The Kjeldahl method is used in agricultural chemistry to...

The Kjeldahl method is used in agricultural chemistry to determine the percent protein in natural products. The method is based on converting all the protein nitrogen to ammonia and then determining the amount of ammonia by titration. The percent nitrogen in the sample under analysis can be calculated from the quantity of ammonia produced. Interestingly, the majority of protein molecules in living matter contain just about $16 \%$ nitrogen. A 1.250 g sample of meat is heated with concentrated sulfuric acid and a catalyst to convert all the nitrogen in the meat to $\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} .$ Then excess $\mathrm{NaOH}(\mathrm{aq})$ is added to the mixture, which is heated to expel $\mathrm{NH}_{3}(\mathrm{g}) .$ All the nitrogen from the sample is found in the $\mathrm{NH}_{3}(\mathrm{g}),$ which is then absorbed in and neutralized by $50.00 \mathrm{mL}$ of dilute $\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq}) .$ The excess $\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq})$ requires $32.24 \mathrm{mL}$ of $0.4498 \mathrm{M}$ NaOH for its titration. A separate 25.00 mL sample of the dilute $\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq})$ requires $22.24 \mathrm{mL}$ of $0.4498 \mathrm{M}$ NaOH for its titration. What is the percent protein in the meat?

The Kjeldahl method is used in agricultural chemistry to determine the percent protein in natural products. The method is based on converting all the protein nitrogen to ammonia and then determining the amount of ammonia by titration. The percent nitrogen in the sample under analysis can be calculated from the quantity of ammonia produced. Interestingly, the majority of protein molecules in living matter contain just about $16 \%$ nitrogen.
A 1.250 g sample of meat is heated with concentrated sulfuric acid and a catalyst to convert all the nitrogen in the meat to $\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} .$ Then excess $\mathrm{NaOH}(\mathrm{aq})$ is added to the mixture, which is heated to expel $\mathrm{NH}_{3}(\mathrm{g}) .$ All the nitrogen from the sample is found in the $\mathrm{NH}_{3}(\mathrm{g}),$ which is then absorbed in and neutralized by $50.00 \mathrm{mL}$ of dilute $\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq}) .$ The excess $\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq})$ requires $32.24 \mathrm{mL}$ of $0.4498 \mathrm{M}$
NaOH for its titration. A separate 25.00 mL sample of the dilute $\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq})$ requires $22.24 \mathrm{mL}$ of $0.4498 \mathrm{M}$ NaOH for its titration. What is the percent protein in the meat?

Key Concepts

Kjeldahl Method

A classical analytical technique used to determine the nitrogen content of organic compounds, particularly in agricultural and food chemistry. The method involves digesting the sample with acid and a catalyst to convert all the nitrogen into ammonium ions, which are then transformed into ammonia and quantified. This overall process is the foundation for calculating the protein content in a sample.

Acid Digestion

A critical initial step in the Kjeldahl method where the sample is heated with a concentrated acid (usually sulfuric acid) and a catalyst to break down organic materials. This process converts all the nitrogen present in the sample into ammonium sulfate, facilitating its subsequent conversion into a measurable form.

Ammonia Liberation and Absorption

After digestion, a base (commonly NaOH) is added to the mixture to release ammonia gas from the ammonium ions. The liberated ammonia is then distilled and absorbed into an acid solution. This step is essential for isolating the nitrogen as ammonia, making it available for quantitative analysis.

Acid-Base Titration

A quantitative analytical method in which the absorbed ammonia is neutralized using a standard acid or base solution. In the context of the Kjeldahl method, titration helps determine the exact amount of ammonia (and thus nitrogen) present by measuring the volume of titrant required to reach the endpoint of the reaction.

Back Titration

A titration technique used when the analyte is not directly titratable or the reaction is slow. In this method, an excess of a standard reagent is added to the analyte, and the remainder is titrated. This approach is often used in Kjeldahl analysis to accurately quantify the nitrogen content after ammonia absorption.

