[Jono360]

As an assessment item I am currently in the process of writing-up a recent titration I did attempting to determine the pH of 5 soft-drink varieties. To calculate the values we were told to use m1v1 = m2v2 which for the most part seems reasonably correct. However the final results that I am now calculating are proving to be seriously inaccurate.

The five soft-drink varieties tested are Sprite, Sprite Zero, Schweppes Lemonade, Tru-Blu Ginger Ale and Mountain Dew. Each of these has a theoretical pH between 2.88 (Sprite Zero) and 4.2 (Ginger Ale). However all of the results I have determined thus far have been in the range of 1.1 to 1.24 with soft-drink that are meant to be the most acidic now appearing to be the least and visa versa. I am unsure as to the source of this error so I will include the method and calculations of my report thus far.

As a side note, the carbonic acid was removed from the soft-drinks so I was actually expecting a higher pH than the theoretical with only citric, ascorbic and benzoic acids being tested. Furthermore, the sodium carbonate solution described below was not freshly prepared, it had in fact been on a shelf for 2 weeks, however it was shaken prior to use to dissolve any collecting solids. The hydrochloric acid solution was standardised against the sodium carbonate of a known concentration, this was then used to standardise the hydroxide which acted as the base in the soft-drink titration.


Preparation of solutions:
Sodium carbonate (Na2CO3)
The mass of sodium carbonate required to make up 500ml of 0.05mol L-1 solution was first calculated using stoichiometry. The solid was then dissolved in 100mL of water and transferred into a 500mL volumetric flask. This was then filled to exactly 500mL with distilled water and mixed thoroughly through inversion. The sodium carbonate solution was then transferred to a storage bottle.
Hydrochloric acid (HCl)
The volume of 12mol L-1 hydrochloric acid required to make up 500ml of 0.1mol solution was first determined using molarity / volume equivalency calculations. The acid was then transferred into 500mL volumetric flask which was then filled to exactly 500mL with distilled water. The hydrochloric acid solution was then mixed thoroughly through inversion and transferred to a storage bottle.
Sodium hydroxide (NaOH)
The mass of sodium hydroxide powder required to make up 500ml of 0.1mol L-1 solution was first calculated through stoichiometry. The solid was then dissolved in 100mL of water and transferred into a 500mL volumetric flask. This was then filled to exactly 500mL with distilled water and mixed thoroughly through inversion. The sodium hydroxide solution was then transferred to a storage bottle.
Standardisation / Titration:
Hydrochloric acid (with Na2CO3)
Initially, the sodium carbonate solution was shaken to dissolve any precipitate that had formed in the storage bottle. A volume of both hydrochloric acid and sodium carbonate was then transferred into two separate beakers to allow for the sodium carbonate to be poured into the burette and the hydrochloric acid to be accurately moved between the glassware with the pipette. Using the digi-pipette, 20mL of the acid was then transferred into a conical flask and three drops of phenolphthalein indicator added to the solution. 50mL of the sodium carbonate from the beaker was then poured into a glass burette that was clamped to a retort stand. The conical flask containing the acid was placed on top of a piece of filter paper underneath the burette tap. The base was then gradually released into the acid until a lingering shade of light pink was obtained due to the presence of the indicator, as this denoted excess base meaning that the equilibrium point had been reached. The volume of sodium carbonate in the burette at this point was then recorded. If the colour of the solution was uncertain, the volume was noted and a single drop was released to test its effect on the shade of the solution. This process was then repeated multiple times until an adequate number of precise results were collected.
Sodium hydroxide (with HCl)
A volume of both hydrochloric acid and sodium hydroxide was transferred into two separate beakers to allow for the sodium hydroxide to be poured into the burette and the hydrochloric acid to be simply and accurately moved between the glassware with the pipette. Using the digi-pipette, 20mL of the acid was then transferred into a conical flask and three drops of phenolphthalein indicator added to the solution. 50mL of the sodium hydroxide from the beaker was then poured into a glass burette that was clamped to a retort stand. The conical flask containing the acid was placed on top of a piece of filter paper underneath the burette tap. The base was then gradually released into the acid until a lingering shade of light pink was obtained due to the presence of the indicator, as this denoted excess base meaning that the equilibrium point had been reached. The volume of sodium hydroxide in the burette at this point was then recorded. If the colour of the solution was uncertain, the volume was noted and a single drop was released to test its effect on the shade of the solution. This process was then repeated multiple times until an adequate number of precise results were collected.
Commercial Soft-Drink (with NaOH)
48 hours prior to the titration the soft-drinks were opened, shaken and allowed to rest in order to eliminate any dissolve carbon dioxide (CO2) in the drink and thus remove the carbonic acid (H2CO3) from the drink. A volume of both the tested soft-drink and sodium hydroxide was then transferred into two separate beakers to allow for the sodium hydroxide to be poured into the burette and the soft-drink to be simply and accurately moved between the glassware with the pipette. Using the digi-pipette, 20mL of the drink was then transferred into a conical flask and three drops of phenolphthalein indicator added to the solution. 50mL of the sodium hydroxide from the beaker was then poured into a glass burette that was clamped to a retort stand. The conical flask containing the tested soft-drink was placed on top of a piece of filter paper underneath the burette tap. The base was then gradually released into the drink until a lingering shade of light pink was obtained due to the presence of the indicator, as this denoted excess base meaning that the equilibrium point had been reached. The volume of sodium hydroxide in the burette at this point was then recorded. If the colour of the solution was uncertain, the volume was noted and a single drop was released to test its effect on the shade of the solution. This process was then repeated multiple times until an adequate number of precise results were collected and all soft-drink varieties had been tested.

