auto-titrator method

University of KentuckyCollege of Agriculture

Division of Regulatory Services

Comparison of Manual and Automatic Titrametric Analysis of

Potassium in Fertilizer using AOACI Method 958.02

C. Wayne Ingram, Dr. Melton Bryant†, Dr. David Terry*

University of Kentucky

John Griffiths

Mettler Toledo Inc.

AAPFCO Midyear MeetingCharleston, SC

February 23, 2010

† - Presenter * - Retired

Flow Analyzer Method

• Routine and quality samples are analyzed with the Automated

Flame

• Photometric Method (AOAC Official Method 993.31,

• Phosphorus (Available) in Fertilizers, Direct Extraction Method

• and AOAC Official Method 971.01, Potassium in Fertilizers

• Automated Flame Photometric Method).

Titration Method

• Routine and quality samples as well as any sample outside the

• investigational allowance are analyzed by titration

• (AOAC Official Method 958.02, Potassium in Fertilizers

• Volumetric Sodium Tetraphenylboron Method I).

Regulatory Services Potash Analysis

Objective of Investigation

Automated Method

Mettler Toledo T-50 auto-titrator

with DP5 phototrode™ probe

(T-50 loaned to UK Regulatory Services)

Manual Method

Brinkman Metrohm dispenser

with visual observation

Comparison of a new automated titration method to the

standard manual titration method for determining the

amount of potassium (as soluble potash, K2O) in fertilizer.

Potash Analysis Method

The fertilizer samples were analyzed using the Official

Methods of Analysis of AOAC International 18th ed.

AOAC Official Method 958.02

Potassium in Fertilizers

Volumetric Sodium Tetraphenylboron Method I

Reagents: Volumetric Sodium Tetraphenylboron Method I

• Ammonium oxalate solution (4%)

• Tributyl Citrate

• Sodium hydroxide solution (20%)

• Formaldehyde, 37% W/W

• Sodium Tetraphenylboron Solution (STPB) (1.2%)

STPB binds soluble K

• Benzalkonium chloride (BAC) solution-approximately 0.625%

Used to back titrate for excess STPB

• Clayton Yellow (Titan Yellow) indicator (0.12%)

In titration - yellowish brown turns to pink color

Sample ListTable 1

Description of Sample % Guarantee UKRS Report

Sample Source N:P:K

Commercial (liq) 9-24-3 3.00 2.81

Commercial (liq) 9-24-3 3.00 3.17

Commercial (organic) 5-3-4 4.00 3.22

Commercial 4-12-4 4.00 4.38

Commercial (organic) 4-10-6 6.00 9.20

Commercial (liq) 2-1-6 6.00 6.88

Commercial 11-7-7 7.00 9.55

Magruder Check Sample 16-3.6-7.76 7.80 7.73

Magruder Check Sample 24-0-9 9.98 9.89

Commercial 10-10-10 10.00 9.60

Commercial 10-10-10 10.00 9.78

Commercial 11-23-10 10.00 9.96

Commercial 10-10-10 10.00 9.31

Commercial 5-10-10 10.00 10.10

Commercial 10-10-10 10.00 11.42

Commercial 10-10:-0 10.00 10.75



Commercial (liq) 4-15-12 12.00 13.29

Commercial 6-12-12 12.00 11.97

Commercial 13-13-13 13.00 13.65

Custom Mix 27.3-13.6-13.6 13.63 13.35


Commercial 15-30-15 15.00 12.90

Commercial 15-30-15 15.00 16.16


Custom Mix 17.7-17.8-17.8 17.78 18.87

Commercial. 19-0-19 19.00 21.72





Custom Mix 9.8-24.9-25.0 25.01 24.40

SOP 0-0-50 50.00 51.04

MOP 0-0-60 60.00 60.99

MOP 0-0-60 60.00 60.63

MOP 0-0-60 60.00 59.54

Sample Source Number of Samples

Specialty 14

Specialty (liquid) 4

Specialty (Organic) 2

Custom Mixture 3

Fertilizer Ingredient 4

Magruder Check Sample 10

Total Sample 37

The samples represent the typical

range of fertilizers that UK Regulatory

Services (UKRS) analyzes on a routine

basis with guarantees ranging from low

(3%) to high (60%) levels of soluble

potash in the fertilizer.

STPB Procedure ReviewThe percent guarantee determines the amount sample used for analysis.

