Determination of the proportion of nitrogen in a fertiliser

When boiling the mixture until no more ammonia is evolved, an indicator was needed to identify the point at which no more ammonia was being created. The indicator used was damp blue litmus paper and in the presence of ammonia, it would turn red. This was used sporadically to check whether ammonia was still being released. Only when no more ammonia was being evolved could the experiment be carried on.

For the titration, the suitable indicator being used was methyl orange. The different levels of color changes is the main indication. When the solution with the indicator is yellow, it means that it’s still alkaline, when it turns orange, it means that the solution has turned neutral and when it turns to pink, it means that the solution is then acidic. This is a good indicator to tell when too much acid has been used. When too much acid is used, it turns pink and therefore, the color which is to be obtained is orange, in order to accurately measure on which exact point does the solution become neutral.

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Raw Data:

Weight of empty weighing bottle: (g)

13.888

Weight of bottle + fertilizer: (g)

15.438

Weight of bottle after emptying and re-weighing: (g)

13.890

Weight of fertilizer used in experiment:(g)

1.548

Attempts/ Trials:

Amount of NaOH alkaline solution used in the conical flask: (cm�)

Amount of initial volume being used in burette with the 0.1M of HCl: (cm�) � 0.05 cm�

Amount of final volume being used in burette with the 0.1M of HCl: (cm�) � 0.05 cm�

Volume of hydrochloric acid used in each attempt: (cm�) � 0.05 cm�

Trial/ Rough

25

0

23.40

23.40

1

25

0

23.50

23.50

2

25

0

23.40

23.40

3

25

0

23.50

23.50

Mean Amount of solution required: (cm�)

25

0

23.47

23.47

Uncertainties:

Class A Burette: (50 cm�) � 0.05(cm�)

Class A Volumetric Pipette: (25 cm�) � 0.03(cm�)

Class A Volumetric Pipette: (10 cm�) � 0.02(cm�)

Class A Volumetric Flask: (250 ml) � 0.12(cm�)

Attempts/ Trials:

Volume of 0.1M hydrochloric acid used in each attempt: (cm�) � 0.05 cm�

Percentage uncertainty for the burette measurements of the amount of HCl used with absolute uncertainty as � 0.05(cm�):

Trial/ Rough

23.40

0.214%

1

23.50

0.213%

2

23.40

0.214%

3

23.50

0.213%

Mean Amount of solution required: (cm�)

23.47

0.213%

Percentage uncertainty of the use of class A volumetric pipette to pipette the 25ml solution of 1M NaOH into the conical flask to be boiled: (this had to be done twice to get the exact 50cm�)

0.03 x 100 = 0.12% x 2 = 0.24%

25

Percentage uncertainty of the use of class A volumetric flask to collect and dilute the 250ml of sea water:

0.12 x 100 = 0.048%

250

Percentage uncertainty of the use of class A volumetric pipette to pipette 25cm3 into conical flask to be titrated:

0.03 x 100 = 0.12%

25

Total Percentage Uncertainty:

0.2130690429 + 0.12 + 0.048 + 0.24 = 0.6210690429

0.6210690429 x (Percentage of nitrogen in a fertilizer)

100

Calculations:

NH4+ + OH- NH3 + H2O

As we know, one mole of NaOH releases 1 mole of NH3 gas and 1 mole of ammonia gas contains 1 mole of nitrogen atoms (14.01g). Therefore, 1 mole of NaOH is equivalent to 14.01 grams of nitrogen.

So in order to find out the proportion of nitrogen in the fertilizer we must find out the exact amount of nitrogen (in grams) in the fertilizer. To do that, we need to find out how many moles of NaOH is being used.

Using the equation,

Number of moles in a solution = concentration x volume

1000

= 1 x 23.47

1000

= 0.02347

Amount of left over NaOH = 0.02347 moles

Therefore, to move on, we need to calculate the number of moles of NaOH added in the first place: (using the same equation)

Number of moles in a solution = concentration x volume

1000

= 1 x 50.0

1000

= 0.05

In order to find out the amount of moles required to displace the ammonia, it is required to take the total number of moles of NaOH added in the first place minus the amount of NaOH left over.