4.1 Calculations
Preparation of solutions:
The mass of sodium carbonate powder required to make up 500ml of 0.05mol L-1 solution:
 n = mL
 n = 0.05 × 0.5   
 n = 0.025 mol   
 m = n × M   
 m = 0.025 × (2(23) + 12 + 3(16))
 m = 2.65 g
The volume of 12mol L-1 hydrochloric acid required to make up 500ml of 0.1mol solution:
 m_1 v_1  = m_2 v_2
 0.1 × 0.5 = 12 × x
 x = 0.004167 L
The mass of sodium hydroxide powder required to make up 500ml of 0.1mol L-1 solution:
 n = mL
 n = 0.1 × 0.5
 n = 0.05 mol   
 m = n x M
 m = 0.05 x (23 + 1 + 16)
 m = 2 g

Standardisation / Titration:
Standardisation of hydrochloric acid solution:
 2HCl+ Na_2 CO_3→2NaCl+H_2 O+CO_2
 m_1 v_1  = 〖2(m〗_2 v_2)
 x×0.02=2(0.05×0.0309)
 x=0.1545 mol∙L^(-1)

Standardisation of sodium hydroxide solution:
 HCl+ NaOH→NaCl+H_2 O
 m_1 v_1  = m_2 v_2
 0.1545×0.02=x×0.0176
 x=0.1756 mol∙L^(-1)
Sprite Titration:
 m_1 v_1  = m_2 v_2
 x×0.02=0.1756×0.0075
 x=0.06585 mol∙L^(-1)
 H^+=-Log(.06585)
 H^+=1.18
Schweppes Lemonade:
 m_1 v_1  = m_2 v_2
 x×0.02=0.1756×0.00655
 x=0.05751 mol∙L^(-1)
 H^+=-Log(.05751)
 H^+=1.24
Tru Blu Ginger Ale:
 m_1 v_1  = m_2 v_2
 x×0.02=0.1756×0.007416
 x=0.06511 mol∙L^(-1)
 H^+=-Log(.06511)
 H^+=1.19
 
Sprite Zero:
 m_1 v_1  = m_2 v_2
 x×0.02=0.1756×0.0065
 x=0.05707 mol∙L^(-1)
 H^+=-Log(.05707)
 H^+=1.24
Mountain Dew:
 m_1 v_1  = m_2 v_2
 x×0.02=0.1756×0.0067286
 x=0.05908 mol∙L^(-1)
 H^+=-Log(.05908)
 H^+=1.23
 
I would greatly appreciate ANY reason why the pH values are so low. Thanks in advance! (Seriously, if you answer this I will be eternally grateful)

[Borek]

As an assessment item I am currently in the process of writing-up a recent titration I did attempting to determine the pH of 5 soft-drink varieties. To calculate the values we were told to use m1v1 = m2v2 which for the most part seems reasonably correct. However the final results that I am now calculating are proving to be seriously inaccurate.

Titration is not a way of determining pH, it is a way of determining amount of acid. These are two different things. Your calculations would work for strong acids only, soft drinks contain mainly weak acids .

[DrCMS]

To determine the pH stick a pH probe in the drinks.