Titration: positive aspects of experiment in terms of accuracy -Checking to see if any air bubbles were present and if the burette was leaking before doing the titration. If I did not do this, it would alter the end point and hence the titre results.
-Recording the volume reading before doing the titration.
-Making sure I was using the burette correctly, to deliver a stream of titrant to within a few millilitres each point till reaching the endpoint.
-Observing the colour change carefully, as end point was reaching.
– adding 2 to 3 drops of indicator, not excess.
-Ensuring calibration of glassware, for volumetric glass accuracy. This can be overcome by using a class volumetric glass. -Misjudging the colour of the indicator as the endpoint is reached, as this can be overcome by asking a few other students to help identify whether the end point has been reached. -not filling the burette properly, as if an airlock is present in the burette stopcock, it can block the flow of the titrant, but it can also flow with the titrant. I can overcome this limitation, by checking the equipment properly before proceeding with the experiment. -Spillage of the solution, as losing some of the solution, by shaking the conical flask to swirl the contents of the flask, may alter the end point. This human error can be overcome by swirling the conical flask carefully. -Leaking burette- affect the titre results of titration, this error can be overcome by testing the burette before using it for titration.
Positive aspects of the experiment in terms of accuracy
-Cleaning the wire loop before starting the actual experiment. I ensured the wire loop was not contaminated by dipping the wire loop into hydrochloric acid or nitric acid, followed by rinsing with distilled water. If colour was produced, the loop was not clean. -Placing the salt (metal) in the blue region of the flame of the Bunsen burner, as it gave more effective colours, that could be distinguished more easily to differentiate. In this experiment involving the flame test of many salts as I observed the colour change of the flame of various solutions of chloride (salts) involving a Bunsen burner, although I positively identified the colour of each salt by comparing it to a known set of results, there were a few limitations such as the test could not detect the low concentration of some ions such as magnesium chloride.
The brightness of a colour change varies from one salt to another for example the green emission from copper is brighter than the red emission from the same amount of lithium. The presence of contaminants in the wire loop affect the test results also the test could not differentiate between all elements such as magnesium chloride did not produce any colour in the flame at all. The cations changed colour, confirming the identification of the specific cation involved. Another problem encountered, the colour of the salt on a metal loop was all different types of colour from substances before the test, so I had to clean the metal loop thoroughly. During the experiment I used the metal loop twice on three different salts which led to contamination giving inaccurate results.
-Not cleaning the loop, may give inaccurate results. To overcome this, I cleaned the wire loops with distilled water and to ensure cleanliness, I put the wire through the flame, and if any colour appeared, I would repeat the process again. -Chemicals (metals/salts) may enter the Bunsen burner lead to contamination and affect the accuracy of results due to giving off mixed colours. This limitation can be overcome by cleaning the Bunsen burner or changing it to a different one. Testing for sugars, protein and starch evaluation
Positive aspects of the experiment and limitations
I obtained accurate colour change detecting the presence of each biological molecule. I ensured the pipettes were clean, to avoid contamination, as contamination would have led to different colour change, hence the presence of molecules would not be distinguished properly. When I added biuret reagent to protein in milk powder, I didn’t observe lilac colour at first, due to using the pipette for iodine solution, so I used a different set of pipettes to avoid the error. I repeated the experiment to ensure accurate results. The limitations in this experiment were using the same pipette for different reagents affecting colour change. I overcame this by using a different pipette each time.
I would improve this experiment, by using a variety of food starch such as for iodine solution can be added to different solid foods to detect starch. Biuret test only determines whether the protein is present in sample, it will not give quantitative result of how much is present. Several other colours can interfere with test. Using more of the sample for example filling the palate with more quantity of spaghetti and milk powder separately to see colour change properly, because more nitrogen atoms of the peptide bond in protein can bind to copper ions by forming coordinate bonds in biuret reagent to give positive result.
Testing for cations and anions: precipitations
Contamination in chemistry describes a single constituent, when a chemical contains an impurity, and can cause an issue if the chemical is mixed with other chemicals or mixtures and cause additional chemical reactions. Contamination of pipettes, measuring cylinder would affect the reaction taking place between anion and cation for example the nitrate was anion with silver and sodium the cation with chloride, altering the precipitation formed, giving inaccurate colour change.
Positive aspects of practical and limitations:
In this experiment I was able to distinguish precipitates in each reaction. I made sure I used a different pipette each time, to avoid contamination, as using the same pipette would change the colour of the precipitate formed, hence giving inaccurate results. Using a different measuring cylinder to measure 10cm3 for each reaction, was important, to avoid human error and increase accuracy of results. Adding one or two drops nitrate to sodium hydroxide solution instead of at least 10-15 drops, would affect the ability to determine whether a precipitate is formed, due to less visible colour change, so results would be less reliable. Repeating precipitation tests for each reaction to observe the same colour change, just in case I haven’t used a different reagent since most regents were colourless such as sodium chloride and sodium hydroxide, so labelling the test tubes was essential. I used distilled water instead of tap water, as it contains many ions, which can lead to contamination.
The limitations in this precipitation experiment were using the same pipettes for each reaction contaminating the solution in the test tubes, affecting the colour and formation of precipitate. I overcame this error by keeping the used pipettes aside and labelling the test tube, to avoid contamination due to clear reagants. Cleaning the apparatus with distilled water to avoid mixing the reagent solutions. Using the same measuring cylinder for different solutions also affected colour change. Rather than holding the test tube, it was better to use a test tube rack to avoid spillage and confusion. Not shaking the test tube, may affect the ability to distinguish the precipitate more clearly, especially in the reaction between iron sulphate and sodium hydroxide.
Also to confirm that silver ions and chloride ions react to form silver chloride solid, as the insoluble silver halide, would have added ammonia, the precipitate would dissolve, to form a colourless solution, indicating the silver halide, ammonia combines with silver ions to produce a complexion, which is a transition metal ion bonded to one or more ligands. In this case ammonia is a monodentate ligand [Ag(NH3)2]+. Presence of blue precipitate indicated the cu2+ ion (cation). Addition of sodium hydroxide to copper sulphate and silver nitrate gave precipitate. I sometimes forgot to mix the solution thoroughly by shaking the test tube, and added more of the precipitating agent than required, as this had caused undesired side reactions affecting observation.