Stoichiometry and Percent Nitrogen/Protein Calculation

Underpinning the calculations in the Kjeldahl method, stoichiometry provides the mathematical relationship between the reagents and products in the chemical reactions. This concept allows the conversion of titration data (amount of titrant used) into the amount of nitrogen present, which is then used to calculate the protein content in the sample based on the typical nitrogen percentage found in proteins.

Transcript

00:01 So we have a meat sample of 1 .2050 grams.

00:03 We're trying to find the percent of protein in the meat sample.

00:06 We know that if we drive out the nitrogen by having ammonia, we can figure out the percent of nitrogen and this, the mass of nitrogen and this.

00:15 And then we know that the percent of nitrogen and protein is about 16 percent, which means there's 16 percent grams of nitrogen and 100 grams of protein.

00:31 And then we can figure out the percent of protein.

00:33 Protein, right, by figure out what this mass is, and then we can figure out how much of percent protein as in the meat sample.

00:42 So to do that, you go through several steps to infuse ammonia into a sulfuric acid, and then you react that with an oh to figure out how much of the sulfuric acid does not react.

00:55 So you get sodium sulfate and water, and you have to balance it.

01:02 So those two here and then we also have a standard that is going to tell us how much of that superior acid reacts with the sodium nitroxide.

01:16 When you subtract those two, this will give you how much excess and this will give you how much you actually should have all the sulfuric acid reacted and subtracting those two will tell you how much in each superic acid actually reacted within h3, which then will tell us how much nitrogen reacted which then we can use to figure out the percent of protein so we do the same here so what we're gonna do is we're gonna try to figure out the moles of sephoric acid in both of them and we're gonna subtract it so let's start with this one so remember that to go from milliliters to liters we're going to divide about a thousand we have 0 .03 2 to 4 liters of studying my then i do all my units first, liters of sodium hydroxide, two moles of sodium hydroxide, moles of sodium hydroxide, two moles of sulfuric acid.

02:24 Moles to molecules, use your coefficients.

02:27 This is one sepurec acid for every two sodium hydroxides.

02:30 In multi -leaders, you're going to use molarity, which is most per liters.

02:34 So this is 0 .4498 moles in one liter.

02:40 Liters of sodium hydroxide cancel, moles of sodium hydroxide cancel, giving you moles of a sulfuric acid, which is equal to 0 .007251 to 4 -6 -5s, moles of h2s -o4.

02:56 And then we're going to do the standard.

02:58 So what this is, is this is the moles of suphoric acid that did not react within h3, because they reacted with the sodium hydroxide.

03:05 What the standard is going to tell us is how many, if none of the sulfuric acid reacted with an h3, how many moles of a superic acid would we have? now, the only thing that i want you really notice is that the volumes are different.

03:18 So we're going to have to do an extra step at the end to make sure that the moles are comparable in the same size container.

03:25 And i'm going to show you how to do that in a second.

03:26 So 0, let's convert first to moles.

03:29 So 2 to 4 liters of sodium hydroxide, 2 liters to sodium hydroxide, 2 moles of sodium hydroxide, moles of sodium hydroxide, to moles of sulfuric acid and then to get the volumes to be for you to be able to compare the moles that have to be in the same volume so we're gonna take the volume of what we have in this titration and then compare it to the volume of what the other titration is and then millators cancel and then we do for every one more spheric acid you have two moles of sodium hydroxide and then most liters is your concentration which is 0 .4498 moles and 1 liter.

04:26 Leter is the sodium hydroxate cancel, moles of sodium hydroxide cancel, and it gives you moles of sulfuric acid.

04:32 So you have 0 .01 and then 4 -6 -figgs, that's 2, 3, 4 moles of sulfuric acid.

04:40 So this is all the moles of sulfuric acid that you have, right? this is how much of it reacted with sodium hydroxide.

04:47 We're going to figure out how much reactive with n.

04:49 H3...

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