DETERMINATION OF PHOSPHORUS BY GRAVIMETRIC ANALYSIS

Group 1, WAD, Chem 27.1

Introduction

a set of methods of quantitative analysis in which the mass of the analyte is determined using the mass of a

compound to which it is chemically related

Introduction

most inorganic anions and cations

neutral species

(e.g., water, sulfur dioxide, carbon dioxide, and iodine)

organic substances

(e.g., lactose, salicylates, nicotine, cholesterol)

Introduction

Introduction

Introduction

Properties of an ideal precipitating agent: –  Specific: reacts with only one chemical species –  Selective: reacts with a limited number of chemical

species

–  Produces a precipitate that is: •  Sparingly soluble •  Stable and unreactive •  Easily filtered and washed •  Of known chemical composition once dried or ignited

Introduction

Colloidal Suspension Crystalline Suspension Smaller particles Larger particles

Do not settle spontaneously (Brownian movement)

Tend to settle spontaneously

Not easily filtered Easily filtered

Crystalline suspensions have greater particle size, making them easier to filter and wash, while avoiding

loss of sample that could lead to errors.

Introduction

Objectives of the experiment: 1.  To isolate phosphorus in the unknown

sample as magnesium pyrophosphate (MgP2O7)

2.  To calculate the percent of phosphorus in the unknown sample from the weight of magnesium pyrophosphate

Methodology

Weigh Whatman no. 40 filter paper in analytical balance.

Record weight.

Store in desiccator until needed.

Use gloves when handling filter paper!

Methodology

Calculate and weigh amount of magnesium chloride hexahydrate (MgCl2•6H2O) needed to prepare 100 mL of 5% (w/v) solution.

Place solid into beaker with 60 CO2-free distilled water. Stir until the solid was completely dissolved.

Transfer solution to a 100-mL volumetric flask.

Methodology

Calculate and measure amount of concentrated NH3 needed to prepare 250 mL of 2 M solution.

Transfer into a beaker with 150 mL of CO2-free distilled water and mix thoroughly.

Transfer to 250-mL volumetric flask and dilute to mark, then transfer to a plastic bottle.

Methodology

Weigh 0.30 – 0.31 grams of unknown sample by difference using the analytical balance.

Transfer into 500-mL beaker with 50 mL of CO2-free distilled water and stir until dissolved.

Add 60 mL of the 5% (w/v) MgCl2•6H2O solution to beaker. Then, slowly add 200 mL of 2 M NH3 mixture, while stirring.

Discussion

Precipitating agents: –  Magnesium chloride hexahydrate –  Ammonia

•  optimal conditions require basic solution (pH 9) •  magnesium ammonium phosphate is slightly soluble in

alkaline solutions

NH4(aq) + H2O(l) ↔ NH3(aq) + OH-(aq)

OH-(aq) + H2PO4

-(aq) ↔ H2O(l) + HPO4

2-(aq).

MgCl2·6H2O(s) + HPO42-

(aq) + NH3(aq) ↔ [Mg(NH4)PO4·6H2O](s)

OR

5H2O(l) + HPO42-

(aq + NH4(aq) + Mg2+(aq) + OH-

(aq) → [Mg(NH4)PO4·6H2O](s)

Discussion

•  Accounts for the effect of certain variables on particle size of precipitate formed

•  Relates the particle size to a single property of the system, the relative supersaturation (RS) –  Q = solute concentration at any time –  S = equilibrium solubility

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RS =Q − SS

Discussion

colloidal precipitate

crystalline precipitate

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RS =Q − SS

Discussion

Mechanisms of precipitate formation –  Nucleation: few ions, atoms, or molecules joining into a stable

solid that usually forms on the surface of suspended contaminants

–  Particle growth: ions, atoms, or molecules deposit on existing particles instead of forming new nuclei

The dominating mechanism determines the particle size of freshly formed precipitate.

The rate of nucleation varies directly with relative supersaturation.

Discussion

NUCLEATION

(large number of small colloidal precipitate)

PARTICLE GROWTH

(smaller number of crystalline precipitate)

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RS =Q − SS

Discussion

•  Higher temperatures increase solubility (S) of the precipitate •  Dilute solutions of reactants decrease Q •  Slowly adding reagents to the solution prevents high

concentration of solute at one point in time (Q) •  Vigorous stirring induces colloid to coagulate into larger

particles •  Controlling pH (e.g., magnesium ammonium phosphate is

sparingly soluble in alkaline solutions)

(Precipitates such as hydrous oxides and sulfides cannot form crystals due to their low solubility.)

Minimized relative supersaturation

Particle growth

Crystalline suspension

Larger particle

size

Easier to filter

Methodology

Cover beaker with watch glass and allow to stand for 20 minutes.

Filter out precipitate using Whatman no. 40 filter paper and an appropriately sized funnel.

Wash completely-filtered precipitate twice using 10-mL portions of CO2-free distilled water and 95% ethanol.

Discussion

Why was the solution left to stand undisturbed? –  Digestion: process in which a freshly formed precipitate is

left in the precipitating solution –  Produces larger, cleaner particles and a denser mass –  Results from the dissolution and recrystallization occuring

constantly in solution while digesting –  Recrystallization bridges adjacent particles together,

producing larger crystals that are more easily filterable

Discussion

Whatman No. 40 filter paper –  Used specifically for

quantitative analyses

–  Medium porosity –  Ashless (does not affect mass

of sample during ignition)

Discussion

Why was the solution washed? –  CO2-free distilled water removes impurities and breaks up

counter-ion layer of colloid –  Peptization (aggregated colloid returns to its dispersed

state) may occur –  To counteract peptization: 95% ethanol recovers dispersed

particles yet easily evaporates upon drying/igniting

Methodology

Remove filter paper from funnel and spread onto a Petri dish.

