What factors could cause pH readings to shift when using red cabbage indicator?

[Jack_B]

I’ve been trying to measure the pH of my homemade vinegar using red cabbage indicator, but my results keep shifting. I can’t tell if it’s my lighting conditions affecting how I see the color, or if the solution itself isn’t stable.

[Mila.D]

Yeah I tried something similar last month. The color shifts drove me crazy until I started treating the indicator like a tiny color experiment rather than a final judge. I kept the extraction in a small glass jar, filtered it, and used a white cardstock as a background. I used a daylight lamp and checked colors on a neutral card to compare against. Also I kept the sample at room temp and used the same drop volume every time. The readings still drifted a little, but the trend was clearer.

[MatthewM]

Tried making indicator fresh every time and it helped a bit. If you reuse old cabbage extract, pigments break down and color response changes. I tossed a batch after a day in the fridge; it was more stable for a few hours, then wandered. Also I noticed tiny bubbles or turbidity changed the shade.

[Mark_J]

I wonder if the real issue is not lighting but the indicator itself being unstable or non linear across that pH range. Red cabbage pigments swing with temperature, metal ions, and extraction strength. If your vinegar is mildly carbonated or has dissolved solids, that can tint the result too. The best you can do is use a fresh, single batch of indicator and test a reference solution of known acidity to see how your color map looks. Do you actually have a reference color chart to compare against?

[Ronald.R]

I drifted off topic once and tried comparing to store bought strips, and the numbers were closer when I used glass cups and kept things still. I think the pigment reacts a bit with stainless or plastic too, so I stuck to plain glass and a stationary setup. It wasn’t perfect, but it made the drift less tiring to chase.