FR Experiment1rev

P O T E N T I O M E T R I C M E A S U R E M E N T SP a g e | 1

Volume No. 3 Issue No. 1

POTENTIOMETRIC MEASUREMENTS

Dr. Kathlia De-Castro Cruza ; Elijah P. Ugaddanb

ABSTRACT

Potentiometry is an electroanalytical technique which uses the potential difference

developed across the electrodes that make up the electrochemical cell to

quantitavely analysed the amount of the analyte in a particular sample such as the

concentration. In the case of this experiment, the pH of shampoo, conditioner and

shampoo + conditioner were evaluated using the pH meter. The pH obtained in all

samples ranges from 4.0 – 6.8 which is relatively acidic. Diluting these samples,

the pH increased to less than 7. The experiment also determined the different

amount of fluoride in the following samples of: toothpaste, mouthwash, tap water

and NaF stock solution without Total Ionic Strength Adjustment Buffer (TISAB)

using the fluoride ion selective electrode (ISE). These were statistically treated

with two analytical techniques namely the External Standard and Standard

Addition Method. The sample of the highest fluoride content is toothpaste which

accounts for (0.1756 M-0.18 M) while the least of them is the stock solution

without the buffer with a concentration of (5.046E-19 M). Buffer plays a vital role

in potentiometric measurements. Between the two analytical techniques used,

standard addition method showed a more accurate and defined result.

Keywords: buffer, electrode, external standard, potential, potentiometry, pH,

standard addition

INTRODUCTION

Electroanalytical techniques relate

two different field one of which is

governed by physical science and the

other transpires as a physical

phenomenon; chemistry and

electricity respectively. These

techniques evaluate the various

electrical quantities such as potential,

current, or charge at which it relates

to quantitative chemical parameters

for instance that of the concentration

of the analyte in the solution. Various

a. Professor of CHM115L, School of Chemistry and Chemical Engineering, Mapua Institute of Technology.b. Bachelor of Science in Chemistry and Chemical Engineering Student of CHM115L, Mapua Institute of Technology;


electroanalytical techniques have

been developed including;

voltammetry, coulometry,

electrogravimetry and potentiometry.

The latter is extensively studied in

this experiment.

Potentiometry is an electroanalytical

technique which utilizes potential (V)

of electrochemical cells for

quantitative analysis such as that of

concentration, neglecting the effect of

current applied in the system. A

typical cell for this analysis is

represented by:

Reference electrode | Salt Bridge |

Analyte Solution | Indicator Electrode

The reference electrode is an

electrode independent of the quantity

of the analyte in the solution and it is

used as a reference because of its

known potential. The reference

electrode used in the experiment is

the Standard Hydrogen Electrode

(SHE). Moreover, the indicator

electrode is dependent on the activity

of the analyte to which the solution is

immersed and to which a potential

(Eind) is obtained. The indicator

electrode used is an ion-selective

electrode (ISE) particularly the pH

electrode and a fluoride electrode.

The overall measured potential is

expressed as Equation 1.1:

Ecell=Eindicator−Ereference+Ejunction

(Equation 1.1)

For an unknown standard potential of

the Ecell, Nernst Equation can then

be applied given in Equation 1.2



(Equation 1.2)

Where n is the number of electrons

involved in the reaction; [A] is the

concentration in molarity; a is the

numerical coefficient in the reaction

The two analytical techniques used to

treat the data in the experiment are

the Standard Calibration Method and

Standard Addition Method. The first

determines the potential from

different standard solution with

known concentration. These data are

then used to plot a linear graph of

potential and the logarithm of the

concentration.

