Determination of anionic surfactants in a washing powder bymeans of potentiometric two-phase titration

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					                                                             Recent applications



Notes from our application laboratory


When you need quick results:


Determination of anionic surfactants in a washing powder by
means of potentiometric two-phase titration
For this particularly rapid two-phase titration we used the new 6.0507.140 Surfactrode
Refill.
Approx. 1.4 g sample is dissolved in about 500 mL hot water. After cooling down the
solution is transferred to a 1000 mL volumetric flask, made up to the mark and carefully
mixed.
50 mL of this sample solution is pipetted into a beaker and the pH value adjusted to 2 ...
3 with c(HCl) = 0.2 mol/L. 20 mL of a solvent mixture* is then added and the titration is
carried out against cetyltrimethylammonium bromide at a concentration of 0.004 mol/L
under vigorous stirring (use 722 Rod Stirrer with a propeller!).


* A mixture of 100 mL MIBK + 100 mL ethanol + 2 mL TEGO add (order no. 6.2317.1X0) is used
  as the solvent.



Remarks:
• Appropriate amounts of representative samples of homogenised washing powder can
  also be weighed out directly into the titration beaker and dissolved in 50 mL water        Fig. 1:   Parameter report of 716 DMS Titrino.
  (without intermediate dilution).
• As the approximate content of anionic surfactants is normally known for washing
  powders the titration can be carried out with a starting volume and a stop volume.
• The average titration time was 108 s.
• It must be pointed out that it may not be possible to achieve such short titration times
  with all washing powder formulations.
• The titration results obtained with the Surfactrode Refill were compared with those
  obtained by a manual Epton two-phase titration:
     Surfactrode Refill: AVG(5) = 21.57 ± 0.05% anionics
     Epton titration:   AVG(3) = 21.24 ± 0,04% anionics
• The 716 DMS Titrino, 736 GP Titrino, 751 GPD Titrino or the 726 Titroprocessor can be
  used as the titrator.




                                                                                             Fig. 2:   Titration curve for the determination of
                                                                                                       anionic surfactants in a washing powder.




18                                                                                                        Metrohm Information No. 1/1999
                                                         Recent applications


                                                     Vitamin C and total acid content of instant fruit drinks
                                                     These titrations can be carried out with the 702 SET/MET Titrino, 716 DMS Titrino, 736 GP
                                                     Titrino, 751 GPD Titrino or 726 Titroprocessor.
                                                     The vitamin C content of the fruit drink is determined by titration with iodine solution
                                                     c(1/2 I2) = 0.1 mol/L. Endpoint indication is bivoltametric (MET, Ipol = 1 µA) at a double
                                                     Pt electrode. The total acid concentration is determined by an endpoint titration (SET pH)
                                                     to pH = 8.5. The titrant is c(NaOH) = 0.1 mol/L; a combined pH glass electrode is used
                                                     for potentiometric indication.

                                                     Determination of vitamin C
                                                     Approx. 5 g sample is dissolved in 60 mL dist. water and titrated with the iodine solution.

                                                     Determination of the total acid
                                                     Approx. 2 g sample is dissolved in 60 mL dist. water and titrated with NaOH to pH = 8.5.
                                                     The results obtained from the analysis of various instant fruit drinks are summarised in Table
                                                     1. Each of the values shown is the average of three determinations.


                                                     Table 1:   Vitamin C and total acid content of various instant fruit drinks
Fig. 3:   Parameter settings on the 716 DMS Ti-
          trino for the vitamin C determination.       Taste                                 Vitamin C [mg/25 g]              Total acid [meq./g]
                                                       Lemon                                 60.34 ± 0.00                     0.792 ± 0.10
                                                       Mango                                 64.73 ± 0.29                     0.551 ± 0.08
                                                       Mandarin                              60.40 ± 0.00                     0.246 ± 0.11
                                                       Peach/apricot                         68.48 ± 0.39                     0.490 ± 0.00




Fig. 4:   Titration curve for the determination of
          vitamin C in a lemon drink.




                                                                                                                Fig. 5:      Parameter settings on the
                                                                                                                             716 DMS Titrino for the to-
                                                                                                                             tal acid determination.




                                                                                                                Fig. 6:       Report for the determina-
                                                                                                                              tion of the total acid con-
                                                                                                                              tent of the lemon drink.




Metrohm Information No. 1/1999                                                                                                                          19
                                                                Recent applications


Determination of terminal amino groups in polyamide (nylon)
by non-aqueous potentiometric titration
This analysis was carried out according to the specifications of a user in the Far East.

Sample preparation
Approx. 0.5 g of the sample material is weighed out into an Erlenmeyer flask. To this are
added 35 mL freshly vacuum-distilled m-cresol and 10 mL methanol. The mixture is heated
under reflux at 50 °C for 1 h and then allowed to cool down to room temperature. A blank
solution (solvents but no sample material) is treated in the same way.

Analysis
The cool solution is titrated with c(HCl) = 0.02 mol/L in ethanol using a combined pH glass
electrode (e.g. 6.0204.100).

