EXPERIMENT 6: Measurement of potassium hydroxide number (KOH number) of

NR latex

6.1 Objective

6.1.1 To determine the KOH number of NR latex according to ISO 127: 1984 (E).

6.1.2 To study the quality-assurance of NR latex by using KOH number.

6.2 Introduction

KOH number is a measure of the content of anions in latex, including volatile fatty acids,

higher fatty acids, phosphates, carbonates and bicarbonates. Most of the volatile fatty

acids are the products of bacterial activity on latex due to inadequate preservation and

normally measured separated by the VFA number. The presence of carbonate and

bicarbonates may be due to the absorption of carbon dioxide during exposure of latex to

air. The other anions are inherent components of the latex systems, the contents may be

varied from clone to clone. The KOH number is therefore defined as the number of

grams of potassium hydroxide equivalent to the acid radicals combined with ammonia

containing 100 grams of total solids. A higher KOH number indicate the poorer

preservation of latex.

The test is usually used to measure the formation of volatile fatty acids by bacterial or

microorganism’s activities on the non-rubber substances during the storage and the

higher fatty acids formed by alkaline hydrolysis of lipids in the natural rubber (NR) latex.

The latter can be explained by latex contains protein-like substances, carbohydrates and

lipid that tend to undergo hydrolysis process during the storage. In other words, the KOH

number measurement provides the indication of the total concentration of acidic

substances present in the latex. Although the concept of potassium hydroxide number is

in principle applicable to all types of aqueous lattices, in practice it is of interest only for

natural rubber lattices which are preserved wholly or partly with ammonia and with no

fixed addition of alkali: thus it is not applicable for natural rubber lattices that preserved

with potassium hydroxide.

In simply word, the KOH number is determined by reducing the ammonia content of the

latex to a specified level by reaction with formaldehyde, diluting with water to a given

total solids content and then titrated with carbonate-free aqueous potassium hydroxide

solution of specified concentration. The purpose is to determine the concentration of

ammonium ions in the latex aqueous phase by titrating with hydroxide ions according to

the reaction as shown follow. Based on ISO 127: 1984(E), KOH number testing, the

KOH can be calculated based on the formulation below by obtained the volume of the

potassium hydroxide solution that required to reach the titration end-point or the

neutralize point.

KOH No=561x c x VW TS x m

Where,

c is the concentration of the potassium hydroxide solution, expressed in moles of KOH

per cubic decimeter.

V is the volume of the potassium hydroxide solution required to reach the end- point in

cm3.

WTS is the total solid content of the latex, as percentage of mass.

m is the mass of the latex sample in gram.

6.3 Materials

New natural rubber (NR) latex, Old natural rubber (NR) latex, distilled water and

potassium hydroxide solution (0.5 M)

6.4 Equipments

pH meters, beaker, magnetic stirrer, glass electrode and burette

6.5 Procedure

1. The pH meter is calibrated by using the method specified in ISO 976.

2. 50.00 g of new natural rubber (NR) latex is weighted. Then, the latex is poured into a

400cm3 beaker.

3. The latex is diluted with water to about 30% (m/m) total solids content by the addition

of distilled water. The solution is stirred homogeneously by the mechanical stirrer.

4. The glass electrode of pH meter is inserted into the latex and 5cm3 potassium

hydroxide solutions (KOH) is added slowly while stirring slowly with the magnetic

stirrer.

5. The resultant equilibrium pH reading is recorded.

6. 1cm3 of potassium hydroxide solution is added with continued stirring in interval of

about 30 seconds.

7. The equilibrium pH after each addition is recorded until the end-point has been

passed, where the pH of the sample is constant for 3 consecutive times.

8. Step 1 to 7 is repeated with old NR latex sample until the result of duplicate

determinations is within 5%.

6.6 Result and Discussion

Continuous addition of alkaline solution of potassium hydroxide will cause an increment

of pH value in the latex sample with an increment of 1cm3 of the solution every time, as

shown in Figure 1. The end point of the titration is the point of inflexion of the titration

curve of the pH value against the volume, in cubic centimetres, of potassium hydroxide

solution. The end point can be characterized by the slope of the first derivative curve

dpHdV

which reaches a maximum and the drop back, as shown in Figure 2.