Figure 1.1 Graph of Standard Calibration

Method

The equation used to express this

graph is given by Equation 1.3;

E=Slog (C )+K

(Equation 1.3)

E is the potential at respective

concentration; S is the slope of the

equation; C is the concentration of the

solution; K being the y-intercept

Standard Addition Method is another

analytical technique that measures

the potential of the system before and

after adding a known volume of the

standard to a fixed volume of the

sample. The response is given by the

graph and linear plot is extrapolated

to the concentration axis as shown in

Figure 1.2

Figure 1.2: Graph of Standard Addition

Method

The objectives of the experiment are

to have an insight of potentiometric

measurements using various kind of

electrode, to be able to determine the



different pH measurements of

shampoo, conditioner and

shampoo+conditioner (2-in-1)

samples. Lastly is to be able to

determine the fluoride content of

mouthwash, tap water and toothpaste

using the fluoride meter electrode and

compare using the analytical

technique of standard calibration and

standard addition method.

MATERIALS AND METHODS

The apparatus used are beakers of

specification 25 mL, 100 mL and 250

mL, iron stand, pipets with speciation

10 mL, iron clamp, burette, pH meter,

volumetric flask with spec. 100 mL

and 250 mL and fluoride-ion selective

electrode. The reagents are buffer

solutions of pH 4, 7 and 9 , TISAB

(total ionic strength adjustment

buffer) and 0.1 M F stock stock

standard solution. Samples used are

shampoo (Sunsilk), conditioner

(Creamsilk), shampoo + conditioner

(Black Beauty), toothpaste (Close-Up),

mouthwash (Colgate Plax) and tap

water.

Methodology

I. pH determination of hair shampoosA. Calibration of electrode (pH meter)

B. pH measurements of the samples ( Shampoos, Conditioner, Shampoo + Conditioner)


Use buffer solutions pH 4, 7 , 9

Adjust pH to read pH of buffer sol'n


II. Determination of fluoride in various samples A. Sample Preparation (Toothpaste)

B. Sample Preparation (Mouth wash and tap water)


5 mL sample to 25 mL beaker ; measure pH

After using electrode: rinse with water & blot with soft tissue paper

5 mL sample + 45 mL dis water (*burette) in 100 mL beaker

Measure and record pHAfter using electrode: clean and

calibrate

0.20 g toothpaste in 300 mL beaker + 50 mL TISAB sol'n. Stir and boil for

2 min

Cool. Transfer to 100 mL vol flask. Dilute to mark with dis water. Mix


C. Determination of fluoride samples using Calibration Curve Method

D. Determination of fluoride samples using Standard Addition Method 1. Prepare in 100 mL vol flask

2. Measure potential 3. Plot the Response ((Vo + Vstd)10⌃((E2-E1)/S))) vs (CV)std4. Determine concentration of fluoride in sample

RESULTS AND DISCUSSION


25 mL of mouth wash + 50 mL TISAB sol'n in 100 mL vol flask

Dilute to mark. Mix

25 mL of 100 ppm std. to 500 mL vol flask. Dilute to mar

Transfer 5 mL, 10 mL, 25 mL, 50 mL of 5 ppm solution to 100 mL vol flask

Add 50 mL TISAB sol'n each and dilute to mark

Determine potential of each standard. Prepare calibration Curve

(E vs. log C)

Determine potential of each sample (toothpaste, mouthwash)

From graph deterine concentration of each sample

Do 3 trials. Get mean.

Confidence limit at 95% CI


I. pH Measurements of Shampoo, Conditioner, Shampoo +

Conditioner

Tag Number Concentrated DilutedSunsilk 1 5.61 6.19Pantene 2 5.70 6.12Dove 3 4.98 5.83Creamsilk 4 4.01 4.63Pantene 5 4.05 4.59Dove 6 4.34 5.12Black Beauty 7 5.71 6.33Palmolive 8 6.76 6.92Palmolive 9 6.56 6.9

pH

Shampoo

Conditioner

Shampoo + Conditioner

Sample

Table 1.1 Table of pH of samples

Figure 1.3: Graph of pH measurements of shampoo, conditioner, shampoo + conditioner

Initially, the pH electrode was

calibrated with a buffer solution of pH

4,7 and 9 in order to examine a more

accurate results in the pH of the

samples to be subjected as well as to

ensure that the electrode is working

properly. Buffer solution was used to

calibrate the pH electrode because

this solution is pH sensitive and

maintains a constant a pH through a

period of time as compared to a pure

acid or base. Introducing any ions in

the buffer solutions for instance

allows the buffer solution to prevent it

from adjusting its pH from rising or

falling. Buffer solutions are suitable

for calibrating the pH meter.