Potash Guarantee (%) Sample Weight (g) Weight Range (g)

0-24 2.50 0.01

25-40 1.40 0.01

40 + 1.05 0.01

Sample DigestionWeigh samples into a 250 mL volumetric flask (add charcoal if sample is organic)

Add approximately 175 mL of Ammonium Oxalate

Boiling chips and 2-3 drops of Tributyl Citrate (defoaming agent)

Swirl solution in flasks, place onto hotplate 450° F (232.2°C)

When all samples have started boiling (some flasks may take longer than others to

start boiling), turn temperature back to 400°F (204.4°C)

Let samples boil for 20 minutes

Cool samples in a water bath

Bring to volume with deionized water

Mix and let settle for 2 hours

Procedure (continued)

Routinely, the sample aliquot is taken from the 250 mL

volumetric flask and transferred into a 50 mL volumetric flask.

10 mL for the manual method

•Add 2 mL sodium hydroxide

•Add 5 mL formaldehyde

•Add STPB to complex potash

Calculated based on sample weight

and guarantee (added in excess)

•Bring to volume with deionized

water (50 mL)

(Use 25 mL for analysis)


Modification for replicates:

Make a 5X sample volume using a 250 mL volumetric flask

50 mL sample for comparison analyses

•Add 10 mL sodium hydroxide

•Add 25 mL Formaldehyde

•Add STPB to complex potash

Calculated based on sample weight

and guarantee (added in excess)

•Bring to volume with deionized

water (250 mL)


• Filter the samples into another 250 mL flask

• Pipette 25 mL samples

from the filtration

flask into the titration

vessels for the manual

and automated

analyses.

25 ml of solution + 6 drops of Clayton Yellow (indicator)

Manual Method

125 mL erlenmeyer flask

3 replicates

Automated Method

80 mL titration cup

3 replicates

Procedure: Titration Solution

Manual AnalysisThe manual analysis using Brinkman Metrohm with

electronic dispenser (not a manual burette) used as the basis

for the comparison of the two titration methods.

•Three replicates analyzed for each

sample analysis.

•Manual delivery of titrant.

•Use Potassium Dihydrogen Phosphate ( KH2PO4 )

from National Institute of Standards and Technology

(NIST) (SRM 200a), as the reference sample.

Manual Analysis

Note:

Potash calculations via electronic spreadsheet

Manual Electronic Dispenser (25 mL sample)

• Add a magnetic stirring bar

• Back titrate excess STPB with BAC to a light

pink endpoint that holds for at least 20 seconds

• Record titrant volume onto the spreadsheet

(Visual observation of pink color on

the white particles)

Preliminary Data

100 mL vessel

Additional 20 mL water required

to cover the phototrode

Investigation Data

80 mL vessel

No additional water

80 mL 100 mL

Automated Analysis

Note:

Potash calculations via electronic spreadsheet

Automated Titration (25 mL sample)• Back titrate excess STPB with BAC

to a point of inflection on the T-50 auto-titrator (pink color formation).(The phototrode is set at 555 nm)

• Rinse probe and stirrer between samples with Triton X-100 solution (0.1% surfactant)

• The titrant volume is recorded by the computer and printed as a permanent record.

Titration: Preliminary and Investigation

Data

Preliminary Data

from the 100 mL vessel

(25 mL sample + 20 mL water)

was done to investigate

the DP5 Phototrode™

probe and absorption of

indicator on solution

particles

Investigation Data

from 80 mL vessel

used with a direct

comparison of

manual (25 mL) and

automated (25 mL)

methods based on

absorption of the

indicator on solution

particles

Automated Analysis

DP5 Phototrode™

The DP5 Phototrode™

probe is routinely used for

the automated

determination of metal

ions (Fe2O3, Al2O3, MgO)

with EDTA. The light

transmission of the

solution is measured

during the titration.

Automated AnalysisDP5 Phototrode™

The DP5 Phototrode™ directs

light into the solution where it

is reflected back to the photo

sensor by a mirror. In this

transmission mode, the

indicator color changes in

solution at the titration

endpoint allowing the

determination of the metal

ion concentration.

Normal Phototrode Application

• In the potassium analysis, STPB develops a turbid

solution with particles that reduces the light being

detected. The analysis can not be performed using the

DP5 Phototrode™ in the transmission mode, since the

light is reduced and an endpoint is not detected.

• To deal with this issue, our idea was to remove the

mirror and measure the reflected light from the

particles, thus using the DP5 Phototrode™ in a reflection

mode.

• In this reflection mode, the light absorbed by the

indicator on the surface of the particles at the endpoint

results in a reduction of the reflected light making it

possible to measure an endpoint.

