ThepH

Meter

Submitted to: Mrs. Andrewin Submitted by: Christine Dawson Class: 3AS

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Before we startBefore we embark on a lesson about the pH meter, it is necessary to know what pH is. pH is the term indicating the hydrogen ion (positively charged hydrogen atom) concentration of a solution, a measure of the solutions acidity. In fact, pH stands for potential of hydrogen. Using the diagram below, you can be able to know what the numerical value (pH unit) displayed on the pH meter indicates.

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Content PageTITLE PAGE NUMBER(S) pH MeterWhat Is It?...4 How It Came About...4 How It Is Made. 5-6 How It Works.6-7 The pH meter vs. other pH indicators...7-8 Bibliography.8

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pH meterwhat is it?Since you have previously learnt what pH is, it is quite obvious what a pH meter is. A definition of a pH meter would be an electronic instrument used to measure the pH of a liquid. Special pH meters, can measure semi-solid substances. That explains why there are soil pH meters. pH meters range from simple and inexpensive pen-like devices to complex and expensive laboratory instruments with computer interfaces and temperature sensors. Some come with such temperature sensors because temperature does somewhat effect pH. Concisely, its main function is to measure the potential difference between its electrodes and convert it to a pH display.

how it came aboutIn the mid-1930s, Glen Joseph, a chemist at a research laboratory run by the California Fruit Growers Exchange, visited an old friend, a professor of chemistry by the name of Arnold Beckman. The CFGE controlled more than 75% of Californias citrus output. Joseph sought Beckman's aid in solving a problem confronting California citrus growers: how to get an accurate and rapid measure of the acidity of lemon juice. This friendly request then led to Arnolds invention of the Beckman pH meter. This inventor was once quoted saying: The pH meter was just a chance development that I did as a favor for Glen Joseph. So the pH meter was actually the result a kind deed for a friend. In the 1970s Jenco Electronics of Taiwan designed and manufactured the first portable digital pH meter, like the one below.

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how it is madeThere are all kinds of pH meters; some are used to test the pH of soil, pool water and even wine. They come with varying sizes and shapes. Following is the basic construction of a pH meter:

A basic pH meter consists of a specially-developed measuring probe (electrode) that is connected to an electronic meter that displays the pH reading. There are two main types of electrodesthe glass electrode and the reference electrode. In the following photo , both are shown:

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Glass Electrode-This is the most simplest and common electrode. It consists of a strong glass tube with a thin glass bulb welded to it. Inside is a known solution of potassium chloride (KCl) buffered at a pH of 7.0. The silver electrode with a silver chloride tip makes contact with the inside solution. To minimize electronic interference, the probe is shielded by a foil shield, often found inside the glass electrode. Calomel Reference Electrode- It consists of a cavity containing mercury and calomel ( Hg2Cl2 ) which are in contact with each other. Contact between the mercury and the calomel is made by a platinum wire while the small internal chamber is surrounded by a solution of saturated KCl. Contact between the saturated KCl and the measured solution, the liquid junction, is made through a porous ceramic pin. The potential which occurs at this point is constant and is determined by the solubility product of the calomel and the concentration of the KCl solution. *More advanced pH meters consist of both of these probes, though the cheaper ones just have a glass electrode.

How it worksWhen a positive and a negative electrode are placed in a solution containing ions, and an electric potential is applied to the electrodes, the cations move toward the negative electrode, or cathode, and the anions to the positive electrode, or anode. As a result, electric current flows between the electrodes. The strength of the current depends on the electric potential between the electrodes and the concentration of ions in the solution. Hence, it is often used to measure the ion concentration in a solution.

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When it comes to a pH meter, electrodes are specially constructed to accept H+ and are used to determine the potential of hydrogen of the solution being analyzed. A pH meter works by measuring the change in electronic voltage potential when it is immersed in the solution being tested. The pH unit is taken by measuring the concentration of free, positively charged hydrogen ions in the solution. These important steps show how the pH meter performs its function: The electrode is filled with an electrolyte solution that responds to the presence of positively charged hydrogen ions. It responds by accepting or releasing electrons in an effort to keep the environment charge-neutral (with no more cations than anions). During the above step, the voltage potential1 of the solution changes. This change is what the meter measures and infers into the number that you display of the meter. This demonstrates why the electrode is the most important part of the meter (and also the most expensive!) *Take note that the most electrodes produce a small voltage of about 0.06 volt per pH unit. This voltage is measured and displayed as pH units by the meter. In simpler terms, the probes voltmeter does not measure voltage, but pH units.

pH meter vs. other ph indicatorsAlthough sometimes expensive and delicate, pH meters have quite a number of advantages over that of other pH indicators. Here are some good stuff about this instrument.

1 the amount of electrons available to flow between the solutions

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Precision- pH meters are much more precise than any other method of pH measurement. They are so precise that they can measure down to 0.01th of the pH unit. Although, there are some pH paper that can measure ranges smaller than 0 to 14, they are still not more precise than that of a fellow pH meter. Objectivity-Whereas pH paper and other indicators give a range of colors that can be subject to being read differently by different observers, reading pH meters are far less subjective. Also pH meters do not limit color-blind individual, unlike like other color strips and indicators. Ease of Use-It is obvious that simply inserting a probe into a liquid is much easier (and less messy) than using pH paper and other indicators. There is no requirement of even using chemicals which may be unavailable for most people. This is too bad for chemical indicators like phenolphthalein. Interpretation of Results- A pH meter gives you a numerical value of the pH after being inserted into the liquid, this is also makes it easier to use. Other pH indicators do not give a numerical pH value. Longer Usage- Unlike other pH indicators, pH meters are not disposable. Each time you measure pH in another way besides using a pH meter, you need to restock your supply. That means that litmus paper, pH paper, methyl orange, alizarin etc., as they can only be used once. Now here is the bad side: Cost- A pH meter is much more costly than that of pH paper and other indicators. Faulty pH Readings-A pH meters measuring capability can easily be thrown off by temperature, by metal deposits on the electrode, protein, salts etc. Require Calibration- To ensure uniformity and accuracy, pH meters are calibrated according to the accepted standards of measurement for the given electrical unit. This process is quite problematic for first-timers. Other pH indicators do not require this preparation.

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Cleaning- As tedious as it seems, pH meters need to be cleaned after each use so that you can attain accurate pH readings the next time it is used. Other indicators do not require this.

So which do you prefer?

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