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Lic. en Química Carlos R. Arano
Buenos Aires 660 - Bº Sta. Brígida -1748 Gral. Rodríguez - Pcia de Bs. Aires - Argentina
Tel. + 54 (0) 237 484 1480
Lic. en Química Carlos R. Arano
Buenos Aires 660 - Bº Sta. Brígida -1748 Gral. Rodríguez - Pcia de Bs. Aires - Argentina
Tel. + 54 (0) 237 484 1480
pH measurements can be done either by colorimetric or by electrometric methods. pH-meters, instruments for electronic testing, are used commonly to-day. However, some people still like the old colorimetric methods, simple and practical procedures with an important advantage: they do not get out of order as from time to time do the pH-meters. This page is designed to illustrate what should be done in order to use the colorimetric method as a measurement procedure.
The colorimetric method employs the use of complex organic dyes which change to distinctive colors through a specific pH range of the aqueous solutions. These dyes are known as indicators.
For most purposes four dyes suffice. These are:
brom-cresol green
chlorophenol red
brom-thymol blue
& phenol red.
Using these four dyes, as it will be later explained, a pH range from 4.0 to 8.4 can be reached.
There are other dyes to be used in order to extend this range of coverage but it is not necessary to describe them here.
Actually only one dye is absolutely necessary,
chlorophenol red,
which reacts in a pH range of 5.2 and 6.8. This dye is easily read within the range of 5.8 to 6.6 which is the usual range of fluctuation in a well regulated nutrient solution. Remember that the optimum pH point for a hydroponics solution is between 5.8 to 6.2.
It is well known that some people prefer brom-thymol blue as the only one indicator dye. Its color change range is from 6.0 to 7.6. In spite it is not too bad, it is much better chlorophenol red dye.
Four-dye Series
The colorimetric method employs the use of complex organic dyes which change to distinctive colors through a specific pH range of the aqueous solutions. These dyes are known as indicators.
For most purposes four dyes suffice. These are:
brom-cresol green
chlorophenol red
brom-thymol blue
& phenol red.
Using these four dyes, as it will be later explained, a pH range from 4.0 to 8.4 can be reached.
There are other dyes to be used in order to extend this range of coverage but it is not necessary to describe them here.
Actually only one dye is absolutely necessary,
chlorophenol red,
which reacts in a pH range of 5.2 and 6.8. This dye is easily read within the range of 5.8 to 6.6 which is the usual range of fluctuation in a well regulated nutrient solution. Remember that the optimum pH point for a hydroponics solution is between 5.8 to 6.2.
It is well known that some people prefer brom-thymol blue as the only one indicator dye. Its color change range is from 6.0 to 7.6. In spite it is not too bad, it is much better chlorophenol red dye.
Four-dye Series
Dye solutions preparation
A.- 40 mg brom-thymol blue + 5 ml ethyl alcohol 95% + Dissolve + Add 95 ml distilled water. Adjust to pH 6.6 .
B.- 40 mg brom-cresol green + 5 ml ethyl alcohol 95% + Dissolve + Add 95 ml distilled water. Adjust to pH 4.6 .
C.- 40 mg chlorophenol red + 5 ml ethyl alcohol 95% + Dissolve + Add 95 ml distilled water. Adjust to pH 5.6 .
D.- 40 mg phenol red + 5 ml ethyl alcohol 95% + Dissolve + Add 95 ml distilled water. Adjust to pH 7.4 .
Note: 1) Use very dilute sodium hydroxide solution drop by drop to adjust.
2) Use the standard tube for comparison.
3) If the proper pH point is passed use a very dilute hydrochloric acid solution drop by drop to re-adjust.
A.- 40 mg brom-thymol blue + 5 ml ethyl alcohol 95% + Dissolve + Add 95 ml distilled water. Adjust to pH 6.6 .
B.- 40 mg brom-cresol green + 5 ml ethyl alcohol 95% + Dissolve + Add 95 ml distilled water. Adjust to pH 4.6 .