It can be generalized from Figure 1.3

that the pH of shampoo, conditioner

and shampoo + conditioner ranges

from 4-6.8 which varies between

acidic to slightly acidic. The reason

behind the acidic pH is because of the

nature pH of the hair which is also

relatively acidic and this makes it

more compatible to penetrate through

the hair scalp. The hair follicle is a

sebaceous gland that secretes sebum,

an oily substance which protects the

hair from drying out. Though it is

beneficial in desirable amounts, it also

poses a disadvantage if present in

excess by clogging the pore of the

hair roots causing dandruff and hair

loss.

In order to eliminate this; aqueous

shampoo, conditioner and shampoo +

conditioner of relatively low pH are

used. Shampoos, conditioners and

shampoo + conditioner just like

detergents contain surfactants. These

surfactive agents form into micelles

which can then solubilize sebum. The

primary surfactants in the shampoo

contain lauryl sulfate. Since the

natural pH of hair is acidic, the acidity

of the shampoo, conditioner and

shampoo + conditioner allows the

moisture to be sealed in the hair

cuticle, close it and help it return to

its natural state. If the pH of the

following samples mentioned were

relatively high, this may take effect in

the opening of the hair cuticle

allowing moisture to diffuse out from

the system causing hair damage and

dry, frizzy hair.

The reported pH of the samples after

dilution showed an increase in its

value. A lower pH value does not only

indicate that the solution is acidic but

also corresponds to the concentration

of hydronium ions [H+] being

significantly in greater amount than

its counterpart hydroxide ion [OH-], in

which the latter causes a higher pH

value and increased alkalinity in the

solution.

Dilution uses water which is a polar

protic solvent with a pH of 7, falls on

the neutral region thereby plays a role

of either a base or an acid depending

on pH nature of the analyte. The case

of which with respect to the shampoo,

conditioner and shampoo +

conditioner samples, the water acted

as a base which deprotonated the

[H+] ions from the samples giving it a

more negative charge on its ion. This

implies that more [OH-] coming from

water dissolves into the lesser amount

of [H+] in the sample. Although an

increase of pH may occur, the pH



value after dilution will not be greater

than 7 since the water was never

basic in the first place.

II. Determination of Fluoride

A. Direct Potentiometry

ppm M

1 0.5 2.63E-05 96.332 1.0 5.26E-05 94.973 2.5 1.32E-04 93.274 5.0 2.63E-04 92

ConcentrationStandard Solution E (mV)

Table 1.2: Standard Calibration Method using Standard Solutions of NaF

Figure 1.4: Calibration Curve of Standard Calibration Method

From Table 2.2, the calibration curve

was constructed by plotting the

logarithm of the concentration and its

potential. The reason behind taking

the logarithm of its concentration is

because of the log parameter in the



Nernst Equation (Equation 1.2). The

plot shows a linear and indirect

relationship of concentration and its

potential. These standard solutions

prepared from a mother solution of

100 ppm NaF are used to identify the

concentration of the unknown

samples of toothpaste, mouth wash

and tap water.

The fluoride ion selective electrode’s

mechanism is a double junction

electrode or a combination electrode

wherein the reference and indicator

are built into one single probe. In this

electrode, an active membrane made

up of a single crystal LaF3 which is

fixed with europium (II) to facilitate

the mobility of ion transportation.