Automated Analysis of Potassium

Mirror

Phototrode™

Automated Analysis

• This modified

configuration of the

instrument provides a

measurable endpoint for

the potassium method.

• Operation of the

instrument in the reflection

mode is a novel application

of the DP5 Phototrode™

and T-50 Titrator that was

developed for this

application.

Automated Analysis

Endpoint color

Particle color

DP5 Phototrode™ directs

light into the solution and

light is reflected back to

photo sensor from the

particles. The millivolt signal

is determined by the amount

of light reflected from the

particles. At the endpoint,

light is absorbed by the

indicator on the particles

reducing the millivolt signal.

Comparison: Manual vs. Automated Methods

Sample + STPB

+ indicator

Sample + STPB + BAC

Particles develop

Sample + STPB + BAC

Color on particles

Endpoint intensities

Manual:

BAC is added to the sample

until the eye recognizes a pink

endpoint that holds at least 20

seconds.

Automated:

BAC is added to the sample and the T-50 with DP5 Phototrode™ will record

the data and calculate the endpoint.

BAC Volume Used by the T-50 for Titration Endpoint

5.850

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

0.0 0.3 0.5 0.8 1.0 1.3 1.5 1.8 2.0 2.3 2.5 2.8 3.0 3.3 3.5 3.8 4.0 4.3 4.5 4.8 5.0 5.3 5.5 5.8 6.0 6.3 6.5

BAC (mL)

Sig

na

l S

tren

gth

(mV

)

Results: Graph of Automated Titration

Reflecting light from particles →

Absorption of light from indicator on particles

Begin with initial titrate volume 0.5 mL

Add 0.025 mL increments

18.7 minutes to reach endpoint

Endpoint*

Long analysis time

BAC Volume Used by the T-50 for Titration Endpoint

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

4.0 4.3 4.5 4.8 5.0 5.3 5.5 5.8 6.0 6.3

BAC (mL)

Sig

na

l S

tren

gth

(m

V)

Results: Graph of Routine Automated Titration

Reflecting light from particles →

Absorption of light from indicator on particles

Begin with initial titrant volume 4.0 mL


3.0 minutes to reach endpoint

Endpoint* 5.850

Shorter analysis time

Results: Average K2O – Manual vs. AutomatedSample # Guar

(%)

Manual

Avg % K2O

3 replicates

Automated

Avg % K2O

3 replicates

Rel % Diff

(Auto-Manual)

Manual

1 81002 3 2.85 2.98 4.56%

2 81094 3 3.16 3.08 -2.53%

3 81177 4 3.45 3.40 -1.26%

4 81227 4 4.47 4.73 5.66%

5 80037 6 9.37 9.58 2.21%

6 81095 6 6.96 7.12 2.25%

7 80022 7 9.78 9.71 -0.75%

8 80244 8 8.14 8.02 -1.43%

9 80017 9 10.31 10.43 1.23%

10 70105 10 10.47 10.51 0.35%

11 80240 10 9.26 9.29 0.29%

12 80954 10 9.90 9.79 -1.11%

13 81029 10 10.00 10.13 1.37%

14 81065 10 9.61 9.64 0.35%

15 81161 10 10.42 10.57 1.44%

16 81173 10 11.71 11.88 1.39%

17 81222 10 10.69 10.84 1.37%

18 80781 11 11.18 11.17 -0.09%

19 81028 12 13.50 13.49 -0.05%

20 81160 12 12.20 12.68 3.91%

21 80067 13 14.22 14.18 -0.30%

22 81171 13 13.87 13.98 0.82%

23 81190 14 13.53 13.59 0.47%

24 80154 15 13.18 13.10 -0.56%

25 80949 15 16.22 15.99 -1.42%

26 80990 16 16.92 17.09 1.00%

27 81187 18 19.45 19.38 -0.34%

28 81399 19 21.16 21.36 0.93%

29 70023 20 19.71 19.82 0.56%

30 70572 21 22.65 22.64 -0.07%

31 80579 22 21.47 21.59 0.56%

32 80015 22 23.10 23.14 0.17%

33 81181 25 25.42 25.40 -0.09%

34 81204 50 51.56 51.90 0.66%

35 80273 60 60.67 60.80 0.20%

36 81014 60 60.43 60.12 -0.51%

37 81059 60 60.58 60.04 -0.89%

Average %K2O

( 3 replicates)