C.- 40 mg chlorophenol red + 5 ml ethyl alcohol 95% + Dissolve + Add 95 ml distilled water. Adjust to pH 5.6 .
D.- 40 mg phenol red + 5 ml ethyl alcohol 95% + Dissolve + Add 95 ml distilled water. Adjust to pH 7.4 .
Note: 1) Use very dilute sodium hydroxide solution drop by drop to adjust.
2) Use the standard tube for comparison.
3) If the proper pH point is passed use a very dilute hydrochloric acid solution drop by drop to re-adjust.
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Colorimetric method for pH measurements
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Preparation of permanent standards
The pH of a nutrient solution is checked by comparing the color developed in a sample against a serie of permanent standards. This serie of standard tubes can be previously prepared with 9 glass tubes of 12 ml each one. However, if somebody wants to have all the four-dye series, 36 tubes shall be required.
Really, only one serie is needed for hydroponic measurements: the C indicator serie made with chlorophenol red. Of course, to prepare it only 9 tubes will be necessary.
The practical grower should purchase ready-to-use commercial pH kits. Those use standard tubes in a block comparator. Either the complete set of dyes suggested above may be
obtained or only the chlorophenol red series may be bought. Any chemical supply shop is a good source of information and materials.
In case the grower may wish to prepare their own pH series set:
1.- Put 200 ml distilled water in an Erlenmeyer flask or in a bottle.
2.- Add 10 ml selected indicator dye solution prepared as it was explained above. Stir slowly. Specific color will be developed, being it inside of the scale. If chlorophenol red dye was used, color will be near to neutral pH color (distilled water).
3.- The pH of this solution shall be tested with a well calibrated pH meter (by sure somebody around you will have one).
4.- Fill one of the 12 ml tubes with this colored solution. Seal the tube and label it with corresponding pH value.
5.- Now continue working with the rest of the bulk solution from point 2. First of all, add drop by drop a very dilute solution of hydrochloric acid, stirring between adds. After each small
change on the solution color, test the new pH value (it will be lower than the previous one), fill a new tube, seal and label it. Follow same procedure until you get the lower extreme of pH
scale for that indicator dye. As an example, if you are working with chlorophenol red, you end point will be a yellow color at pH = 5.2.
6.- After that, same procedure shall follow using highly diluted sodium hydroxide solution. The rest of the solution in the flask, yellow in our example, shall be returned to the
higher side of the standard set by adding drop by drop this alkaline liquid. Following with our example, an intense red color will be the end point at pH = 6.8.
7.- The 9 tubes permanent color serie will be there. Color from yellow to red increasing pH values if using chlorophenol red or phenol red, and from yellow to blue increasing pH values if working with brom-thymol blue and/or brom-cresol green.
Same steps shall be followed in order to prepare other series.
Note: All colors printed in this page will not show correct. Comparisons shall be done against standards prepared as indicated above.
Comparations
The advisable rate is very practical:
10 ml nutrient solution sample + 0.5 ml Indicator dye (10 drops)
Use same type of tube. Add 10 ml nutrient solution sample using a syringe and 10 drops of dye solution. Stir it carefully. If chlorophenol red is used, yellow, red or an intermediate color will be shown. If the last one is a case, comparison with standard block will reveal the solution pH in some place between 5.4 and 6.6 immediately.
If color is deep yellow, pH will be 5.2 or lower. To know exactly the value it will be necessary to use the B indicator serie prepared with brom-cresol green. Anyway, being pH 5.2 or lower hydroponic nutrient solution should be corrected. To do it, potassium hydroxide diluted solution shall be added to reach a desired value of 5.8 to 6.2.
If color if deep red, pH will be 6.8 or higher. To know the exact value shall be necessary to use the A indicator serie prepared with brom-thymol blue or, in an extreme case, D indicator serie (phenol red). Anyway, being pH 6.8 or higher, hydroponic nutrient solution should be corrected. To do it, a hydrochloric acid diluted solution shall be added to reach a desired value of 5.8 to 6.2.
