How would I perform a titration for quality control analysis on sodium levels in a soda and vit. C levels in coconut water?

Question

In chemistry lab we are supposed to perform quality control analysis on a basic product and an acidic product. I chose Sprite and coconut water, but I'm trying to figure out how to do the experiment. We are told to figure out what we will do on our own, and I want to do it correctly.

For the Sprite - I would like to test the sodium levels to determine how close they are to the amount listed on the bottle.

For coconut water - I would like to test if the vitamin C content is close to the amount listed on the bottle.

Can someone help explain how to do the procedures and the amounts to use, etc.?

I am a bit confused since Sprite is generally an acidic product, but if we are testing for sodium, which is basic, wouldn't we need to titrate using HCl? Can you titrate acid into an acidic product to find out sodium levels?

Same with coconut water - it is generally alkaline, but vit. C is acidic. So how do we titrate?

 

I am hoping someone can give me a precise procedure to follow.

Please help me! :)

Answer 1

Vitamin C (ascorbic acid) in coconut water can be determined by an oxidation-reduction reaction called iodimetry (titrate with iodine in the form of I₃^-)  Google " method to determine ascorbic acid in coconut water" to find a ppt file at ycp.edu  -- might say webpage not found, by try refresh)  This will give you a procedure.

Be aware that an acid-base titration would not be specific enough to distinguish ascorbic acid from other acidic components.

 

Determining sodium content by a titration method is not easily done.  I did see a complexometric titration method online using the complexing agent trans-1,2-diaminocyclohexane-N,N,N',N'-tetra acetic acid (abbreviated CyDTA).  However, this method might not have the sensitivity needed to determine the sodium content in Sprite (reported as about 65 mg per 12 oz can, or 183 mg/L).  The sodium ion itself is neither acidic nor basic.  In Sprite, the sodium is introduced as sodium citrate and sodium benzoate.  Commercially, the sodium content of soda is probably determined by atomic emission spectrometry (AES), inductively-coupled atomic emission spectrometry (ICPES). or some other spectroscopic method.

 

When designing an experiment to quantify components in a material, it is important to find methods which have both the sensitivity (ability to measure the levels expected) and specificity (ability to distinguish the analyte from other components).  For sodium, specificity can be achieved by using either sodium's atomic emission/absorption spectrum or it's atomic mass.

 

The simple answer to this assignment is to choose a product to analyze which contains only one acid and a product which contains only one base.  Suggestion:  analyze for acetic acid in "distilled" or "white" vinegar, and analyze for ammonia content of household ammonia. 
 

The titration of an acid can be done with a commercially-available sodium hydroxide solution of known molarity, or by making a solution of sodium hydroxide and "standardizing" it by titration with a "primary standard" such as potassium hydrogen phthalate (KHP), a solid acid which can be obtained in high purity.  A known mass of sample (preferably an aqueous solution) to be analyzed is placed in an Erlenmeyer flask.  An indicator which changes color at the appropriate pH at the equivalence point is added to the solution.  Add standardized base with a burette, and stop when the indicator changes color.  Determine the volume of standard base solution added, and use the volume value and the molarity of the solution along with the mass (or volume) of the sample to calculate the acid concentration of the sample.

 

The titration of a base can be done using a commercially-available standard solution of HCl, or make an HCl solution from a concentrated HCl solution, then determine it's exact molarity using a primary standard base such as primary standard grade sodium carbonate or tris-(hydroxylmethyl)aminomethane, both of which are solids available in high purity.  Then place a known mass of sample (or volume) in an Erlenmeyer flask, add an appropriate indicator, and titrate with the standardized HCl solution.  Determine the base concentration in the sample using the volume of standard acid, it's molarity, and the mass (or volume) of the sample.