0 2 4 6 8 10 12 149

9.5

10

10.5

11

11.5

12

12.5

New NR latexOld NR Latex

KOH Volume (cm3)

pH

Figure 1: pH latex versus volume of KOH added to 30% (m/m) TSC of new and old NR

latex (zero-derivative curve)

0 2 4 6 8 10 12 14 160

0.050.1

0.150.2

0.250.3

0.350.4

0.45

New NR LatexOld NR Latex

KOH Volume (cm3)

dpH/

dV

Figure 2: dpH/dV latex versus volume of KOH added to 30% (m/m) TSC of new and old

NR latex (first-derivative curve)

KOH volume at the point of inflexion in the titration curve can be observed in Figure 2

which is 9.0 cm3 for the new NR latex sample and 13.0 cm3 for the old NR latex sample.

The KOH number of both new and old NR latex is 0.83 and 1.24 respectively as

calculated in Appendix. The KOH number obtained shows that new NR latex have better

quality (more colloidal stable) than old NR latex before both NR latex samples were

taken to do the testing. This result is satisfactorily to accept since the microorganism and

bacterial activity and the rate of spontaneous hydrolysis in the old NR latex is vigorous

which will liberate more acid radical and destabilize the colloidal latex system. Hence,

old NR latex shows higher KOH number.

In general, the potassium hydroxide (KOH) number can be defined as the number of

grams of potassium hydroxide equivalent to the anions present as ammonium salts in a

quantity of latex which contains 100 grams of total solids. The test is usually used to

measure the formation of volatile fatty acids by bacterial or microorganism’s activities on

the non-rubber substances during the storage and the higher fatty acids formed by

alkaline hydrolysis of lipids in the natural rubber (NR) latex. The latter can be explained

by latex contains protein-like substances, carbohydrates and lipid that tend to undergo

hydrolysis process during the storage. In other words, the KOH number measurement

provides the indication of the total concentration of acidic substances present in the

latex.

The KOH number of the latex is used as a rough quality-assurance criterion for

ammonia-preserved NR latex. These acids will react with ammonia and produce

ammonium salts, as shown in Equation 1. Generally, the test is assumed that the quality

of the NR latex is poorer when the KOH number of the latex is higher due to the higher

concentration of acidic substances in the latex need to be titrated by KOH.

HA + NH3 A- + NH4+ NH3 + H20 (Eqn

1)

Since the ammonium ions are weakly acidic, the end-point of the titration occured at high

pH, which is about 11.98 as in case in old NR latex which shown in Figurre 1. Figure 1

illustrates that the pH of both latex increases rapidly with the addition of KOH solution

initially. However, the gradient of the graph decreases with increasing volume of KOH

after 13.00 cm3 of KOH solution being added in old NR latex sample. This is probably

due to the acid contents in the latex depleting after neutralized by potassium hydroxide.

Besides that, 13.0 cm3 of KOH solution added into the latex is the indication of the end-

point reached in the determination, as shown in Figure 2.

There are no suitable colour-change indicators by which the end-point can be

satisfactorily detected. The problem is not the high pH at which the end-point occurs;

there are indicators such as alizarin yellow R and brilliant crescyl blue, which change

colour in the region pH 9-11. The problem is that the rate of change of pH with alkaline

solution is comparatively sluggish, even in the vicinity of the end-point. Thus, pH meter is

used to measure the end-point of the titration

According to the ISO 127, KOH number of the NR latex should not exceed 1.0. The

calculated KOH number of the NR latex sample is found to comply with ISO 127 which is

0.617. The accuracy of the result obtained might be caused by several factors:

1. The NR latex used is fresh latex. The storage of latex is not too long and the

formation of acids and other anions from the long-chain carboxylic acids by

hydrolysis of lipids is lesser. Thus, lesser amount of KOH is being added to the

ammonia-preserved latex.

2. The presence of the inorganic polymer in the NR latex such as phosphate and

magnesium in minor amount will affect the KOH number for the latex. For this

reason, the presence of magnesium and phosphate will produce higher

concentration of ammonium salt, such as ammonium phosphate salt when react with

ammonia in the latex. The ammonium salts can react with the hydroxide ions during

the titration and subsequently influence the KOH number of the latex.

