Ph and Buffer

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PH and Buffer
PURPOSE
The purpose of this experiment is to use the Henderson-Hasselbach equation to prepare buffers of known PH and composition. The exercise also seeks to compare and predict observed pH of buffers prepared. This laboratory exercise also aims at using pH electrode during titration of sodium phosphate in order to obtain an acid-base titration curve. It also involves calculating the pKa value of a buffer from its titration curve (Chem354 Biochemistry Notes, 6).
MATERIALS AND METHODS
Materials. They include a stirring plate, a digital pH meter, magnetic stirring bars, a clamp for pH sensor, Standard pH solutions at 4, 7 and 10, sodium acetate and glacial acetic acid. It also requires a wash bottle with distilled water, pipet and pipet aid, plastic waste beaker, and a p-1000 adjustable micro-pipette.
Methods
Preparing a 0.2m sodium acetate and 0.2M acetic acid stock solutions. The first part of the involved preparing a 250 mL of 0.2M sodium acetate. This was done by adding distilled water (200mL), dissolving and stirring. The next step was preparing 400mL of 0.2M of acetic acid with a molar weight of 60.1g/mol. Glacial acetic acid was added into distilled water (300mL) followed by stirring.
Preparation of acetic buffers. It involved preparation of 0.2M acetic buffer at pH 4 from 0.2M acetic acid and 0.2M sodium acetate. The solution was prepared in a beaker and a pH meter was used to measure the mass of the acetic buffer after calibrating the pH electrode.

The predicted and observed pH were recorded in a table. The next step involved preparation of 0.2M acetate buffer by mixing 70mL of 0.2M sodium acetate and 55mL of 0.2M acetic acid. The pH meter was used to measure the pH of the acetic buffer after re-calibration. Predicted and observed pH values were recorded in a table.
Preparation of Phosphate buffer. This part involved preparation of 100mL of 0.1M sodium phosphate buffer at pH = 11.0. The amount of sodium phosphate base (g) was calculated, weighed and dissolved in water 150 mL beaker. The solid was dissolved by stirring at a gentle rate. The initial pH of phosphate buffer was measured using a digital pH meter after calibration. Sodium phosphate was titrated to pH 11 by adding 0.5mL aliquots of 1.0M NaOH using a micropipette. The volume of sodium hydroxide added and pH were noted after every addition. The total volume of solution was measured in a clean cylinder and the pH confirmed.
RESULTS AND ANALYSIS
Part I. A. The amount of sodium acetate needed is given by; Moles = Volume * Molarity
= 0.25 L*0.2 = 0.05 Moles, therefore amount needed is; 136.08 *0.05 = 6.804 g
B. The molar concentration of glacial acetic acid is given by;
1.05gml*99.7100*1000mL1L*1mL60.1g=17.37mol/LAmount of glacial acetic acid added is given by; Moles = 17.37 * 0.4 = 6.948 moles
Part II. A. Ratio of sodium acetate and acetic acid to yield a pH of 4.5 is determined using Henderson-Hasselbach equation as follows;
pH=pKa+log(A-HA)Where HA molar concentration of undissociated weak acid and A- is is the molarity of the acids conjugate base
The ratio A-HA is unitless.
pKa=-logbase10Ka
Where Ka is the acid dissociation constant or acidity constant.
The acidity (pKa) of acetic acid is given as 4.76
Therefore, ratio is given by; = 10pH-pKa=104.5-4.76=10-0.26=1.82The predicted and observed pH values were 4.9 and 4.83 respectively.
B. The pH of the buffer is given by Henderson-Hasselbach equation as;
pH=pKa+log(A-HA)= 4.76+log0.20.2=4.76+0=4.76
The predicted pH is 4.76
Table.
Part III. Preparation of Phosphate Buffer
The amount of sodium phosphate base (g) is given by;
Mass = Grams/Moles *Moles = 120gMol*MolesMoles of sodium phosphate = Volume * Molarity = 0.1L *0.1 M = 0.01 Moles
Therefore;
Mass of sodium phosphate is given as; = 120*0.01= 1.2g.
Table 1. Predicted and Observed pH Values
Buffer Predicted PH Observed PH %Error
Acetate I 4.50 4.43 -1.56
Acetate II 4.9 4.83 -1.4
Phosphate 11 11.04 0.036
Sample error is given by;
%Error=[Measured-ExpectedExpected]*100
% Error for Acetate I is given as; = 4.43-4.504.50*100=-1.56%Error for Acetate II is given as; =( 4.83-4.94.9)*100=-1.42%Error for Phosphate is given as; = ( 11.04-1111)*100=0.036Table 2. Titration of Sodium Phosphate
Volume of 1.0M NaOH added(mL) Measured
pH Volume of 1.0M NaOH added(mL) Measured
pH Volume of 1.0M NaOH added(mL) Measured
pH Volume of 1.0M NaOH added(mL) Measured
pH
0 4.47 3.5 6.60 7.0 7.34 10.0 10.59
0.5 5.57 4.0 6.70 7.5 7.48 10.5 10.61
1.0 5.90 4.5 6.80 8.0 7.67 11.0 10.97
1.5 6.10 5.0 6.90 8.5 7.95 11.25 11.04
2.0 6.25 5.5 7.00 9.0 8.65 2.5 6.38 6.0 7.10 9.5 10.2 3.0 6.50 6.5 7.22 9.75 10.44 Fig 1. Titration Curve (pH vs. Volume of Na)

From the graph, at pH 8, the volume of NaOH added is obtained as 8.238.
From the relation; pH=pKa+log(A-HA)The pKa value of sodium phosphate (NaH2PO4 is given as; =8-(log(A-HA)Molarity of NaOH is given as; Molarity = Moles/Volume = 0.01/11.25 = 0.00089M
Therefore, pKa = 8-log0.000890.1=8+2.05=10.05
Pre-Lab Questions
1. Goals of the experiment
The aim of the experiment was to prepare acidic and alkaline buffers of known pH. This is done by employing the Henderson-Hasselbach equation and lab techniques like titration.
2. Key theoretical principles
Buffers are used to maintain optimal pH conditions. They mimic properties which are found in the physical environment and don’t chemically with the system under investigation. They also don’t absorb light at a wavelength which will interfere with spectrophotometric properties. Acid and alkaline buffers are made from weak acids, weak salts and their corresponding salts. The most accurate way of measuring pH is electrochemically using a pH meter.
3. Key Equation
The Henderson-Hasselbach equation reveals the relationship of pH and pKa (affinity of a molecule to its proton). It helps in calculating pH from the ratio of salt to acid.
The equation is given as follows;
pH=pKa+log(A-HA)Where HA molar concentration of undissociated weak acid and A- is is the molarity of the acids conjugate base
pKa=-logbase10Ka
Where Ka is the acid dissociation constant or acidity constant
4. Key Instrumentation
pH meter is the main instrument used in the lab exercise. It records very small differences of electrical potential due to difference in hydrogen concentration for different pH values.
DISCUSSION/CONCLUSION
The observed pH values for acetate I and Acetate II are an indication of acidic buffers. This is because acidic buffers have a pH of less than 7. These are obtained from weak acids. Sodium phosphate was confirmed to be an alkaline buffer since the pH obtained was more than 7. There was a significant difference between the observed and predicted pH for the Acidic buffers. The observed pH value of alkaline buffer showed an insignificant error. In general, the measured results and experimental results showed minimal differences which means the lab exercise was a success.

Works Cited
Chem354 Biochemistry Notes. Laboratory exercise 2: Introduction to pH and Buffers. 1-10