lab. There are large and small models of pH meters.

Portable models are available to take out in the field and larger models, such as this one, are used in the lab. Inside the thin glass bulb at the end of the probe there are two electrodes that measure voltage.


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