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A Titrimetric Study of the Effect of pH on Carboxyl Content of Oxidised Starch

1. Introduction

1.1 Background information

My mom likes to cook and sell her food, and she often uses food packaging such as plastic, but  this  led  to  a  huge  amount  of  waste  that  was  non-biodegradable.  After doing  some research,  I found that starch which was oxidised could be made into wrapping films as a biodegradable alternative, and I wondered how this typical foodstuff (starch) can be modified through chemical reactions.

The results of this experiment would contribute to the general understanding of the reaction condition, specifically pH, that is necessary to maximise the oxidation of starch as measured by the carboxyl content. While it may be intuitive to know that temperature can increase the rate of oxidation, using high temperatures may incur significant electricity and energy costs to be fully maintained. It is worth investigating other variables that can be used to maximise oxidation, like pH level which can be easily changed by the addition of certain alkaline or acidic substances.

During the investigation itself, some aspect of the methodology was modified to apply my learning.  The  common  method  of  starch   oxidation  is  to  use  sodium  hypochlorite  (in industries), but hydrogen peroxide may be a better reagent for this experiment because it decomposes into oxygen and water. Thus, it is more environmentally friendly. Discovering more about starch oxidation using hydrogen peroxide may benefit research into the possible transition from using sodium  hypochlorite to  hydrogen  peroxide as a  reagent  in  relevant industries.

1.2 Literature review

Apart from creation of biodegradable films, oxidised starch has many applications in other industries, such as paper printing and textile industries. Starch itself can be typically oxidised by  a  variety  of  oxidising  agents  such  as  sodium  hypochlorite  and  hydrogen  peroxide (Tolvanen et al., 2011).

Figure 1.2 - Oxidation of starch by hydrogen peroxide with an iron catalyst (Tolvanen et al., 2011)

The proposed mechanism is seen in Figure 1.2 above, though it should be noted that the reaction mechanism may change depending on the reaction conditions (Dias et al., 2011). At first, the hydroxyl groups undergo oxidation to form carbonyl groups then carboxyl groups (Sukhija et al., 2017). The extent of starch oxidation can therefore be measured by how many  carboxyl  groups  are  formed  at  the  end  of the  reaction.  This  is expressed as the number of carboxyl groups per 100 glucose units (COOH/100 GU) (Fonseca et al., 2015) or as a percentage.

Table 2.1.2 - Controlled variables

2.2 Apparatus and materials

15. Place this slurry in 100.0 cm3  of distilled water.    in conical flask

16. Heat this starch slurry in a boiling water bath for 15 minutes to gelatinise.

17. Add 0.005 mol dm-3 NaOH into a burette.

18. Add 3 drops of phenolphthalein as an indicator.   in the flask

19. Immediately titrate the heated sample with 0.005 mol dm-3 NaOH.

20. Measure the volume of NaOH needed to make the solution reach pH 8.30 in step 17.   neutralized

Figure 2.3 - Computer drawn figure of the experimental setup

2.4 Evaluation of procedure

Several    important    modifications    were    considered.    Originally   the   titration    utilised phenolphthalein as an indicator for the first few trials. This was preferred because the colour change of the indicator would be sharp and quick, allowing the titration to stop immediately close to the equivalence point, in comparison to a pH meter that has a lag time. However, the opaque nature of the starch slurry made the colour change hard to see at times. Hence the titration is done until a pH of 8.30 is reached, and this is in line with other methodologies by   Matsuguma   et   al.   (2009).   Notably   the   range   pH   8.30   to   10.00   is   also   when phenolphthalein would sharply turn pink. To prevent overshoot, NaOH should be added in small increments (not continuously) and the reading on the pH meter should stabilise before adding more NaOH.

Additionally, after several preliminary trials, it was found that using 0.010 mol dm-3 NaOH for the titration led to very small titre volumes. Thus the solution was diluted in order to obtain titre  values  within  the  20.0  cm3   to  increase  precision  of  the  readings  (more  significant figures).

2.5 Risk assessment

● Safety considerations: H2O2  is a strong oxidiser. Also, skin exposure may result in severe skin burns (Lab Chem, 2017). Though NaOH, HCl and AgNO3  are non-toxic at smaller concentrations, they can still be irritants or caustic in nature. Thus rubber gloves are utilised in handling all chemicals throughout the experiments. A lab coat is also worn along with safety goggles.

● Ethical considerations: There were no ethical considerations to take into account.

● Environmental considerations: Avoid  release of chemicals into the environment. Chemicals should  not  be  poured down the sink.  Instead  all  hazardous chemicals should be collected in a specific container for hazardous substances, to be disposed of professionally by the school lab.

3. Data collection

3.1 Qualitative observations

Before  heating, starch  is  insoluble  in water.  Most of the starch  would  settle  to  the  bottom  of  the  beaker  if  not periodically mixed. The resulting suspension is an opaque white  colour.  In  terms  of  colour,  oxidised  starch  was indistinguishable from unmodified starch.

Figure  3.1 shows  the  suspension,  with  all the  oxidised starch, after initial titration with NaOH until pH 8.30. A faint pink colour is produced in the titration, due to the presence of  the  phenolphthalein  indicator,  thus  showing  that  the acidic carboxyl groups  have  been  neutralised.  However, this pink colour is faint due to the colour of the starch not being fully colourless. Additionally, it was observed that the pink colour would form, then after stirring, disappear if the colour was not strong enough.

3.2 Raw data

Table 3.2 - Initial and final burette readings of the 0.005 mol dm-3 NaOH titre against oxidised starch.