0.05 – 0.02347 = 0.02653

= 0.02653 = number of moles needed to displace the ammonia

As we know from before, 1 mole of NaOH is equivalent to 14.01grams of nitrogen, therefore to find out the total amount of grams of nitrogen in the fertilizer, we need to multiply the number of moles required to displace the ammonia with the 14.01 atomic weight of nitrogen.

0.02653 x 14.01 = 0.3716853 grams

To figure out the exact proportion between the amount of nitrogen and the total amount of fertilizer, the percentage needs to be calculated:

0.3716853 x 100 = 24.01067829 %

1.548

Therefore, the percentage of nitrogen in the fertilizer = 24.01 %

From this, the overall uncertainty can then be calculated =

0.6210690429 x 24.01 % = 0.1491228899

100

= 0.15

Therefore,

Overall uncertainty = 24.01% � 0.15%

Conclusion:

From the results obtained, it can be seen that there is approximately 24.01% of nitrogen found in the fertilizer we used. This means that in all the different kinds of salts in this fertilizer, 24.01% of it had nitrogen in it. There are not exactly accurate literature values because in this experiment we didn’t know what kind of fertilizer we were dealing with. The exact compound formula wasn’t given and there so many different types of fertilizers. Therefore, no comparison with literature values can be obtained. However, from one of literature values, there is one type of fertilizer that comes close to this value. Ammonium sulfate carries 20.5% of nitrogen, and that is fairly close to this result obtained from this experiment. Therefore, it can only be predicted that the type of fertilizer being used in this experiment was ammonium sulfate. The multiple number of steps needed to be conducted in this experiment causes an increase in the chances of errors which could affect the final result.

Evaluation:

Errors and Limitations:

o There could’ve been instrumental errors or a slight fault in the measuring of the bottle and the fertilizer. Left over salts and substances on it from previous experiments could’ve caused the weight to alter slightly or greatly.

o Contamination in the weighing bottles would definitely affect the experiment because it would mean that we would be mixing the sodium hydroxide with fertilizer and possibly other substances as well which would have a large affect during the boiling of the solution. This contamination could’ve occurred by other people previously using the weighing bottles and not cleaning it properly. Even though it was washed out properly with distilled water, sometimes, not all the left over substances and chemicals are washed out.

o During the transfer of the fertilizer into the conical flask it is easy to accidentally lose some of the solids. Human errors can always occur at any point and this is a situation where human error is very likely and would have a dramatic affect on the results. Less salts being poured in would be less fertilizer actually being used and measured.

o When 50cm� of 1M NaOH is being collected by the pipette, it is done twice and this causes more chances of human error and more uncertainties for the experiment itself, as shown in the uncertainties calculations. Accidental spills not caught by the eye or other simple errors would have double the affect due to doing this twice.

o When boiling and trying to identify the point at which no more ammonia is being evolved, it is extremely hard to know the exact point at which this occurs. It is hard because the litmus paper must be placed close enough to the tip of where the ammonia is being released and at the position, it is extremely hot. Sometimes, one might rush into the situation at not leave the litmus paper in that spot long enough to see that there is still ammonia left and believe that there is no more being evolved and end the boiling right there. That would be wrong because it would leave left-over ammonia in the solution during titration. The ammonia would have a large impact on the titration.

o When rinsing the inside and outside of the flask after heating, and pouring the solution together with this rinsed water into the measuring flask, it is possible that not all the instruments were rinsed enough and complete. There could be some left-over solutions on the instruments that weren’t rinsed properly.

o The instruments have been designed and created to function at its optimum level at 20�C. The room temperature on the day this experiment was conducted wasn’t at that exact temperature. Therefore, the instruments could act a little differently than is expected by them. The instruments wouldn’t be as accurate as hoped to be, including the class A and all other instruments.

