PH MEASUREMENT AND BUFFER PREPARATION

Christian D. Aceret, Harold James O. Alcantara, Ixzi Thia G. Alforque, ​Jewel Hannah M. Angel​,
Rischthelle S. Aton, Amiel Rovic T. Babaran
Group 1 2B Medical Technology Biochemistry Laboratory

ABSTRACT
The experiment was done by first preparing the buffer solution, then the procedure modified and determined the
buffer’s pH along with other two samples which was done with Colorimetric and Electrometric determination methods.
The Phosphate buffer solution was prepared by dissolving 11.37 grams of the weak acid (​H​3​PO​4​) with 7.3mL of its
conjugate base (​NaH​2​PO​4​), then mixed with distilled water to obtain the desired pH of 2. In Electrometric
determination, the pH of the samples of distilled ​H​2​O, Coca-Cola, and the Buffer solution showed acidic values of 5.56,
2.95, and 2.03 respectively; the amounts of [​H+​​ ] ions of the samples were ​2.75x10​-6 ​, ​1.12x10​-3​, and ​0.0169
respectively. Results have shown that the Buffer solution was the most acidic compared to the other two samples.
Qualitative Colorimetric analysis shows that distilled water has a pH range of 3.125 - 6.5, while the buffer solution
prepared had a pH range of 2 - 4.63. Both qualitative and quantitative analysis of pH are consistent with each other.

INTRODUCTION 2. Buffer solution

In 1909, Søren Sørensen, developed the pH ● Phosphoric acid H​3​PO​4
scale to determine the extent of acidity or ● Monobasic Sodium phosphate NaH​2​PO​4
basicity of a substance [2]. ​pH ​is the
measurement used to determine the ​ 3. Electrometric Determination of pH
concentration of Hydrogen ions in a solution. It is ● Prepared Buffer solutions
expressed as the negative logarithm of the ● Distilled water
Hydrogen concentration. ● Coca-Cola soda
A buffer is added to a solution to resist changes
in pH whenever acids and bases are added. It 4. Acid-Base Indicators
has a neutralizing property, maintaining ​the pH ● Bromophenol blue
of the solution relatively stable [3]. A buffer ● Bromocresol green
solution consists of a weak acid and its conjugate ● Bromocresol purple
base or a weak base with its conjugate acid. ● Phenol red
The Henderson - Hasselbalch equation ● Methyl red
determines the pH of a solution by correlating the ● Methyl orange
pK of an acid and the molar ratio of the ● Phenolphthalein
concentration of the acid and its conjugate base ● Thymol blue
[4]. This equation may also be used to determine
the concentration of either the acid or its B. Procedure
conjugate base depending on a specific pH level. 1. Preparation of reagents
This experiment aims to prepare different buffer ​First, the amounts needed to prepare
solutions, depending on the desired pH assigned. the reagents were computed using the
Moreover, the pH of the buffer and samples are dilution equation formula. For 250mL of
to be determined colorimetrically using 6.0 M of HCl, 120mL or 0.12 L conc. of
indicators, and electrometrically, by using the pH HCl was used for dilution.
meter. 2. Preparation of buffer solution
The buffer solution was prepared using
the following measurement of reagents.
EXPERIMENTAL PROCEDURE
Table 1. ​Buffer Components
A. Compounds used in the experiment
1. For the Preparation of Reagents
● 6.0 M of HCl Acid Base pH pka vol
● 6.0 M of NaOH pellets
 4. Colorimetric Determination of pH
H​3​PO​4 NaH​2​PO​4 2.0 2.12 250
mL
A. Preparation of color standards using
the buffer solution
The specific amounts of the acid and conjugate
6 test tubes were prepared. 5 mL of
base was determined through the use of
each of the prepared buffers were put into
Henderson-Hasselbalch equation
a test tube separately. Each test tube was
11.37 grams of monobasic Sodium phosphate
treated with 2 drops of an acid-base
(NaH​2​PO​4​) powder reagent was dissolved with 7.3
indicator. Each mixture was then mixed.
mL of Phosphoric acid (H​3​PO​4​), then the solution
The colors produced were noted. Another
was added and mixed to a 500 mL beaker with
set of 6 test tubes with 5 mL buffer was
distilled water resulting in a 250mL solution. The
prepared, and 2 drops of the 8 different
prepared solution was transferred to an amber
acid-base indicators were added
bottle and was labeled properly. The pH meter
separately to each of the test tubes.
was utilized to measure and ascertain that the pH
of the buffer was 2.
B. Determination of the pH of the
samples
Computations
The pH of distilled water and CocaCola
Moles of buffer = (0.250L)(0.1 M)
soda was measured using a pH meter. 2
= 0.025 moles
drops of each of all the 8 acid-base
indicators was added to 5 mL of a
Henderson-Hasselbalch Equation:
CocaCola sample separately. Each sample
pH = pka + log [conjugate base] / [weak acid]
was shaken and mixed, and the color
2.00 =2.12 + log (0. 379 M) / (0.5 M)
change was recorded.
1. To get Moles of the base , first is to
​ 5.​ ​Determination of buffer capacity
multiply the molarity of the base to the
100 mL each of 0.1M HCl and 0.1M
moles of the buffer and divide it by the
NaOH was prepared. Using the buffer, the
total molarity of both the acid and the
buffer capacity was determined by using a
base. The same steps were done in
buret to add 1 mL portions of 0.1M HCl to
getting the moles of the acid.
20 mL of the buffer until its pH changes.
2. To get the volume of the acid. Use the
The volume added was recorded. The
density formula where in the specific
same was done with NaOH.
gravity is divided by the moles of the acid.
3. To get the grams of the base. Divide the
moles of the base to the molar mass of RESULTS AND DISCUSSION
the compound. The experiment focused mainly on colorimetric
and electrometric determination of pH. The
3. Electrometric Determination of pH results reveal that in comparison, both methods
A pH meter was the apparatus used to of determination were consistent with each other.
measure the pH via electrometry. It was
calibrated at pH 4, 7, and 10, then the pH of Table 2. ​Electrometric Determination of PH
20mL of each sample was measured. A small Samples pH at °C [H​+​]
amount of the Coca-Cola soda was diluted with
20 mL of distilled water. Once the pH was Distilled Water 5.56 at 22.4°C 2.75x10​-6
determined, it was used to calculate the
Assigned 2.95 at 22.3°C 1.12x10​-3
hydronium ions [H+] of the samples used. The Sample
pH of the buffer solutions were then adjusted to (Coca-cola)
the desired pH using Hydrochloric Acid (HCl) and
Buffer Prepared 2.03 at 23°C 0.0169
Sodium Hydroxide (NaOH).
 After the experiment was conducted, the Without the use of a redox-based pH meter, a
results showed that each sample has a varying qualitative analysis of the sample is used to
pH. For distilled water the pH is 5.56, for the determine the approximate color change of a
sample (Coca-Cola) the pH is 2.95, and for the given sample in specific pH level. Colorimetric pH
buffer solution the pH is 2.03. The pH of each determination showed that the distilled water had
sample was measured using a pH meter that was a pH range of 3.125 - 6.5 based on the color it
calibrated to ensure correct measurements. The exhibited for each of the indicators. The pH
temperature of each sample was also measured. determined from the meter, 5.56, is within this
The buffer solution specifically assigned was to range confirming that the colorimetric
be at pH 2. However, the solution prepared did determination is consistent. The buffer solution
not reveal to be exactly at pH 2. Possible errors meanwhile, had a pH range of 2 - 4.63 which is
arise from the weighing of the salt and the also where the electrometric determined pH, 2.03
pipetting of the acid to be able to reach an exact falls under and thus the results of the analysis
pH. Distilled water revealed to be slightly acidic are confirmed.
which was due to it absorbing carbon dioxide
from the atmosphere which dissolves in it, and
created a very dilute solution of carbonic acid REFERENCES
[1][5].

