Biological Sciences
pH Test
pH test is a method used to measure the acidity or alkalinity of a substance. It is based on the pH scale, which ranges from 0 to 14, with 7 being neutral. In biological sciences, pH tests are commonly used to assess the acidity or alkalinity of biological samples, such as blood, urine, or soil, which can have significant implications for various biological processes.
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4 Key excerpts on "pH Test"
eBook - ePub- M. Bazin(Author)
- 2018(Publication Date)
- CRC Press(Publisher)
l . In the strictest sense, activity of hydrogen ions should be used in the definition, however, for practical purposes, molar concentrations approximate activities. The pH scale ranges from 0 to 14 with pH 7 representing neutrality. Those pH values which are less than 7 are acidic, and those greater than 7 are alkaline.A. Acid-Base Equilibria
According to commonly used Bronsted-Lowry theory, an acid is defined as a proton donor and a base as a proton acceptor. As a general rule, any species (for example, HA) that can donate a proton, on dissociation, gives a species (A- ) that can accept a proton to form HA, as shown in the following equilibrium reaction:H AIn the Bronsted-Lowry system, the above reaction is considered to involve a proton transfer from HA to a molecule of solvent:H ++A - (2)H A +H 2O ⇌++H 3OA −( 3 )Every Bronsted-Lowry acid is paired with a Bronsted-Lowry base and the two are said to comprise a conjugate acid-base pair. For example, HC1 is the conjugate acid of C1- ; C1is the conjugate base of HC1.An acid such as HC1 dissociates completely in solution into H+ and C1- ions, and hence is considered a strong acid. On the other hand, a weak acid such as acetic acid dissociates only partly, being in equilibrium with its representative ions. For example, the undissociated acetic acid is in equilibrium with acetate ions in a solution as described by the following reaction:Similarly, strong bases and weak bases can be defined based on the degree of their ionization in a solution.C O O H ⇌C H3C OC H3O −+H +( 4 )B. Buffers
For wastewater biological treatment processes, it is important that the pH not deviate significantly from an optimum value that is suited. A nearly constant pH may be maintained due to what is called the buffering action of acid-base equilibria and can be achieved by the addition of buffering compounds, if needed, or as a result of natural chemical reactions such as those involving carbonates and bicarbonates (discussed in a later section). A buffer contains both an acid and a base, and can respond to the addition of either an acid or alkali, with a minimal change in pH. As an example, a solution containing acetic acid molecules and acetate ions in equilibrium (as represented in Equation 4
- Ian Peate, Muralitharan Nair(Authors)
- 2011(Publication Date)
- Wiley-Blackwell(Publisher)
it is slightly alkaline. However, a blood pH lower than 7.35 is considered to be acidic, whereas a pH greater than 7.45 is alkaline, and when either of these events occurs, it can have a serious effect on the body, as is discussed in other chapters within this book. The reason for this is that because the scale is a logarithm, then just a small change in pH indicates a very significant alteration in H + concentration. The reason for this is that every change in one pH unit represents a tenfold change in H + ion concentration. Thus an alteration of pH from pH 7.4 to pH 7.3 results in a doubling of the H + ion concentration. In other words: pH 8 contains 10 -8 mol/Lor 10nmol (nanomoles) pH 7 contains 10 -7 mol/L or 100 nmol pH 6 contains 10 -6 mol/L or 1000 nmol and so on. Homeostasis Homeostasis is the body’s attempt to maintain a stable internal environment by achieving some sort of balance. The body is normally able to achieve a relatively stable internal environment even though the external environment is constantly changing – from cold to hot, or from dry to wet, etc. The body uses various homeostatic mechanisms to monitor and maintain a dynamic state of equilibrium within the body – i.e. a balance in which the internal environment conditions can react to external environmental conditions by changing within quite narrow limits. The homeostatic mechanisms include: Receptors – these receptors sense external and internal environmental changes and provide information on the changes to the control centre Control centre – the control centre determines what a particular value (e.g
eBook - ePub- Tony Farine, Mark A. Foss(Authors)
- 2013(Publication Date)
- Routledge(Publisher)
When using the pH scale, a number of points must be considered. First, since it is a logarithmic scale, every change of one unit in pH represents a tenfold change in hydrogen ion concentration, a change of two units in pH a 100-fold change in hydrogen ion concentration, and so on. For this reason, the normal range of blood pH (7.35–7.45) is not as narrow as it first appears, and apparently small changes in blood pH represent large changes in hydrogen ion concentration, which you may need to report. Second, the pH scale is a negative scale – that is, a falling pH represents a rise in hydrogen ion concentration and a rising pH represents a falling hydrogen ion concentration.Pure water has a pH of 7 and an identical concentration of hydrogen ions and hydroxide ions, and therefore is referred to as neutral . If hydrogen ions are added, then [H+ ] rises and pH falls – that is, acids have a pH of less than 7. In contrast, if hydrogen ions are removed, then [H+ ] falls and pH rises – that is, bases have a pH of greater than 7.Salts as acids and basesWhen acids and bases react together, the salt (ionic compound) formed may be neutral, acidic or basic, depending on the strengths of the acid and base used in the reaction. If a strong acid is added to a strong base, or a weak acid is added to a weak base, then the resultant salt is neutral. In contrast, the reaction between a strong acid and a weak base results in the formation of an acidic salt, while the reaction between a weak acid and a strong base produces a basic salt.Acid–base balanceIn-text review Acids are substances that donate hydrogen ions during a chemical reaction.Bases are substances that accept hydrogen ions during a chemical reaction.Acids and bases are described as weak or strong, depending upon the extent of their dissociation.A solution of an acid or base can be concentrated or dilute, irrespective of whether the acid or base is strong or weak.Acids and bases react together to produce a salt and water.The concentration of hydrogen ions is described in terms of pH.We have already looked at the concept of homeostasis in Chapter 2
eBook - ePub- Odd-Ivar Lekang(Author)
- 2019(Publication Date)
- Wiley-Blackwell(Publisher)
Below this point the carbonate system will only be represented by carbonic acid. A definition of total alkalinity can also be the requirement in the number of equivalents per litre of a strong acid to titrate the water sample down to a pH of 4.5. There can still be alkalinity down to these low pH's. Alkalinity can also be expressed as P alkalinity, meaning alkalinity above phenolphthalein indicator with endpoint pH around 8.2–8.4 but this is used in aquaculture. 5.3.3 A buffer A buffer may be added to increase the alkalinity in water when it is low. A good buffer keeps the pH constant at the required pH. For aquaculture purpose, buffers that keep pH stable at around 7 are good buffers. What is happening is that the buffer builds up alkalinity and the alkalinity is typically used/consumed in a narrow pH interval for a good buffer. Outside this interval, the pH will drop fast. The buffering capacity is strongest when the pH is close to the p K (equilibrium constant) for the media, acid buffer system. As said the typical natural buffer in water is made of the carbonate system. Here the carbonic acid–bicarbonate equilibrium has a p K value around 6.3 and the bicarbonate–carbonate equilibrium has a p K value of 10.3. This means that the titration curve flattens out in those areas (Fig. 5.4)
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