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Industrial chemicals & solvents · Clear, nearly colourless liquid (solvents, glycols, plasticisers, water-white chemicals)

Colour (APHA / Hazen / Pt-Co)

The colour of a clear liquid is compared with a series of yellow platinum-cobalt standards in matched tubes, or measured in a colour spectrophotometer. The result is the Pt-Co (APHA or Hazen) number of the standard that matches best.

  • Chemicals & materials
  • Visual comparison with Pt-Co standards, or instrumental colorimetry
  • Colorimeter
  • Burette & glassware
  • Analytical balance

Reference: Typical method based on ASTM D1209 and ISO 6271. The instrumental option uses a colour spectrophotometer that reports Pt-Co units.

PRINCIPLE

Why this method works

Many 'water-white' chemicals turn faintly yellow as they age, oxidise or pick up contamination. The Pt-Co scale is a set of standards made from potassium chloroplatinate and cobalt(II) chloride in dilute hydrochloric acid. The 500-unit stock has 500 mg/L of platinum; each unit is equal to 1 mg Pt per litre. The sample is viewed down the length of a tube against a white background next to the standards, and the nearest match is reported. Instruments calculate the same number from transmittance measurements.

Instruments used

MATERIALS

Reagents & standards

ReagentGrade / specification
Potassium chloroplatinate (K₂PtCl₆)Analytical grade, 1.246 g per litre of 500-unit stock
Cobalt(II) chloride hexahydrateAnalytical grade, 1.000 g per litre of stock
Hydrochloric acidConcentrated, 100 mL per litre of stock
WaterPurified water, also used as the 0-unit reference
SAMPLE PREPARATION

Preparing the sample

  1. 01

    Prepare stock

    Make the 500-unit Pt-Co stock solution, or use a certified commercial stock.

  2. 02

    Dilute standards

    Dilute the stock to a series of standards that bracket the expected colour.

  3. 03

    Fill tubes

    Fill matched Nessler tubes with sample and standards to the same mark.

  4. 04

    Compare

    Look down the tubes against a white background and pick the nearest standard.

Procedure

  1. 1Prepare the 500-unit stock: dissolve 1.246 g potassium chloroplatinate and 1.000 g cobalt(II) chloride hexahydrate in water with 100 mL hydrochloric acid and dilute to 1000 mL. Store in the dark.
  2. 2Prepare working standards in 100 mL volumetric flasks (for example 5, 10, 15, 20, 25 and 50 units) by diluting the stock with water.
  3. 3If the sample is hazy, let it settle or filter as the method allows; haze makes colour look darker.
  4. 4Fill a clean, matched 100 mL tall-form Nessler tube with sample to the mark, and fill tubes with the bracketing standards.
  5. 5Compare by looking vertically down the tubes against a white surface in good diffuse light.
  6. 6Report the matching standard. If between two, report the higher or the nearer as your method states.
  7. 7For the instrumental method, calibrate the colorimeter, blank with water in the specified cell and read the Pt-Co value directly.

Standard preparation

  1. 1Volume of 500-unit stock per 100 mL = required units × 100 ÷ 500 (for example 2.0 mL for 10 units, 10.0 mL for 50 units).
  2. 2Working standards are stable for a limited time; prepare fresh as your SOP states (often monthly) and keep them stoppered and dark.

Notes for analysts

  • APHA, Hazen and Pt-Co are names for the same scale. The Gardner scale (darker, for resins and oils) and Saybolt scale (petroleum) are different and cannot be converted exactly.
  • Only clear liquids with a yellowish hue suit this scale; samples of a different hue cannot be matched well.
  • Diluting a dark sample assumes the colour is proportional to concentration; state the dilution in the report.
TEST CONDITIONS

Method parameters (typical)

Scale range0–500 Pt-Co units
TubesMatched tall-form Nessler tubes, 100 mL, with equal liquid height
ViewingVertically through the liquid column against a white background
Instrumental optionTransmission measurement in a colour spectrophotometer with Pt-Co scale (path length as the instrument requires, often 50 or 100 mm)
BEFORE YOU REPORT

System suitability & sequence

ParameterLimit
Standard checkA Pt-Co standard (for example 25 units) read on the instrument within about ±2 units, or as your SOP states
Tube matchTubes filled with water look identical side by side

Typical sequence

  1. 1Prepare or check standards
  2. 2Calibrate instrument (if used)
  3. 3Measure check standard
  4. 4Measure sample(s)
CALCULATION

Calculation with a worked example

Formula
Volume of stock (mL) = C × V ÷ 500; Colour of diluted sample = reading × (final volume ÷ sample volume)

Where

C
Required standard colour (Pt-Co units)
V
Final volume of the standard (mL)
500
Colour of the stock solution (Pt-Co units)

Given

Standard needed
15 Pt-Co units in 100 mL
Dark sample
10.0 mL diluted to 50.0 mL with water-white solvent; diluted reading 70 Pt-Co

Working

  1. 1
    Stock volume for 15 units = 15 × 100 ÷ 500 = 3.0 mL of stock made up to 100 mL
  2. 2
    Dark sample: dilution factor = 50.0 ÷ 10.0 = 5
  3. 3

    Colour of sample = 70 × 5 = 350 Pt-Co units

Take 3.0 mL stock per 100 mL for the 15-unit standard; the diluted dark sample has a colour of about 350 Pt-Co

Try it with your numbers
ACCEPTANCE CRITERIA

What result passes

  • Compare with the product specification, for example ≤ 10 or ≤ 15 Pt-Co for many water-white solvents and glycols (check your specification).
METHOD VALIDATION

Validation parameters (ICH Q2(R2))

ParameterTypical requirement
PrecisionRepeatability between analysts and between visual and instrumental results
AccuracyInstrumental readings on prepared Pt-Co standards across the range
In LimsPulse · Sample Management

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