Electronic sources
Table 3. ​Colorimetric Determination of pH

pH samples
[1] Deziel, C. (2019, March 2). What Is the pH of
Acid-base
indicators
Distilled Water? Retrieved September 19 2019,
2 3 5 7 7.5 8 12 A B from
1​Thymol blue https://sciencing.com/ph-distilled-water-4623914
O O Y Y Y Y Y Y O
.html
2​ Bromophenol Y Y Y G B B B Y Y
blue [2] Irfan, U. (2018, May 29). S. P. L. Sørensen
3 ​Bromocresol invented the pH scale by experimenting with
Y Y G B B B B G Y
green beer. Retrieved September 19 2019, from
https://www.vox.com/2018/5/29/17404820/spl-s
4 ​Bromocresol Y Y Y P P P P Y Y
purple orensen-google-doodle-ph-scale

5 ​Phenol red Y Y Y R O O R Y Y [3] Libretexts. (2019, June 3). Buffers. Retrieved

September 19 2019, from
6 ​Methyl red R R O Y Y Y Y O R https://chem.libretexts.org/Bookshelves/Physical
7 ​Methyl _and_Theoretical_Chemistry_Textbook_Maps/Su
R R R Y O O O R R
Orange pplemental_Modules_(Physical_and_Theoretical_
Chemistry)/Acids_and_Bases/Buffers
8 C C C C C C R C C
Phenolphthalein
[4] Libretexts. (2019, June 5).
pH 5.56 2.03 Henderson-Hasselbalch Equation. Retrieved
September 19 2019, from
https://chem.libretexts.org/Ancillary_Materials/R
Legend:
eference/Organic_Chemistry_Glossary/Henderson
A ​- Distilled water
-Hasselbach_Equation
B​ - Buffer prepared
C​ - Colorless
[5] Why You Should Drink Distilled Water. (n.d.).
O​ - Orange
Retrieved September 19 2019, from
P ​- Purple
https://www.drdavidwilliams.com/distilled-water
R ​- Red
Y - ​Yellow