PH indicator

Indicators (Latin indicare, "display" ) are generally aids that are intended to identify certain information. They allow the determination of conditions and the tracking of processes by the Show or exit from certain states.

In the chemical indicator is meant by a substance or a device that is used to monitor a chemical reaction or a condition. Often, a state of a certain color, the change indicated by a color change. The most common indicators are used in titrations.

Depending on which type of chemical or physical states or reactions you follow with an indicator, a distinction is made between the following types of indicators:

• PH indicators ( in acid - base titrations and the determination of the pH value )
• Redox indicators (for redox titrations )
• Complex indicators ( in the complexometric )
• Thermal indicators (to display a temperature range )

One example often used are the indicators that indicate the pH of certain substances on the basis of a comparison with a color scale. Often used here Litmus, bromothymol blue or phenolphthalein. The latter shows a color change only when adding an alkaline solution. There is also universal indicators for a more accurate measurement of pH. These are usually in the form of a strip of paper, wetted with the solution to be tested, followed by comparing the staining of the paper with the color scale. We speak now of either an acidic, alkaline or neutral solution.

The following table shows the color of the different indicators in dependence on the pH.

They differ

• In the colors that they have in acidic, neutral or alkaline solutions
• In the pH range in which the change occurs between the two colors ( this pH range is also called fold-over area, the pKa is ± 1).

Everyday acid -base indicators

And cabbage juice can be used as pH indicator. The dye contained in the cabbage juice can cyanidin colors of red = blue = acidic to alkaline accept ( in even more alkaline environment it is green and at pH > 10 even yellow ). Depending on the preparation takes cabbage vegetables a more red coloring (such as by the addition of malate pieces of apple or some vinegar ), or a violet to blue staining ( preparation with alkaline -reacting household baking soda), which brings the also common name red cabbage expressed.

Tea drinkers know the dependence of the tea color from the acid: Is the black added lemon juice, then the color changes from dark brown to light reddish brown. This color change is also due to pigments in tea that act as indicators.

Operation

The molecules of the acid -base indicators are themselves weak acids ( or bases ), that is they can give or receive protons. The molecule of the acid indicator is simply referred to as HInd. After donating a proton remains Ind, the corresponding indicator base called back.

The protolysis equilibrium for the delivery or acceptance of a proton in an aqueous indicator solution is the following:

The indicator molecule can thus donating a proton, but also record again. At a high concentration of H3O ions ( that is, in an acidic solution ), amplifies the reaction takes place to the left, whereby the concentration of hind ( indicator acid ) is greater than the concentration Ind. At a very low concentration of H3O ions ( ie, in an alkaline solution ) is reinforced, the reaction to the right place, whereby the concentration of Ind is greater than the concentration HInd.

This fact is particularly clear if one applies the law of mass action to the general equation above. Then, with a constant:

As usual, the constant concentration of the water is included in the constant of. Note that the concentration of H3O in a different order of magnitude than that of the indicator. Now, if this concentration is greatly increased or decreased - depending on whether the solution is acidic or basic - the balance has to adjust, because Ks is constant. Therefore, the concentration of Ind - strong needs to change, thereby simultaneously since HInd from Ind - formed, the concentration of HInd must move correspondingly strong in the other direction. Therefore, the indicator change generally takes place very quickly.

The actual effect of the indicator based on the fact that the compound has a different color than HInd Ind. As a result of protonation or deprotonation of the indicator at its resonance stabilization changes. In acid solution, the concentration of predominant Hind so that the solution takes on the color of the protonated form. By increasing the pH, the concentration of growing Ind, while the concentration in Hind decreases until finally the former is predominant, and the solution takes on the color of Ind (see, e.g. phenolphthalein). The different color of protonated and non-protonated dye molecules are called halochromism.

The transition point of the indicator is characterized by that. At this point, therefore also applies because the two terms are then wegkürzen in the above -mentioned equation of the law of mass action, and. The pH of the solution at the transition point corresponds theoretically the constant of the indicator. The perceptible to the human eye turning point deviates from them often, since the human eye can detect the color change until a concentration ratio of 9:1 .. The pH range, which can be seen as a mixed color envelope interval is called.

Error indicator

During titration, the pH of a buffer system is shifted so far by the quantitative exactly known addition of an acid or an alkaline solution ( titrant ) that to be determined base or acid ( Titrand ) is completely neutralized. However, the pH indicator also constitutes a buffer system that simultaneously consumes hydroxide or hydronium ions from the titrant. The concentration of indicator in the titration solution is usually in the range of 10-4 mol / L. At much more concentrated titrant the indicator hardly plays a role. In the analysis of natural water, however, the buffer concentration of the substances in the water, depending on water hardness at the same to about 10 - fold higher concentration. Therefore, the indicator may cause a significant error here.

Another problem may arise in that the indicator dye is normally added in the form of an alcoholic solution. This alone can the whole buffer system change its properties.

Redox indicators

The simplest use of redox indicators is the determination of the end point in redox titrations ( Oxidimetrie ).

Common redox indicators are:

• Methylene blue
• Neutral
• Ferroin
• Red prussiate of potash
• Yellow prussiate of potash
• Dichlorophenolindophenol ( DCPIP )

Complex indicators ( metal indicators)

Possible application is the volumetric determination of the concentration of metal ions, for example the complexometric titration. A typical application is the water hardness determination.

Known complexometric indicators:

• Murexide
• Tiron
• Thorin
• Calconcarboxylic acid
• Eriochrome black T
• Diphenylcarbazone
• Xylenol orange
• Fura-2
• Calcein

Solvatochromic indicators

Change Solvatochromic Indicators their color after the surrounding solvent.

• Brooker's merocyanine

Thermal indicators ( thermochromic )

Thermal indicators are often used where the temperature can not be easily measured with the thermometer. For example, a melting pot is marked with chalk thermocouple and displayed to reach a desired temperature in the flame by the discoloration of the thermal chalk. Also, a sufficient cooling can be displayed by Thermochromic.

Thermal indicators as a sticker based on liquid crystals. There are reversibly and irreversibly reacting variants. The latter are particularly suited to register when operated invisible places exceeding certain temperature values.

See also Seger cones.

Mixed indicator

Mixed indicators are mixtures of various indicators, which cover the range is expanded or more envelope regions are created. Among the mixed indicators include the contrast indicators. A common example of a mixed indicator is Tashiro.

Contrast indicator

Contrast indicators usually comprise a counter and a dye, which retains its color. This addition of the contrast of the envelope area is enhanced. A common example of a contrast indicator Eriochrome black T in a mixture with methyl orange.

Humidity Indicators

Humidity indicators perform a color change when certain humidity levels are exceeded, which they are exposed. The color change reaction due to water absorption, therefore, the time of exposure plays also a role. Most famous is the blue, cobalt chloride displaced silica gel ( blue gel ), which is colored or pink at the influence of moisture to violet.

It is applied to paper, added moisture-sensitive shipments or housed in hermetically sealed modules behind sight glasses. Due to the toxicity of the cobalt salt and cobalt-free alternatives are available ( orange gel ).

Fluorescent indicator

Fluorescent indicators are substances that change their fluorescence at the equivalence point of a titration. Also referred to as fluorescent indicators phosphors of the stationary phase ( separation layer) are added to the plates for thin layer chromatography. They make it possible to detect colorless substances under a UV lamp due to fluorescence quenching.