o It was an essential part of this experiment that the glassware was washed with distilled water and not tap water because in tap water, there are many impurities and salts in it. These salts and impurities would definitely affect the experiments titration because some of those salts could have additional chlorine ions or other ions that might react with the silver nitrate. The can be no alterations in the results from what was obtained, if distilled water is used because after all, the distilled water doesn’t contain any impurities, or is at least expected not to.

o The many steps in this titration, the multiple occurrences of having to use the pipette is a large factor because by having more steps to conduct, it’ll create more chances of both human and manipulation errors. Accidental spills or contaminations from the long list of instruments that had to be used could occur multiple times throughout this whole experiment. Any of it could easily affect the results of the experiment by quite a lot. Thus, all this is shown by the uncertainties calculations.

o In each of the attempts, the quickness and speed of moving the conical flask around would be different in each case. The conical flask was being moved around to mix the solutions properly as an act of trying to stir the solutions. However, this caused more errors because it meant that in each attempt, the only correct way was to shake it around at a constant speed in all trials, but that is impossible for anyone to regulate with their arms. Each attempt would have its own speed in strength of shaking.

o Parallax error of not recording down the exact measurement could’ve occurred. Although attempts to minimize the error, by using a white surface behind the burette to see it clearly, there is always room for some minor errors in reading it wrongly.

o When observing and reacting to when the yellow color of the methyl orange in alkaline solution, would change to orange in the presence of a neutral solution, it is difficult because the different shadings is extremely hard to differentiate when one looks at the solution for a long period of time. Different people have their own preferences in what they feel is yellow and orange color. The color change from orange to pink is then so slight at the beginning and is extremely hard to tell. Missing this end-point is a common and easy fault to make due to the suddenness of how quickly the orange precipitate forms. Sometimes, one may accidentally turn the burette too much and too much hydrochloric acid could’ve accidentally poured out and sometimes one might simply just miss it due to slow reaction time.

Improvements:

o The only way of ensuring that the problems of contaminations couldn’t occur in this experiment is if each and every instrument used was decontaminated every time it needed to be used or new sets of instruments would be constantly used, however this idea is impossible. The normal usage of removing left over impurities in the instruments by using distilled water isn’t accurate enough to ensure there is no error of contamination at all.

o During the transfer a funnel could’ve been used to ensure that all the powder would end up in the conical flask. However, for this, the funnel would have to be perfectly dry so that none of the salt ends up sticking onto the sides of it. The funnel would be required to be dry and extremely clean so that no contamination problems could occur.

o There are no 50cm� pipettes at our disposal, so this problem of having to do it twice with the 25cm� will always be an uncertainty and a possible source of error. The only way too try to reduce this error is to take extra safety precautions are to doing it slowly and surely to not spill any of the substances.

o To ensure that all the ammonia has been given off and that no more is being evolved, a more technologically advanced tool should be used, which can withstand high temperatures and still measure accurately the presence of ammonia. With this, it could be fitted in a close position onto the area where the ammonia would be released and it would accurately measure the end point of when no more ammonia is being produced.

o To eliminate the parallax error, one way is to use a more efficient or technologically advanced reading machine, which could read the mark much more accurately than the human eye. Such sorts of apparatus’s aren’t available among the list of instruments at our disposal.

o The error of observing the end point incorrectly is hard to eliminate because there is no machine at our disposal that could be used to identify the exact point when the titration is complete. In this case the error cannot be eliminated and this will always be a constant source of error, not to mention that this is one of the largest errors in the experiment.

o A constant stirring or shaking of the conical flask could be obtained by the means of a more technologically advanced machine, which would ensure a constant movement, and speed of movement of the flask in this experiment.

o Human manipulations and errors will always be inevitable and the only thing, which can be done, is to use other external sources of machines or apparatus’s that could help in the experiment.

o The correct the last limitation and error, the only way possible would be to conduct the experiment at 20�C. In the labs, the use of air-conditioning could be used to obtain such temperatures. Other than that there isn’t much else that can be done to rectify that error.