Calculation
meq. R-NH2 / kg sample = (EP1 – C01) * C02 * C03 / C00
where
EP1   =     titrant consumption in mL
C00   =     sample weight in g
C01   =     titrant consumption for blank in mL
C02   =     0.02 (concentration of titrant in mol/L)
C03   =     1000 (conversion factor g/kg)                                                        Fig. 7:     Result report and titration curve for the
                                                                                                             determination of terminal amino groups
Result of a 4-fold determination:                                                                            in nylon, carried out using a 716 DMS
                                                                                                             Titrino.
AVG(4) = 43.8 ± 0.3 meq. R-NH2 / kg sample


Automatic titration of the phosphate content of animal feed
and mineral salt mixtures after digestion of the samples

Sample preparation
An adequate amount of the sample is initially ashed in a platinum or porcelain basin with
a Bunsen burner. The sample is then heated in a muffle furnace for 1 h at 800 °C. To the
cooled-down ashes one adds 10 mL dist. water, 2 mL conc. H2SO4 and 1 mL w(HClO4) =
60%. The mixture is heated and evaporated almost to dryness. A further 10 mL dist. water
is added, the mixture heated to boiling and then allowed to cool down. The solution is rinsed
into a 100 mL volumetric flask with dist. water, filled up to the mark and mixed.

Automatic titration
In a beaker, 50 mL dist. water is added to 10.0 mL of the prepared sample solution. In a first
SET titration a preliminary neutralisation to pH 3 ... 3.5 is carried out with c(NaOH) = 0.2
mol/L. A second SET titration is then carried out with c(NaOH) = 0.1 mol/L to pH = 4.2.
Then 10 mL c[La(NO3)3] = 0.1 mol/L is added and the third SET titration to pH = 4.2 is
carried out. The resulting consumption of c(NaOH) = 0.1 mol/L is used to calculate the
phosphate or phosphorus content of the sample.


1 mL c(NaOH) = 0.1 mol/L corresponds to 4.7486 mg PO4 or 1.5487 mg P

Results:
Additive:         AVG(3) = 0.0259 ± 0.00005 g PO4 / g sample
Premix:           AVG(3) = 0.0751 ± 0.00020 g PO4 / g sample
The following instruments and accessories were used for this application:
                                                                                                 It is of course possible to carry out the prelimi-
736 GP Titrino, 685 Dosimat, 700 Dosino with Dosing Unit, 728 Magnetic Stirrer, two 20           nary additions manually and the titration itself on
mL Exchange Units (6.3014.223) and 6.0239.100 combined pH glass electrode.                       a more economical Titrino.


20                                                                                                               Metrohm Information No. 1/1999
                                                                Recent applications



   ‘pa                                                   ‘pa                                                   ‘pa
  736 GP Titrino Test00d        736.0012                736 GP Titrino Test00d          736.0012             736 GP Titrino Test00d 736.0012
  date 1998-06-24    time       15:19            0      date 1998-06-24    time         15:24          0     date 1998-06-24    time 15:26                 0
  TIP              P Det.                               SET pH            SET 1                              SET pH            SET 2
  parameters                                            parameters                                           parameters
  > sequence                                            >SET1                                                >SET1
    1.method:           S       1                         EP at pH         4.20                                EP at pH         4.20
    2.method:         SET       1                         dynamics             2                               dynamics             2
    3.method:         DOS       1                         max.rate            10        ml/min                 max.rate         10.0 ml/min
    4.method:         SET       2                         min.rate            25        µl/min                 min.rate         25.0 µl/min
  >statistics                                             stop crit:      drift                                stop crit:      drift
    status:           OFF                                 stop drift          20        µl/min                 stop drift          20 µl/min
  >preselections                                        >SET2                                                >SET2
    req.ident:        OFF                                 EP at pH          OFF                                EP at pH          OFF
    req.smpl size:    OFF                               >titration parameters                                >titration parameters
    meas.mode:        OFF                                 titr.direction: auto                                 titr.direction: auto
    temperature      25.0       °C                        pause 1              0        s                      pause 1              0 s
            ——————                                        start V:          OFF                                start V:          OFF
                                                          pause 2              0        s                      pause 2              0 s
                                                          extr.time            0        s                      extr.time            0 s
   ‘pa                                                    dos.element: internal         D0                     dos.element:internal D0
  736 GP Titrino Test00d        736.0012                  meas.input:          2                               meas.input:          1
  date 1998-06-24    time       15:22            0        temperature      25.0         °C                     temperature      25.0 °C
  SET pH                S1                              >stop conditions                                     >stop conditions
  parameters                                              stop V:          abs.                                stop V:          abs.
  >SET1                                                   stop V          99.99         ml                     stop V          99.99 ml
    EP at pH         3.25                                 filling rate     max.         ml/min                 filling rate     max. ml/min
    dynamics          OFF                               >statistics                                          >statistics
    max.rate          100       ml/min                    status:           OFF                                status:           OFF
    min.rate            20      µl/min                  >preselections                                       >preselections
    stop crit:      drift                                 conditioning:     OFF                                conditioning:     OFF
    stop drift          20      µl/min                    req.ident:        OFF                                req.ident:        OFF
  >SET2                                                   req.smpl size:    OFF                                req.smpl size:    OFF
    EP at pH          OFF                                 activate pulse:   OFF                                activate pulse:   OFF
  >titration parameters                                           ——————                                               ——————
    titr.direction: auto
    pause 1              0      s                        ‘pa
    start V:          OFF                               736 GP Titrino Test00d 736.0012
    pause 2              0      s                       date 1998-06-24     time 15:25                 0
    extr.time            0      s                       DOS                DOS 1
    dos.element: external       D2                      parameters
    meas.input:          1                              >dosing parameters
    temperature      25.0       °C                        dispensing type:volume
  >stop conditions                                        volume          10.000 ml
    stop V:          abs.                                 disp.crit:        rate
    stop V          99.99       ml                        rate              max. ml/min
    filling rate     max.       ml/min                    pause                0 s
  >statistics                                             time interval       10 s
    status:           OFF                                 dos.element: external D1
  >preselections                                          temperature       25.0 °C
    conditioning:     OFF                               >stop conditions
    req.ident:        OFF                                 stop V:            OFF
    req.smpl size:    OFF                                 filling rate      max. ml/min
    activate pulse:   OFF                               >statistics
            ——————                                        status:            OFF
                                                        >monitoring
                                                          meas.mode:         OFF
                                                          temperature:       OFF
                                                          assign output:    none
                                                        >preselections
                                                          req.ident:         OFF
                                                          req.smpl size:     OFF
                                                          activate pulse:    OFF
                                                                  ——————