Procedure is always same in all cases. It is very simple. Practice is the only requirement. Sensibility of dyes is great and measures values are very sure.
Note: To avoid any decoloration it is convenient to keep dyes indicator series in a dark and fresh place when not in use.
The pH of a nutrient solution is checked by comparing the color developed in a sample against a serie of permanent standards. This serie of standard tubes can be previously prepared with 9 glass tubes of 12 ml each one. However, if somebody wants to have all the four-dye series, 36 tubes shall be required.
Really, only one serie is needed for hydroponic measurements: the C indicator serie made with chlorophenol red. Of course, to prepare it only 9 tubes will be necessary.
The practical grower should purchase ready-to-use commercial pH kits. Those use standard tubes in a block comparator. Either the complete set of dyes suggested above may be
obtained or only the chlorophenol red series may be bought. Any chemical supply shop is a good source of information and materials.
In case the grower may wish to prepare their own pH series set:
1.- Put 200 ml distilled water in an Erlenmeyer flask or in a bottle.
2.- Add 10 ml selected indicator dye solution prepared as it was explained above. Stir slowly. Specific color will be developed, being it inside of the scale. If chlorophenol red dye was used, color will be near to neutral pH color (distilled water).
3.- The pH of this solution shall be tested with a well calibrated pH meter (by sure somebody around you will have one).
4.- Fill one of the 12 ml tubes with this colored solution. Seal the tube and label it with corresponding pH value.
5.- Now continue working with the rest of the bulk solution from point 2. First of all, add drop by drop a very dilute solution of hydrochloric acid, stirring between adds. After each small
change on the solution color, test the new pH value (it will be lower than the previous one), fill a new tube, seal and label it. Follow same procedure until you get the lower extreme of pH
scale for that indicator dye. As an example, if you are working with chlorophenol red, you end point will be a yellow color at pH = 5.2.
6.- After that, same procedure shall follow using highly diluted sodium hydroxide solution. The rest of the solution in the flask, yellow in our example, shall be returned to the
higher side of the standard set by adding drop by drop this alkaline liquid. Following with our example, an intense red color will be the end point at pH = 6.8.
7.- The 9 tubes permanent color serie will be there. Color from yellow to red increasing pH values if using chlorophenol red or phenol red, and from yellow to blue increasing pH values if working with brom-thymol blue and/or brom-cresol green.
Same steps shall be followed in order to prepare other series.
Note: All colors printed in this page will not show correct. Comparisons shall be done against standards prepared as indicated above.
Comparations
The advisable rate is very practical:
10 ml nutrient solution sample + 0.5 ml Indicator dye (10 drops)
Use same type of tube. Add 10 ml nutrient solution sample using a syringe and 10 drops of dye solution. Stir it carefully. If chlorophenol red is used, yellow, red or an intermediate color will be shown. If the last one is a case, comparison with standard block will reveal the solution pH in some place between 5.4 and 6.6 immediately.
If color is deep yellow, pH will be 5.2 or lower. To know exactly the value it will be necessary to use the B indicator serie prepared with brom-cresol green. Anyway, being pH 5.2 or lower hydroponic nutrient solution should be corrected. To do it, potassium hydroxide diluted solution shall be added to reach a desired value of 5.8 to 6.2.
If color if deep red, pH will be 6.8 or higher. To know the exact value shall be necessary to use the A indicator serie prepared with brom-thymol blue or, in an extreme case, D indicator serie (phenol red). Anyway, being pH 6.8 or higher, hydroponic nutrient solution should be corrected. To do it, a hydrochloric acid diluted solution shall be added to reach a desired value of 5.8 to 6.2.
Procedure is always same in all cases. It is very simple. Practice is the only requirement. Sensibility of dyes is great and measures values are very sure.
Note: To avoid any decoloration it is convenient to keep dyes indicator series in a dark and fresh place when not in use.