Air-dry filter paper in fume hood overnight, then dry in an oven at 110°C until completely dry.

Cool filter paper in desiccator for 15 minutes and weigh final product.

Discussion

Why was the precipitate dried in an oven after air-drying? –  Removes leftover solvent and volatile species from

precipitate (e.g., ethanol wash) –  Hydrate was removed and magnesium ammonium

phosphate was produced

2[Mg(NH4)PO4·6H2O](s) → MgNH4PO4(s) + 6H2O(l)

Discussion

… but, wait! –  After drying, precipitate should have been ignited

to yield magnesium pyrophosphate –  Some precipitates are ignited in order to

decompose them into a suitable weighing form •  Weighing form: compound with a known composition

that is used to find analyte’s weight

–  This step, however, was not carried out in the experiment.

2[Mg(NH4)PO4·6H2O](s) → Mg2P2O7(s) + 2NH4(g) + 11H2O(g) + 2OH

Results Trial 1

Weight of filter paper 1.548 g

Weight of filter paper and sample 3.460 g

Weight of Mg(NH4)PO4 1.912 g

Weight of P in sample 0.4313 g

Weight of sample 0.3077 g

% P in sample 140.2%

Actual weight of P in sample 30.97 g

Actual % P in sample 21.82%

% relative error 542.4%

Calculations

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mass P = mass MgNH4PO4 ×1mol MgNH4PO4

FW MgNH4PO4×

1mol P1mol MgNH4PO4

×FW P1mol P

mass P =1.912g × 1mol MgNH4PO4

137.31746 MgNH4PO4×

1mol P1mol MgNH4PO4

×30.97376 P1mol P

mass P = 0.4312767591g = 0.4313g

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% P =mass analytemass sample

×100

% P =0.4312767591g

0.3077g×100

% P =140.1614427%P =140.2%P

Calculations

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% actual P =AW P

MW Na2HPO4×100

% actual P =30.97376g141.961514 g

×100

% actual P = 21.818142%actual P = 21.82%actual P

Calculations

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%Er =Xi − XtXi

×100

%Er =21.81842% −140.1614427%

21.81842%×100

%Er = 542.3929872%Er = 542.4%Er

Discussion

542.4% ERROR?! –  Magnesium ammonium phosphate was not ignited

•  Further removes impurities from the precipitate •  Produces magnesium pyrophosphate (more suitable

weighing form than magnesium ammonium phosphate)

–  Magnesium ammonium phosphate as weighing form •  Less stable with indefinite composition (not certain if all

of the hydrate has been removed when the precipitate was dried)

Guide Questions & Answers

1.  Give factors that affect the particle size of a precipitate and explain how each factor affects particle size. A. Temperature: higher temperatures increase solubility, affecting S. B. Reactant concentrations: dilute solutions of reactants have lower

concentrations, decreasing Q. C. Rate at which reactants are mixed: slowly adding reagents to the

solution prevents having a high concentration of solute at one point in time, thus lowering Q.

D. Stirring: vigorous stirring reduces the volume of the outer layer of the colloid, inducing the colloid to coagulate into larger particles; also prevents localized high concentration sites.

E.  Digestion: having the precipitation process occur for a longer period of time allows molecules on the surface of smaller particles to dissolve and then reform on the surface of larger particles.

Guide Questions & Answers

2.  What are the properties of an ideal precipitating agent?

Specificity and selectivity are two properties of an ideal precipitating agent. Specific precipitating agents react with only one chemical species, while selective precipitating agents react with a limited number of chemical species. Additionally, an ideal precipitating agent should react with the analyte to produce a precipitate that is sparingly soluble, easily filtered and washed, and of known chemical composition once dried or ignited.

Guide Questions & Answers

3.  Define the following terms: peptization, digestion, weighing form. –  Peptization: the process in which an aggregated colloid

returns to its dispersed state. –  Digestion: the process in which a freshly formed precipitate

is left (usually with heating) in the precipitating solution, resulting in larger, cleaner particles and a denser mass.

–  Weighing form: the compound of known composition that results from the filtering and drying of the gravimetric precipitate; called as such because it is the form in which the sample is weighed in order to quantify the analyte.

Conclusion & Recommendations

•  Gravimetric analysis is based on the measurement of the analyte’s mass.

•  The analyte in this particular experiment was phosphorus in the form of MgNH4PO4 (theoretically, Mg2P2O7), while the precipitating agents were MgCl2•H2O and NH3.

•  The analyte must first be separated by forming a precipitate, which is then filtered, washed, and dried/ignited until a compound with a definite composition is obtained.

•  The weight of the analyte can be determined from the weight of the sample through stoichiometry.

Conclusion & Recommendations

•  Certain variables, such as temperature, concentration, etc., can be manipulated in order to yield more effective results.

•  Although gravimetric analyses produce accurate results, multiple errors can be introduced during the experiment due to factors like lack of time or materials.

•  The major error in the experiment was the failure to ignite the sample and obtain a suitable weighing form (Mg2P2O7), resulting in a very high percent error.

•  It is recommended that the procedure should be followed at all times, and done carefully while performing gravimetric analysis.