The following points highlight the six experiments on plant in relation to pH. Some of the experiments are: 1. Determination of Total Titratable Acidity of the given Plant Sap 2. Determination of pH of the Cell Taps of the Plant Material 3. Determination of Degree of Dissociation of Cell Sap 4. Determination of Buffering Capacity of Plant Sap and Others.
Experiment 1
Determination of Total Titratable Acidity of the given Plant Sap:
Experiment:
5gm juicy hairs of lemon or orange or grape is taken in a mortar and crushed well with a pestle. This is then filtered using glass wool. The mother liquor thus obtained is measured and taken -in a 100 ml volumetric flask. This is diluted to just 100 ml with distilled
Discussion:
Newly produced in metabolism, acids remain in Free State and increase the titratable acidity (the free acids which can be titrated against an alkali) of the cell sap and tend to lower the pH. Subsequently, however, the titratable acidity may decrease without change in the amount of total acidity.
Organic acids like oxalic, tartaric and Krebs cycle intermediates, viz., malic, isocitric, citric, and other acids are produced and may accumulate in different plant parts, particularly in green fruits.
Accumulation implies a rate of enzymic production exceeding that of consumption. The formation and consumption of oxalic and tartaric acids which are not Krebs cycle acids cannot be explained. There is a wide fluctuation in content of individual acids among different genera of plants.
Experiment 2
Determination of pH of the Cell Taps of the Plant Material:
Experiment:
The original mother liquor of the previous experiment is taken and its pH is determined either by using universal pH indicator paper and comparing the coloured chart, or by the indicator prepared according to the following formula: Phenolphthalein — 0.1 gm, Methyl red — 0.2 gm. Dimethylaminoazobenzene — 0.3 gm. Thymol blue — 0.5 gm, Bromothymol blue — 0.4 gm, absolute alcohol — 500 ml.
A drop of the mother liquor is taken on a spot plate, mixed with a drop of indicator and pH is determined by comparing the colour chart.
For standard comparison, 5 ml of sample and 2 drops of indicator solution give better results.
N.B. pH may also be determined more accurately with the help of a pH-metre.
Experiment 3
Determination of Degree of Dissociation of Cell Sap:
Experiment:
If pH or total acidity of any acid or cell sap can be known, the degree of dissociation can be calculated as follows:
In case of cell sap, the pH and strength in terms of 0.1 N NaOH is to be determined first according to the experiments already described. Then the degree of dissociation is determined as above. The degree of dissociation increases with increase in dilution of acids or alkalis.
Experiment 4
Determination of Buffering Capacity of Plant Sap:
The efficiency of a buffer system (or its buffering capacity) depends upon how many moles of acid or base can be added before we get an appreciable change in the initial pH. In any case the buffering capacity depends on the actual amount of acid and conjugate base available for combining with the added base or acid.
The buffering capacity of cell sap, soil solution or any other liquid can be expressed by a number, the buffer index.
A solution has a’ buffer index of unity when the addition of 1gm equivalent of a strong acid or alkali shifts the pH of 1litre of the solution through one limit. In general, it may be said that the amount of shift depends upon the nature and concentration of the buffer.
Comparative values of the buffer capacity of different solutions may be obtained by determining with pH-meter or indicators, the number of gram equivalent of acid or alkali that must be added to equivalent ‘ amount of a solution to produce unit change in pH. Biological fluids are in general strongly buffered, so that their pH is maintained within narrow limits under physiological conditions.
Experiment:
25 ml of the plant juice is taken in a 100 ml flask pH of the cell sap is determined with the help of a pH-meter or a universal pH indicator paper. 1 ml of 0-5 (N) HCl is added from a burette to the sap and the beaker is gently shaken. Its pH is again determined.
The addition of 1 ml aliquot of acid is continued and pH is determined in each case until pH value of 2 is reached. To a second 25 ml of original juice or sap 0.5 (N) NaOH is added from a burette in 1 ml increments until a pH value of 12 is reached. The pH is determined each time after addition of each ml of 0.5 (N) NaOH.
Results:
The results of the titration are plotted on the graph paper in the form of two titration curves using a single co-ordinate axis taking pH increasing from 2 to 13 horizontally along the abscissa.
At the midpoint of the ordinate zero is placed. Now equivalents of alkali added are placed above and equivalents of acid added placed below it. The buffer capacity can be estimated from the slope of the titration curve.
Sharp slope indicates a large pH change for small change in base or acid concentration, i.e., a poor buffer capacity and vice-versa. A model graph is given below for better understanding (Figure 18).
Experiment 5
Determination of pH of Soil Sample:
20gm soil sample is mixed with 40 ml distilled water. The suspension is stirred at regular intervals for 1/2 hour. pH is then measured in a pH meter. The suspension is to be stirred well before taking reading of the sample.
Experiment 6
Determination of pK Value of Plant Sap and the Possible Buffering Acids Present:
The pH at which the plant sap shows the maximum buffering capacity is equivalent to its pK value. If maximum buffering capacity is shown, say, at pH 4.74, the pK is equal to 4.74. The antilog of this value is equal to 1.8×10-5 and pK is, according to the definition, the negative logarithm of the dissociation constant K.
Now, the dissociation constant is equal to 1-8×10-5 which is typical for acetic acid (pK = 4 74, 5 — 4 74 = 0.26, antilog of 0.26 = 1.82, hence its negative logarithm = 1-82 X 10-5 = K).