Fig. 8:   Parameter settings on the 736 GP Titrino for the automatic phosphate titration. The TIP method used comprises three SET titrations as well as a dos-
          ing step.




                                                           Fig. 9: Report for the determination of the phosphorus or phosphate content of the premix.




Metrohm Information No. 1/1999                                                                                                                             21
                                                            Recent applications


Ion chromatography with chemical suppression:


Traces of fluoride, chloride, nitrate, orthophosphate and sul-
phate in boric acid after sample enrichment


Instruments and accessories
2.732.0X10      IC Detector
2.733.0X20      IC Separation Center with two injection valves
2.709.0X10      IC Pump
2.753.0010      Suppressor module
2.752.0010      Pump Unit
2.714.0310      IC Metrodata for Windows™ 95
6.1006.100      Metrosep Anion Dual 2 separation column
6.1006.200      Enrichment column
6.2620.150      Pulsation dampener

Reagents
• Eluent:
  1.0 mmol/L Na2CO3, 4.3 mmol/L NaHCO3 in ultrapure water
• Anion standard solutions:
  1, 5, 10, 50 and 100 µg/L each of fluoride, chloride, nitrate, orthophosphate and
  sulphate in 2% boric acid
• Regeneration solution:
  20 mmol/L sulphuric acid

Analysis
Solutions with a concentration of w(H3BO3) = 2% in ultrapure water are first prepared by
dilution.
2 mL standard solution or sample solution are concentrated on the enrichment column. This
is then rinsed with 1 mL ultrapure water (to remove the boric acid) and then eluted/
chromatographed.




                                                                                            Fig. 10:   732 IC Detector reports: parameters and
                                                                                                       configuration.




Fig. 11:   Chromatogram of a sample solution.


22                                                                                                        Metrohm Information No. 1/1999
                                                     Recent applications


                                                  In-line sample preparation using the 754 Dialysis Unit:


                                                  Ion chromatographic determination of calcium, magnesium,
                                                  sodium and potassium in soybean products
                                                  Various soybean drinks and desserts based on soybeans were investigated.

                                                  Sample preparation
                                                  The sample package is vigorously shaken manually and the whole contents transferred to
                                                  a plastic beaker. The sample is then mixed thoroughly with a high-frequency mixer. 20 mL
                                                  of the treated sample is removed, diluted with c(HNO3) = 1 mmol/L to 100 mL and mixed.
                                                  5 mL of this mixture is again diluted with c(HNO3) = 1 mmol/L to 100 mL and mixed. This
                                                  results in a total dilution of 1 : 100.

                                                  Analysis
                                                  By using the 754 Dialysis Unit in combination with 766 IC Sample Processor the sampling
                                                  and subsequent dialysis can be automated.
                                                  The standard or sample solutions are dialysed for 10 min and the acceptor solution with the
                                                  enriched ions then injected into the IC system.
                                                  Acceptor solution:      1 mmol/L HNO3 in ultrapure water
                                                  Eluent:                 4 mmol/L tartaric acid, 1 mmol/L dipicolinic acid in ultrapure
                                                                          water, microfiltered
                                                  Separation column:      Metrosep Cation 1-2 (6.1010.000)


                                                  The evaluation of the chromatogram is carried out with IC Metrodata for Windows™ 95.




Fig. 12:   766 IC Sample Processor reports: pa-   Fig.13:    Chromatogram of a soybean drink.
           rameters and configuration


Metrohm Information No. 1/1999                                                                                                           23
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Description: Determination of anionic surfactants in a washing powder by means of potentiometric two-phase titration