Relative Percent

Difference for Manual

and Automated

Methods

Comparison of the Percent K2O from the Manual and Automated Methods

0 . 0 0

10 . 0 0

2 0 . 0 0

3 0 . 0 0

4 0 . 0 0

5 0 . 0 0

6 0 . 0 0

7 0 . 0 0

8 0 . 0 0

0 . 0 0 10 . 0 0 2 0 . 0 0 3 0 . 0 0 4 0 . 0 0 5 0 . 0 0 6 0 . 0 0 7 0 . 0 0 8 0 . 0 0

Automated, K2O (%)

Ma

nu

al,

K2O

(%

)

(70,70)

Results: Comparison Manual and Automated Method

Theoretical Results

Automated = Manual

((Automated avg - Manual avg) / Manual avg) x 100

-3.00

-2.00

-1.00

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

2

Aumated 4.56 -2.53 -1.26 5.66 2.21 2.25 -0.75 -1.43 1.23 0.35 0.29 -1.11 1.37 0.35 1.44 1.39 1.37 -0.09 -0.05 3.91 -0.30 0.82 0.47 -0.56 -1.42 1.00 -0.34 0.93 0.56 -0.07 0.56 0.17 -0.09 0.66 0.20 -0.51 -0.89

3 3 4 4 6 6 7 8 9 10 10 10 10 10 10 10 10 11 12 12 13 13 14 15 15 16 18 19 20 21 22 22 25 50 60 60 60

Guarantee (%K2O)

Results: Comparison of Relative Percent Difference

Using Manual Method as the Basis

Automated > Manual

Automated < Manual

Rel

% D

iffe

ren

ce K

2O

2

0.00

0.50

1.00

1.50

2.00

2.50

3 3 4 4 6 6 7 8 9 10 10 10 10 10 10 10 10 11 12 12 13 13 14 15 15 16 18 19 20 21 22 22 25 50 60 60 60

Guarantee (%)

Rel

Std

Dev

(%

)

Manual Automated

Results: Comparison of Relative Standard Deviation

of Manual and Automated Methods

---------------------------------------------------------------------------------------The RSD of 95% of all the samples was ≤ 1.0%

%RSD = (Std Dev ÷ Avg from Replications) x 100

Statistical Analysis

Method Condition Precision Accuracy

Manual 25 mL Sample

No Significant

Difference

No Significant

DifferenceAutomated

(80 mLvessel)

25 mL Sample

Preliminary Data

Manual 25 mL Sample

Automated

More Precise

No Significant

DifferenceAutomated

(100 mL vessel )

25 mL + 20 mL

H2O Sample

Statistical Analysis with SAS/STAT® Software

Statistical analysis – Dr. David Terry

SAS was used to compare the manual and automated data sets.

Conclusions of Statistical Analysis of the Analytical Data

Results: Particles on the stirring blades

Titration Particles

Cleaning Required

Use Surfactant

Rinse

Results: Automated Titration when Over Titrated

Over Titration with BAC for Titration Endpoint

4.600

0 .0

1.0

2 .0

3 .0

4 .0

5.0

6 .0

7.0

8 .0

9 .0

10 .0

11.0

12 .0

0 .0 0 .3 0 .5 0 .8 1.0 1.3 1.5 1.8 2 .0 2 .3 2 .5 2 .8 3 .0 3 .3 3 .5 3 .8 4 .0 4 .3 4 .5 4 .8 5.0 5.3 5.5

BAC (ml)

Sig

na

l S

tren

gth

( m

V)

Begin with initial titrant volume 4.6 mL


Initial BAC application beyond

Endpoint

The Endpoint was 4.500 mL ↓

Initial Titrant Volume > Endpoint Volume

Conclusions: Advantages and Disadvantages

Advantages of Automated• Does not rely on human eye and color perception to determine the

endpoint.

• Allows analyst to perform other task while the T-50 is unattended and processing the 18 samples on the sample carousel.

• Gives a permanent record of BAC volume with less probability of transposing the data.

• Removes bias from the analyst adding or subtracting BAC drops.

Disadvantages of Automated• Cleaning particles from stirring blades.

• Added expense to the potassium (potash) analysis (Instrument).

• Automated method and manual method give equivalent potash results. Excellent automated analysis with definitive endpoint.

• Automated method and manual method have equivalent precision.

• Future plans:

1. Implement Metter Toledo T-50 system for potassium analysis.

2. Evaluate its application for the analysis of various fertilizer products

Conclusions

Acknowledgments

John Griffiths

Mettler Toledo Inc.

Instrument Sales Specialist

Titration, Density, Refractometry & Automation Systems

Tore Fossum

Mettler Toledo Inc.

Director of Research and Development

Mettler Toledo Inc.

Loan of the T50 Auto-Titrator

QUESTIONS ?

auto-titrator method

Documents