Figure 1.5: Schematic Representation of the fluoride ISE

Fluoride ions (F-) from the electrode

and analyte solution bind to the

crystal in the membrane to form

LaF4-.Because of the activity of the F

in the analyte solution different to

that of the electrode solution, a

measured potential difference is then

obtained across the membrane.

Upon preparation of the standard

solution; Total Ionic Strength

Adjustment Buffer (TISAB) is used.

TISAB controls the pH of the fluoride

solution constant. Since a possibility

of formation of hydroxide ions (OH-)

because of the relative acidity of the

fluoride solution, the ion-selective

electrode (ISE) is somewhat sensitive

to the (OH-) and may account for the

activity of the hydroxide ions giving

erroneous results. TISAB is used to

prevent this from occurring.



Secondly, TISAB is also used because

it contains a complexing agent known

as trans-1,2

cyclohexanedinitrilotetraacetic acid

(CDTA) to combine with

contaminants such as (Al3+ and Fe 3+)

which leaves them inert . These

contaminants may form complexes

with (F-) causing a different

potentiometric measurement.

Al 3+(aq) + 4F-

(aq) <-> AlF4-(aq)

Figure 1.6: Structure of trans-1,2

cyclohexanedinitrilotetraacetic acid

(CDTA

Using the calibration curve method,

an equation of the line can be

obtained and this represents Equation

1.3

E=Slog (C )+K

E is the potential at respective

concentration; S is the slope of the

equation; C is the concentration of the

solution; K being the y-intercept

Although the value of S theoretically

was obtained at 59.2 as compared to

the experimental S of 4.322; this is

still acceptable since the linearity (R2)

is close to unity (1). Using the

equation generated from Figure 1.4;

the following fluoride ions of the

unknown samples are tabulated in

Table 1.3

Sample Tag No. E (mV) [F-], MMouthwash (Colgate Plax) 1 99.7 1.60E-03

Toothpaste (Close Up) 2 95.6 0.18Stock Solution without Buffer 3 32.7 5.046 E-19

Tapwater (N405, Mapua Intramuros) 4 96.3 2.62E-04

Table 1.3: Sample with given potential

and calculated unknown concentration

The fluoride content of the following

samples in decreasing order shows:

Toothpaste > Mouthwash >Tap Water

> Stock Solution without Buffer

As compared to toothpaste and

mouthwash; mouthwash is diluted

with water therefore it gives lesser

amount of fluoride as that of the

toothpaste. Tap water is maintained

to contain at least less than 4ppm of

fluoride content in municipal water.

The stock solution has the least



fluoride content because it does not

contain TISAB. Consequently, without

the buffer solution the hydroxide ions

(OH-) somehow affect the read-out of

the ISE since there is no control of pH

in the solution. The solution is also

prone to contaminants such as Al3+

and Fe3+which gives a different

measurement.

B. Standard Addition Method

Figure 1.7 shows the corresponding linear curve for Standard addition method of Samples

In this method a known volume of the

sample and standard is used. The

concentration of the unknown sample

depends on the response of the

system from its potential. The given

equation formed corresponds to

Equation 1.4;

(Vo+Vstd)Vo

10E2−E1S =Vo+ 1

CunkCstdVstd

(Equation 1.4)

y=b+mx



Where: E2 is the potential after

addition of standard; E1 potential

before standard; S is the slope of the

calibration curve in Figure 1.4

The standard addition graph in Figure

1.7 is linear and passes through the

origin. To get the concentration of the

unknown, the data is extrapolated

until the response reached a zero

signal wherein the point crosses the y

axis and touches the x-axis.