3. The KOH number test might take accounts of cations which present in the latex and

react with the hydroxide ions as ammonium ions. For example, alkylammonium ions

can react with hydroxide ions in a similar way to ammonium ions that can be

included into the KOH number. As a result, the measure on the KOH number of the

latex sample does not reflect the actual concentration of anions present in the latex

that equivalent to the cations.

4. Glass electrode used by the previous user was not sufficiently cleaned, and cause

some latex used by previous user stick on the glass electrode, therefore when we

use the glass electrode to test our latex sample, we need to clean the glass

electrode in order to get more accurate result.

Theoretically, the changes in total solids content (TSC) and alkalinity can affect the KOH

number of the NR latex. The higher the total solids content in the latex concentration, the

lower the KOH number of the latex. The total solid content of NR latex can be attributed

by the presence of proteinaceous substances, carbohydrates and lipid substances in the

latex. When the hydrolysis of these substances occurred in the latex, the TSC of NR

latex will be reduced. As a result, the presence of the higher fatty acid in the latex can

increase the KOH number of the latex. Hence, the KOH number is expected inverse

proportional to the total solids content of the latex.

On the other hand, the KOH number of the NR latex is expected decrease with

decreasing of alkalinity. This can be attributed by the decreasing ammonia content in the

latex lead to lesser of free ammonia to react with fatty acids. Subsequently, lesser

anions of ammonium salts produced in the latex which is readily to react with hydroxide

ions during titration. For this reason, the volume of KOH required to reach end-point was

reduced.

6.7 Conclusion

Reference

Anil K. Bhowmick & Howard L. Stephens (2001). Handbooks of Elastomers- Second

Edition, Revised and Expanded. New York: Marcel Dekker, Inc

Blackley, D.C. (1997). Polymer Latices: Science and Technology Vol 1: Fundamental

Principles. New York: Springer.

Appendix

(1) Dilution of 50g total solid NR latex to 30% total solid content

M1V1=M2V2

For new NR latex:

Initial total solid content (% m/m) 61.1Initial Volume (cm3) 50Final total solid content (% m/m) 30Final Volume (cm3) 101.67

Hence, the distilled water added is V=101.67-50 g = 51.6 cm3

For new NR latex:

Initial total solid content (% m/m) 59.0Initial Volume (cm3) 50Final total solid content (% m/m) 30Final Volume (cm3) 98.33

Hence, the distilled water added is V=98.33 – 50.00 g = 48.33 cm3

(2) Calculation of KOH number for new and old NR latex sample

KOH Noof new NRlatex=561 xc xVW TS xm

¿561x 0.5 x9.030x 101.67

= 0.83

KOH Noof old NRlatex=561x c xVW TS xm

¿561x 0.5 x13.030x 98.33

= 1.24

Where,

c is the concentration of the potassium hydroxide solution, expressed in moles of KOH

per cubic decimeter.

V is the volume of the potassium hydroxide solution required to reach the end- point in

cm3.

WTS is the total solid content of the latex, as percentage of mass.

m is the mass of the latex sample in gram.

Table1: The corresponding pH value, dpH value and dpH/dV value with respect to the

volume of KOH solution added for new NR latex

Volume pH dpH dpHdv

0 10.49 0.00 0.000

5 11.03 0.54 0.108

6 11.19 0.16 0.160

7 11.39 0.20 0.200

8 11.65 0.26 0.260

9 12.01 0.36 0.360

10 12.31 0.30 0.300

11 12.56 0.25 0.250

12 12.75 0.19 0.190

Table2: The corresponding pH value, dpH value and dpH/dV value with respect to the

volume of KOH solution added for old NR latex

Volume pH dpH dpHdv

0 10.14 0.00 0.000

5 10.51 0.37 0.074

6 10.59 0.08 0.080

7 10.70 0.11 0.110

8 10.82 0.12 0.120

9 10.95 0.13 0.130

10 11.12 0.17 0.170

11 11.32 0.20 0.200

12 11.59 0.27 0.270

13 11.98 0.39 0.390

14 12.36 0.38 0.380

15 12.64 0.28 0.280