The obtained concentrations of the

analytes are tabulated below (Table

1.4);

Sample Vo Vstd CstdVstd E,mV Response [F-], M5 0 0 81.6 05 10 0.002631579 78.7 0.2445469155 20 0.005263158 81.4 0.3959207645 30 0.007894737 90.9 0.496865447

5 0 0 127.7 05 10 0.002631579 115.6 0.1489560135 20 0.005263158 117.4 0.2404884035 30 0.007894737 116.5 0.342123897

5 0 0 122.7 05 10 0.002631579 105 0.176415735 20 0.005263158 109.1 0.2763241975 30 0.007894737 109.7 0.383227121

Tapwater (N405, Mapua Intramuros)

1.22E-03

0.1756

1.86E-04

Mouthwash (Colgate Plax)

Toothpaste

Table 1.4 shows the tabulated results for Standard Addition Method

From the graph, toothpaste still has the most fluoride content among the samples

and tap water contains the least of it. Comparing the two analytical techniques

used to determine the fluoride as shown in Table 1.5;

Standard Calibration Standard Addition[F-], M [F-], M

Toothpaste (Close-up) 0.18 0.1756Mouthwash(Colgate Plax) 1.60E-03 1.22-03

Tap Water 2.62E-04 1.86E-04

Analytical TechniqueSample

Stock Solution without Buffer N/A5.046 E-19

Table 1.5 shows the comparison of results between Standard Calibration and Standard Addition



The advantages of using Standard

Calibration or External Standard

allows a series of standard with a

known quantity be compared to that

of multitude of samples to be

analyzed. The standards are analyzed

separately from the analyte. In this

method, recalibration is very much

simple since a single point will be

recalibrated for instance. This is most

commonly utilized for routine

analysis. This method does not

account for the interferences (pH

change, contaminants, temperature

change) that can happen in the

matrix. The matrix is anything in the

unknown except the analyte. To

remedy this problem, standard

addition method is used thus giving

more accurate results as that of the

external standard. Standard Addition

Method reduces these matrix effects

by spiking known amounts of

standard to the analyte giving more

accurate and desirable results.

CONCLUSION

Potentiometry uses the potential

difference developed across the

reference and indicator electrode to

denote the quantity of the analyte in

the sample. Various electrodes were

used in the experiment such as the pH

meter and the fluoride ion selective

electrode (F- ISE). Measured pH of

shampoo, conditioner and shampoo +

conditioner displayed a relatively

acidic pH. The acidic nature of these

samples was due to the compatibility

of the natural pH in the hair cuticle.

Diluting the concentrated sample

increases the pH of the sample.

Moreover, F-ISE determined the

fluoride ion content through external

standard via the calibration curve and

the standard addition method. The

relationships between the potential

remains to be fundamental in

evaluating the concentration of the

toothpaste, mouthwash, tap water and

stock solution without buffer.

Toothpaste has the highest fluoride

content amongst all other sample. The

least of them is the stock solution

without buffer. Buffer such as TISAB

plays a great role in obtaining

accurate potentiometric

measurements. Additionally, standard

addition method provided a more

accurate result of the concentration

obtained as compared to that of the



external standard due to the accounting of the matrix interferences

occurring in the sample.

REFERENCES

[1] D.A. Skoog and J.J. Leary, “Principles of Instrumental Analysis, Sixth Ed., Saunders College Publishing, Orlando, FA, 1992, Chap. 22, particularly pp. 499-511.

[2] T.S. Light and C.C. Cappuccino, "Determination of Fluoride in Toothpaste Using an Ion-Selective Electrode", J. Chem. Ed. 1975 52(4) pp. 247-250.

[3] R.D. Braun and F.H. Walters, Laboratory Experiment No. 35, "Determination of Fluoride in Tap Water With the Fluoride-Selective Electrode" in "Applications of Chemical Analysis", McGraw-Hill Book Co., New York, 1982, pp. 222-226

[4] Willard, Merritt, Dean and Settle, Jr., “Instrumental Methods of Analysis, Seventh Ed., Wadsworth, 1988, Chap. 22 and particularly pp. 682-691

[5] D.A Skoog and West, “Principles of Analytical Chemistry. Ninth Ed., Saunders College Publishing, Orlando, FA, 1992, Chap. 21


FR Experiment1rev

Documents

Transcript of FR Experiment1rev